Halofuginone | Mll Signal

The chemistry and biology of febrifugine and halofuginone
Noel P. McLaughlin a, Paul Evans a,⇑, Mark Pines b,⇑
aCentre for Synthesis and Chemical Biology, School of Chemistry and Chemical Biology, University College Dublin, Dublin 4, Ireland
bAgricultural Research Organization, The Volcani Center, Institute of Animal Science, P.O. Box 6, Bet Dagan 50250, Israel

a r t i c l e i n f o

Article history:
Received 22 December 2013 Revised 14 February 2014 Accepted 18 February 2014 Available online 1 March 2014

Keywords: Natural product Quinazolinone
3-Hydroxypiperidine Isomerization Antimalarial
Fibrosis
a b s t r a c t

The trans-2,3-disubstituted piperidine, quinazolinone-containing natural product febrifugine (also known as dichroine B) and its synthetic analogue, halofuginone, possess antimalarial activity. More recently studies have also shown that halofuginone acts as an agent capable of reducing fibrosis, an indication with clinical relevance for several disease states. This review summarizes historical isolation studies and the chemistry performed which culminated in the correct structural elucidation of naturally occurring febrifugine and its isomer isofebrifugine. It also includes the range of febrifugine analogues prepared for antimalarial evaluation, including halofuginone. Finally, a section detailing current opinion in the field of halofuginone’s human biology is included.
ti 2014 Elsevier Ltd. All rights reserved.

Contents

1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1993
2.Activity guided isolation and gross structural determination of febrifugine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1994
3.Baker’s synthesis of febrifugine and isofebrifugine and their structural misassignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1995
4.Ambiguity surrounding the structure of febrifugine: Kobayashi’s confirmatory synthesis of (+)-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1997
5.Structural analogues of febrifugine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1998
6.Halofuginone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2001
6.1.Halofuginone and fibrosis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2002
6.2.Halofuginone and cancer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2002
6.3.Mode of action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2002
6.4.Clinical status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2003
7.Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2003 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2003 References and notes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2003

1.Introduction

The isomeric alkaloids (+)-febrifugine (1) and (+)-isofebrifugine (2), Figure 1, are found in the roots and leaves of the Chinese medicinal plant Dichroa febrifuga (also called Chinese quinine) belonging to the Saxifragaceae family.1 For centuries the roots (Chang Shan) and leaves (Shuu Chi) of Dichroa febrifuga have been
used in East Asia to treat the symptoms of fevers. Decoctions of Chang Shan, considered one of the fundamental herbs in Chinese medicine, have also been used in the treatment of a variety of ailments including stomach cancer. Its traditional use in the treat- ment of malaria led, in the mid-twentieth century, to investiga- tions aimed at evaluating whether the extracts of Chang Shan might yield a new antimalarial drug.2–4 These investigations also demonstrated that febrifugine is found in at least one other plant

⇑ Corresponding authors. Tel.: +353 17162291.
E-mail addresses: [email protected] (P. Evans), [email protected] (M. Pines).

http://dx.doi.org/10.1016/j.bmc.2014.02.040

0968-0896/ti 2014 Elsevier Ltd. All rights reserved.
belonging to the Saxifragaceae family.5
Ultimately, these efforts failed to result in a human treatment for malaria due to side-effects including nausea, vomiting and liver

N
H

OH
O

(+)-1

N
H

O OH

(+)-2

N
H

OH N
O
N
(±)-3 O

8′

7′

Figure 1. Chemical structures of (+)-febrifugine (1), (+)-isofebrifugine (2) and (±)-halofuginone (3).

toxicity. However, an analogue termed halofuginone (3), designed with the aim of improving the therapeutic potential of 1, has been used in racemic form as an antiprotozoal agent in the poultry industry. Studies investigating the optimal dose of 3 for the treat- ment of poultry led to the discovery that this compound is an effec- tive inhibitor of type 1 collagen biosynthesis6 not only in avian species, but also in humans.7 This observation, in turn, led to the identification of 3 as a lead compound for the development of an agent for the treatment of uncontrolled collagen biosynthesis. Halofuginone (3) has been the subject of a clinical trial aimed at assessing its potential as an anticancer agent8 and is currently the subject of a Duchenne muscular dystrophy clinical trial.9
The aim of this review is to summarise the history of this natu- ral product and to highlight future avenues for possible research efforts particularly related to recent findings concerning the human activities of this type of compound.10

2.Activity guided isolation and gross structural determination of febrifugine

In terms of structure elucidation, when these compounds were first reported in the mid-twentieth century, the determination of their relative and absolute stereochemistry proved to be an extre- mely challenging problem. Indeed, even with the spectroscopic tools of today, the stereogenicity of the hemiketal carbon in isofeb- rifugine (2) remains unknown. This puzzle was compounded by the fact that it is now appreciated that under certain conditions febrifugine (1) and isofebrifugine (2) interconvert via cis-1 by a ret- ro-conjugate addition — conjugate addition — hemiketal formation sequence (Scheme 1).10,11 In relation to the hemiketal present in (+)-2, as shown in Scheme 1, in the cis-configuration, the electro- philic ketone group is proximal to the hydroxyl group in either chair form. In contrast, in either the diaxial, or the diequatorial form, trans-(+)-1 is either precluded from, or less able to form the hemiketal. Due to this unappreciated isomerization, histori- cally, mistakes concerning the precise structure of these alkaloids have been made which were only completely resolved following the elegant synthetic work from the groups led by Kobayashi in 199912 and Takeuchi and Harayama in 2003.12
Febrifugine’s contemporary story began in 1943 when Chou and co-workers reported that a crude extract of Chang Shan (from Dichroa febrifuga Lour.) could be effectively used in clinical cases
of tertian malaria.4,14 Crude extracts of Shuu Chii were found to be five times more active against Plasmodium gallinaceum in chicks than Chang Shan. This difference in activities was later accounted for by Koepfli et al.’s discovery that isofebrifugine (2), which is inactive against Plasmodium gallinaceum, occurs only in the roots of Dichroa febrifuga Lour.1 In 1946, the same Chinese group reported the isolation of two alkaloids from Chang Shan as their hydrochloride salts which they termed ‘dichroine A’ (mp 230 tiC) and ‘dichroine B’ (mp 237–238 tiC).15 The alkaloid dichroine B was found to be active against Plasmodium gallinaceum in chickens whereas dichroine A was found to be inactive against the same malarial strain.
In 1947, the empirical formula of dichroine B was reported to be C16H19O3N3.16 Later that year, Koepfli et al. isolated two isomeric alkaloids from the same plant source and called these alkaloids febrifugine and isofebrifugine the former being isolated from both the roots and the leaves of the plant with the latter being isolated only from the roots.1 The empirical formula calculated for these alkaloids corresponded exactly with that reported by Chou et al. for dichroine B.16 Koepfli et al. determined the melting point and
D
D
(c = 0.35, CHCl3)]. They found that febrifugine was dibasic and that the dihydrochloride salt (mp 220–222 tiC) formed readily upon addition of 10% excess 12 M HCl to an alcoholic solution of the alkaloid. The total alkaloid content of Chang Shan was estimated to be 0.1% of the dry mass of the roots; a figure that was invariably found to be 5–10 times greater than the alkaloid content of the leaf derived Shuu Chii. Febrifugine was observed to be 100 times more active against Plasmodium Lophurae in ducks than quinine (which was the antimalarial drug of choice at the time) whilst isofebrifu- gine possessed only modest activity against the same malaria strain.1
In 1948, Chou et al. were the first to report that dichroine A and dichroine B were isomers, undergoing interconversion under the action of heat, acid, alkali hydrolysis and even in the presence of different solvents.17 The melting points of the free bases of dichro- ine A (136 tiC) and dichroine B (145 tiC) were reported and found to be more consistent with the values reported by Koepfli et al. for isofebrifugine and febrifugine, respectively. Thus, since only one set of alkaloids has ever been isolated from Dichroa febrifuga Lour. and on the basis of the very similar experimental data, one may

N
H

OH
O

(+)-1

N
H

OH N
O
N
cis-1 O

N
H

O OH

(+)-2

Het
O

HN
H

H
OH

Het

H
H
HN
OH
O Het
HN
H

Het
H

OH
O

(+)-2

trans-(+)-1a trans-(+)-1b cis-1a cis-1b
{Het = quinazolinone}

Scheme 1. The interconversion of febrifugine (1) into isofebrifugine (2) (‘wiggly’ bond defines unknown stereochemistry at stereogenic centre).

N
H
X

O OH N

hemiketal

N
H
Y

OH
N

N
H
Z

OH
O

quinazolinone

Figure 2. Koepfli’s proposed structures (X and Y) for febrifugine and isofebrifugine and the American Cyanamid structure (Z).

now conclude that Chou’s dichroine materials and the alkaloids isolated by Koepfli et al. in 1947 were identical.
In their 1948 publication, Chou et al. described, in detail, the extraction and isolation of the ‘dichroine’ Chang Shan alkaloids.17 During this process, finely powdered material was percolated in 90% ethanol for 2 days and the resulting filtrate was concentrated under reduced pressure. These actions afforded a residue which was acidified using HCl, then extracted several times with ether to give an ethereal extract. The acidic aqueous solution was ren- dered weakly alkaline using sodium bicarbonate and was extracted with an 80:20 mixture of ether and chloroform affording a second extract. Finally, the pH of the remaining aqueous solution was made more strongly alkaline via the addition of potassium carbon- ate and extracted several times with chloroform resulting in a third extract. It was from this final extract that the alkaloids were iso- lated and were purified from one another using crystallization techniques. In the same year a research group led by Keuhl reported a modified procedure culminating in the isolation of two alkaloids from both Indian and Chinese Dichroa febrifuga Lour. which were shown to be chemically equivalent to febrifugine and isofebrifugine.18 The alkaloidal content of the roots and leaves of the Chinese plant was found to be consistent with that previously reported by Koepfli et al.1 In contrast, the identical Indian plant was found to contain only one tenth of the alkaloidal content.
During Chou’s 1948 study, the isolation of 3H-quinazolin-4-one, a structural motif that at the time was known to be present in sev- eral alkaloids,19 was also reported. Based on this and several chem- ical degradation studies the conclusion that the dichroine alkaloids both possessed the heteroaromatic 4-quinazolinone entity was made. This thesis was corroborated by Koepfli et al.20 and the com- bined results from these structural investigations led to an insight- ful proposal for the structures of febrifugine and isofebrifugine (Fig. 2). Although at the time it was impossible to be specific about the absolute configuration of the stereocenters, they proposed that the alkaloids existed as the hemiketal diastereoisomers, X and Y, which differed in the configuration of the hemiketal carbon atom.
Alternative natural sources as potential leads for new antima- larial agents were studied by an industrial team in American Cyan- amid and during this work extracts from Hydrangea umbellata (which also belongs to the saxifragaceae family) were analysed. In 1952, antimalarial activity in acidic aqueous extracts of hydran- gea were reported.5 Partial purification of the active component indicated that the antimalarial activity was due to the presence of an alkaloid-like substance and utilising typical extraction proce- dures, 23 mg of a crystalline material was isolated from 1.0 kg of dried plant leaves and stems. Further analysis demonstrated that the hydrangea alkaloid was identical to febrifugine.
ignorance of the sense of relative and absolute configuration of each stereocenter.
Somewhat surprisingly, details concerning the biosynthesis of febrifugine have not been reported although anthranilate derived biosynthesis of quinazolinone containing natural products23 and indeed several related piperidine-based alkaloids24,25 are well- appreciated.

3.Baker’s synthesis of febrifugine and isofebrifugine and their structural misassignment

Since the initial reports concerning their isolation, isofebrifu- gine (2),26 and particularly febrifugine (1) have been popular syn- thetic targets.27 Apart from supplying a means to augment natural supplies and to prepare analogues for biological studies these efforts eventually led to the confirmation of the structures of 1 and 2. Synthetic efforts aimed towards these alkaloids began in the 1950s when Baker and co-workers designed and executed several synthetic strategies which were ahead of their time.21 Fol- lowing Baker’s first reported racemic synthesis,21g subsequent studies relying on the diastereoselective reduction of substituted heteroaromatic species are particularly noteworthy.28–30
Baker’s second route to febrifugine relied on a diastereoselec- tive reduction of a monosubstituted furan followed by elaboration of the products into piperidines.28 Treatment of 4 with hydrogen in the presence of Adam’s catalyst and N-benzoylation of the resul- tant amino tetrahydrofuran gave 5 (Scheme 2). This material pos- sessed a melting point of 117 tiC. In contrast, N-benzoyl protection of 4, to give 6 followed by hydrogenation with Pd/C led to the iso- lation of 7 which gave identical combustion analysis to 5 but had a melting point of 156 tiC. Because the required analytical tools were not available at the time their relative structures were not deter- mined, but both diastereoisomeric materials were subsequently used by Baker to access the febrifugine skeleton (Schemes 3 and 4). At this juncture, it should be mentioned that the stereochemis- try of these isomers (5 and 7) were, until relatively recently, a sub- ject of debate. The structures presented (Scheme 2) are based on Takeuchi’s 200313 report.
Acid promoted ring opening of the tetrahydrofuran 8 led to the formation of an intermediate bromohydrin hydrobromide and its treatment with NaOH resulted in a cyclization which afforded a piperidine (Scheme 3). Subsequent N-protection in the presence of ethyl chloroformate resulted in the isolation of 9 in 70% yield

(ii)

In a sequence of papers also published in 1952, the Williams, O CO2H O CO2H
Hutchings and Baker research team reported an elegantly per- 4 NH2 6 NHBz

formed series21 of studies which ultimately led to the proposal of the structure for the antimalarial alkaloid isolated from hydrangea (Fig. 2).22 To achieve this, guided by Chou and Koepfli’s findings, they painstakingly synthesized isomers and analogues of febrifu- gine which were then compared to the naturally occurring mate- rial in terms of properties. At the time the stereochemistry of the
(i), (ii)

O CO2H
NHBz
5: M.p. 117 °C
(iii)

O CO2H
NHBz
7: M.p. 156 °C

alkaloid was not known and therefore structure Z, featuring, unlike X and Y, an open hydroxyl ketone structure, was postulated in
Scheme 2. Diastereoselective hydrogenation of 3-amino(furan-2-yl)propanoic acids 4 and 6. Reagents: (i) H2, PtO2; (ii) benzoylation; (iii) H2, Pd/C.

O
8

NH2

(i)-(iii)

OH (iv)-(vii) OHO
NOH N
CO2Et 9 CO2Et 10

(viii)
Isomer- ization

OH
O
N
N

N
N
H
O
(±)-1 .2HCl Claimed!

N
H
OOH
N
(±)-2 .2HCl

Scheme 3. Baker’s synthesis of (±)-2ti2HCl from tetrahydrofuran 8. Reagents and conditions: (i) HBr, reflux; (ii) NaOH(aq), reflux; (iii) EtOCOCl, base, 70%; (iv) PCl5, AcCl; (v) CH2N2; (vi) HBr, AcOH 68%; (vii) 3H-quinazolin-4-one, base, 75%; (viii) 6 M HCl, reflux, 21%.

O
BzHN

OH
(i)-(iii)

N
Bz
O

O
(iv)

N
Bz
OH
O

12
(v)
OH

N
Bz
OMe
O

7
N
O OH
N
N
O
H
(±)-2 .2HCl claimed!

N
H
(vii)

OR N O
N
O
15: R = Me;
(±)-1 .2HCl: R = H

(vi) Isomer-
ization

N
Bz
4 steps OMe
O

Scheme 4. Baker’s synthesis of (±)-1ti2HCl from tetrahydrofuran 7. Reagents and conditions: (i) HBr, reflux; (ii) Et3N; (iii) BzCl, base, 27%; (iv) NaOH; (v) NaOMe then MeI, 42%; (vi) 6 M HCl, reflux; (vii) HBr, reflux, then recrystallization from HCl saturated EtOH.

over 3 steps. At this point in the synthesis, it was anticipated that the 3-hydroxyl group would require protection in order to prevent lactonization. In actuality O-protection was not required and the reluctance of the hydroxy acid to undergo lactonization was evi- dence that the relative configuration of the piperidine 9 was trans. Utilizing a methylation, bromination–alkylation sequence, 9 was successfully converted to N-protected (±)-febrifugine (10). In the presence of 6 M HCl heated to reflux, the carbamate protecting group was removed.
Based on the recalcitrance of 9 to undergo lactonization Baker and co-workers assumed they had isolated the trans-alkaloid (±)-1ti 2HCl. However, based on subsequent work carried out by Barringer et al., isomerization (see Scheme 6)11a had in fact occurred following prolonged exposure to HCl, which produced (±)-isofebrifugine dihydrochloride (2) in 7.4% yield over 9 steps! The material obtained was compared to the alkaloid isolated from hydrangea5 with respect to melting point and antimalarial activity and it was apparent from these investigations that both com- pounds were not identical (only febrifugine was isolated from hydrangea). Thus, based on this influential work the relative con- figuration of isofebrifugine was incorrectly concluded to be trans.
Subsequently, the alternative isomer 7, obtained from the stereo-complementary hydrogenation reaction (Scheme 2), was employed in order to assemble what was now, in the light of the study outlined above, the expected structure of febrifugine. In the presence of HBr heated to reflux, 7 afforded the bromohydrin hydrobromide (Scheme 4). Unlike Scheme 3, in which 8 was iden- tically treated, this bromohydrin hydrobromide was isolated as a lactone. Ring closure with triethylamine in chloroform and subse- quent N-protection afforded the piperidine lactone 11. The addi- tion of NaOH to 11 gave the hydroxyl acid 12 which proved difficult to handle and readily underwent re-lactonization regener- ating piperidine lactone 11. Therefore, it proved necessary to pro- tect the hydroxyl group in the shape of a methyl ether 13 (this work pre-dates alternative, more labile O-protecting groups).
Utilizing the same methylation, bromination–alkylation sequence, N,O-protected 14 was synthesized in 4 steps from 13. Finally, 14 was doubly deprotected, via 15, to give what Baker expected, based on the facile lactonization, to be 2.
Indeed, the material isolated from this sequence proved to be different from material obtained from the series of reactions described in Scheme 3 and data, including activity, matched the natural product febrifugine. Therefore, Baker and co-workers incor- rectly assumed that febrifugine possessed a 2,3-cis-relationship. However, it was subsequently also uncovered that, remarkably, an isomerization must have again occurred during the final depro- tection steps. An outcome which resulted in Baker et al., ultimately isolating the trans-alkaloid (±)-1ti 2HCl!
In 2003, the isomerization that led to the erroneous structural assignment was unpicked and explained by Takeuchi and co-work- ers.13 They re-prepared 15 and discovered that the stereochemi- cally pure dihydrochloride salt did not undergo isomerization during demethylation. However, it was found that the diprotected compound 14 did undergo isomerization on treatment with 6 M HCl affording, along with 15 (as a mixture of diastereoisomers), both cis- and trans-14, albeit in small amounts.
Based on the sequence illustrated in Scheme 4, Baker et al. devised and carried out a synthesis of the naturally occurring enan- tiomer of febrifugine in 1953.31 Optically active 7 was obtained from the resolution of N-protected 3-amino-3-(tetrahydrofuran- 2-yl)propanoic acid by crystallization of the brucine salt and this material was then subjected to the same chemical transformations described in Scheme 4 in order to complete the synthesis of (+)- 1ti 2HCl. Synthetic (+)-1ti 2HCl proved identical to the naturally occurring alkaloid.
In 1955, believing the more potently anti-malarial natural product possed a 2,3-cis-relationship, Baker et al. devised a more practical synthesis of (±)-febrifugine 1 which employed the hydro- genation of a 2,3-disubstituted pyridine as the key reaction (Scheme 5).29,30 Thus, cis-2,3-disubstituted piperidine 17 was

OMe
(i)

OMe
OH

(ii), (iii)

OMe
O

isomer. However, a flaw with these syntheses was that brutal conditions were required for the final deprotections which,

N
N

16
N
H
(iv), (v)

17
unwittingly, led to the isomerization of the material and ultimately incorrect assignment of the relative stereochemistry of the alkaloid.

N
OH
O

N
N

(vi), (vii)

N
OMe
O

N
N

4.Ambiguity surrounding the structure of febrifugine: Kobayashi’s confirmatory synthesis of (+)-1

H
(±)-1 .2HCl
O CO2Et
18
O

In 1973, Barringer et al. investigated the equilibrium between

Scheme 5. Baker’s 2,3-disubstituted pyridine-based synthesis of (±)-1ti2HCl. Reagents and conditions: (i) PhLi, then MeCHO; (ii) H2, PtO2; (iii) CrO3; (iv) Br2, HBr; EtOCOCl, base; (v) 3H-quinazolin-4-one, base; (vi) 6 M HCl, reflux; (vii) HBr, reflux.

constructed following the diastereoselective reduction of 16.30 The resulting methyl ketone was then selectively brominated, N-protected and alkylated to generate N,O-protected alkaloid 18. The N-protecting group was removed in the presence of 6 M HCl at reflux generating the corresponding dihydrochloride in a step that we now know is accompanied by isomerization. The material obtained from the reaction was treated directly with 48% HBr (also heated to reflux), resulting in the cleavage of the methyl ether and completing the synthesis of (±)-1ti 2HCl (10 steps with an overall yield of 2%).
In summary, Baker et al. completed three independent synthe- ses of (±)-febrifugine dihydrochloride (1).21g,28,30 In addition, an early stage resolution enabled the preparation of the natural
cis- and trans-3-hydroxyl substituted 2-propanone substituted piperidines 19 and 20 in an attempt to understand the relationship between febrifugine and isofebrifugine (Scheme 6).11a In order to remove the added complication of hemiketal formation from the cis-2,3-diastereoisomer, the hydroxyl group was derivatized as an alkyl ether (R = Et or Me). The equilibrium between these isomers was examined with respect to temperature, pH and solvent effects. This isomerization was found to be facile and a mechanism featur- ing a base catalysed b-elimination, followed by rotation and conju- gate addition from either of the diastereotopic alkenyl faces of 21 was proposed (Scheme 6).
In 1952, the synthesis of (±)-febrifugine (1) and subsequent evaluation as an antimalarial compound revealed that the racemic mixture was about half as effective against Plasmodium falciparum as the natural source.32 This observation suggested that the anti- pode of febrifugine possessed little or no antimalarial activity, thus highlighting the need for a selective synthesis of the natural enan- tiomer of febrifugine. From Baker’s synthetic efforts, it was pro- posed that (+)-1 exhibited a cis-relative configuration. In 1962,

ORO OR OR Si- OR ORO

N
H

cis-19

H
N
H

O
Re-
NH2
O
21

H
N
H

O
N
H
trans-20

Scheme 6. Equilibrium between the cis- and trans-piperidines 19 and 20 (R = Et, Me) accounting mechanistically for the interconversion of febrifugine and isofebrifugine.

O
ent-24
OH O

OH O
TBSO OPh
OBn 25: 96% ee
(i), (ii)
O TBSO OPh
OBn
26
H
N
N
Me 24 Sn(OTf)2, SnO,
EtCN, 70% (iii)
TBSO
TBSO

BnO

27
Sn(OTf)2, 22
SnO, EtCN
OSiMe3
OPh
23

O
(iv)
TBSO
OBn
TBSO
ent-25
(i)-(x)
(+)-1

OBn
O
HN
OMe
30
(v)
OPh
OBn

OPMB

OR’ N
O
N
N
R
O
( )-1: R = H; R’ = H

(ix), (x)

N
Boc
32

OBn
O

(vi)-(viii)

NH2

N
OBn
O

OMe
31

OPMB

OMe OMe
OPMB
28 29

Scheme 7. Kobayashi’s Sn(II) mediated asymmetric aldol condensation-based synthesis of (ti )- and (+)-1. Reagents and conditions: (i) TCDI; (ii) n-Bu3SnH; (iii) Dibal; then Swern; (iv) Yb(DS)3, 28, 29; (v) HF; then CBr4, PPh3; (vi) CAN; (vii) Boc2O; (viii) CBr4, PPh3; (ix) 3H-quinazolin-4-one, base; (x) 6 M HCl, reflux.

Hill and Edwards proposed its absolute structure based on Baker’s cis-relative configuration, which was therefore also incorrect.33
In 1999, Kobayashi et al. completed the first asymmetric syn- thesis of (+)-febrifugine (1) not only supporting Barringer’s relative stereochemistry but also finally determining unambiguously the absolute configuration of the alkaloid.12 This strategy relied on two stereoselective carbon–carbon bond forming reactions to introduce the stereogenic centres. As outlined in Scheme 7, the sequence commenced with a tin(II)-mediated asymmetric aldol condensation between the achiral aldehyde (22) and achiral silyl enol ether (23) in the presence of diamine (24). Following this reaction, asymmetry was successfully induced and esters (25) and ent-25 were isolated in good enantiomeric and diastereomeric excess. Chiral, enantioenriched ester (25) was then converted, using a modified version of the Barton–McCombie deoxygenation, to (26). Reduction and subsequent oxidation afforded the aldehyde (27) in 55% yield over 4 steps.
Aldehyde (27) was then used in a three component Mannich type reaction with 2-methoxyaniline (28) and functionalized enol ether (29), which in the presence of ytterbium tri-dodecylsulfate (Yb(DS)3) gave the desired b-aminomethyl ketone adduct (30) in excellent yield but modest diastereomeric excess (anti:syn; 60:40). This mixture of diastereoisomers was then carried through the next two steps to afford piperidine (31) as the major com- pound. The synthesis of unnatural (ti)-febrifugine (1) continued with an oxidative double deprotection with ceric(IV) ammonium nitrate (CAN) followed by N-tert-butyloxycarbonyl (Boc) protec- tion. An Appel bromination reaction afforded the N,O-protected piperidine (32). This a-bromoketone was then reacted with 3H-quinazolin-4-one in the presence of potassium hydroxide to af- ford the N,O-protected alkaloid. Deprotection was carried out in 6 M HCl held at reflux for 35 min and after neutralization (ti)-feb- rifugine (1) was isolated in 11% over the 13 distinct reaction steps.
The isomerization problems encountered by Baker and co-workers did not feature in this end-game possibly due to re- duced HCl exposure which was facilitated by the use of more acid labile protecting groups (i.e., N-Boc and O-benzyl). When the final compound was isolated, analysis of its optical rotation and antima- larial activity revealed that the antipode of naturally occurring feb- rifugine had been synthesized, that is (ti)-1. Naturally occurring (+)-febrifugine (1) was subsequently synthesized from the chiral ester, ent-25, using an identical strategy to that outlined above. Based on this work the absolute configuration of naturally occurring (+)-febrifugine (1) was unambiguously determined to be (2R,3S).

The antimalarial activity against Plasmodium falciparum and cytotoxicity against mouse mammary FM3A in vitro of synthetic
(+)-febrifugine (1) and its antipode (ti)-1 were investigated. Synthetic (+)-1 (EC50 = 7.6 ti 10ti 11 M) showed antimalarial activity similar to that of natural febrifugine (isolated from Dichroa febrifu- ga) against the chloroquine-sensitive FCR-3 strain. The antimalarial activity of synthetic (ti)-febrifugine (1) (EC50 = 2.0 ti 10ti7 M) against the same strain was significantly less than that of its natu- rally occurring antipode.
Over the last 15 years the research group led by Takeuchi have carried out extensive investigations into the structure of febrifu- gine and how systematic structural changes impact on antimalarial activity. During this period the group has published four distinct syntheses and developed a means to access enantioenriched mate- rial. In 1999, as shown in Scheme 8, Takeuchi et al. published their first synthesis of (±)-febrifugine (1).27f,g This 12 step synthesis utilized a Claisen rearrangement to facilitate construction of the 2,3-disubstituted piperidine moiety (34–36). In the penultimate step, deprotection of the benzyloxycarbamate (Cbz) protecting group afforded (±)-2 (not shown) which underwent efficient isom- erization to (±)-febrifugine (1) in ethanol held at reflux. Again this synthesis is notable and clearly demonstrates that the position of equilibrium between febrifugine (1) and isofebrifugine (2) may be manipulated synthetically.
In 2000, Takeuchi et al. completed an asymmetric synthesis of (+)-febrifugine (1) building on the work carried out during the pre- vious synthesis.27h,i This strategy employed baker’s yeast to carry out an optical resolution of racemic 35 resulting in the isolation of (+)-36 in 98% ee and 40% yield. Epimerization of the chiral centre in (+)-35 was affected by base making the resolution more efficient.
During the course of the syntheses of febrifugine (and isofeb- rifugine), the isomerization of febrifugine to isofebrifugine and vice versa is a recurring theme. Investigative work carried out by Barringer et al. on analogues of the natural products,11 revealed that this type of 2,3-disubstituted piperidine existed naturally as a mixture of cis- and trans-diastereoisomers (vide supra) and that heating this piperidine mixture in water influenced the equilib- rium in favour of the trans-diastereoisomer, whereas the addition of concentrated aqueous acid had the opposite effect.

5.Structural analogues of febrifugine

Febrifugine shows powerful antimalarial activity against Plas- modium falciparum, the most insidious malaria causative agent,

OH (i)-(iii)

(iv), (v)

(iii)

(vi)

O Br

N
33

N
H

OHO

(±)-1

N
N
Bn
34

(ix), (x)
N
Cbz
35

N
Cbz

O OH

O
N
Cbz
36

(viii)
N
Cbz
37
(vii)
O

N
Cbz
38

OH
Br

(xi)
O OH
(vi)-(x)

(±)-35 + (+)-1

N
Cbz
N
Cbz

(xii) (+)-35: 90% e.e (+)-36: 98% e.e

Scheme 8. Takeuchi’s synthesis of (±)-1 featuring the isomerization of (±)-2 and the subsequent asymmetric variant. Reagents: (i) BnCl; (ii) NaOMe, allylbromide; (iii) NaBH4; (iv) CbzCl, base; (v) BF3tiEt2O; (vi) NBS; (vii) KOt-Bu; then NBS, H2O; (viii) 3H-quinazolin-4-one, base; (ix) H2, Pd/C; (x) EtOH, reflux; (xi) Baker’s yeast, sucrose; (xii) K2CO3,
MeOH, reflux.

N
H

OHO

(+)-1

O OH

(+)-2

(i)
74%

(i)
22%

HO H

N
40 HO H

N
41

In 2002, Kikuchi et al. reported a number of transformations on (+)-febrifugine 1 and investigated how these structural changes impacted upon activity (Scheme 10).38 Initially, (+)-febrifugine 1 was bis-acetylated to afford 42 and this analogue showed signifi- cantly lower antimalarial activities (EC50 = 9.1 ti 10ti7 M) than (+)-febrifugine 1 (EC50 = 7.0 ti 10ti10 M). Subsequently, (+)-febrifu- gine 1 was N-protected in the presence of ethyl chloroformate to afford the analogue 43. Similarly, this analogue also showed diminished antimalarial activities (EC50 = 4.8 ti 10ti6 M) compared to (+)-febrifugine 1.
Manipulation of the oxidation states of the hydroxyl and keto-

Scheme 9. The synthesis of bicyclic febrifugine analogues 40 and 41. Reagents: (i) acetone, SiO2.

and to date, no parasite resistant to febrifugine has been reported. However, the use of febrifugine as an antimalarial drug has been precluded due to side effects such as diarrhoea, vomiting and liver toxicity.34 The potent antimalarial activity of febrifugine led to studies dating right back to the 1950s around the time of its first isolation which aimed to use it as a lead compound for the con- struction of analogues as potential antimalarial drugs. Evidence indicates that febrifugine, akin to established antimalarials such as chloroquine, elicts its antimalarial response by impairing the formation of haemozoin required for parasite maturation within the host. In the following section, key studies concerning the design of analogues which uncover the structure activity relation- ship for febrifugine with respect to antimalarial activity are presented.
In the 1970s, the synthesis and evaluation of analogues of febrifugine as potential antimalarial drugs demonstrated that the 4-quinazolinone moiety, the nitrogen atom of the piperidine ring and the hydroxyl group were necessary for antimalarial activ- ity.35,36 It was also demonstrated that the absolute configuration of the compound is important since only modest activity is reported for the unnatural enantiomer (see also above).32
In 1999, Takaya et al. synthesized and evaluated the bicyclo acetone adducts 40 and 41 (Scheme 9).37 These bicyclo analogues were synthesized from the natural alkaloids (+)-1 and (+)-2 via a Mannich-type reaction with acetone in the presence of SiO2, thus presenting a locked conformation of the natural alkaloid. The anti- malarial activity against Plasmodium falciparum and in vitro cyto- toxicity against mouse mammary FM3A was investigated. Both
functionalities were next investigated. Oxidation of (+)-1 in the presence of Dess–Martin periodinane afforded keto-analogue 44 which showed antimalarial activities of EC50 = 2.0 ti 10ti8 M. Reduction of (+)-1 in the presence of NaBH4 afforded 45 which showed similar antimalarial activity to ketone analogue 44. Both of these analogues showed high selectivity for Plasmodium falcipa- rum. The cyclization of the hydroxyl analogue 45 in the presence of dimethoxymethane afforded a mixture of cyclized compounds 46 and 47. Both these analogues 46 (EC50 = 3.7 ti 10ti 9 M) and 47 (EC50 = 8.6 ti 10ti9 M) showed high antimalarial activities with 46 showing excellent selectivity for Plasmodium falciparum. Similar types of analogues derived from (+)-isofebrifugine 2 showed signif- icantly less activity than those derived from (+)-febrifugine 1.
Kikuchi et al. also synthesized racemic analogues 48-51 (Fig. 3) using a 1,3-dipolar cycloaddition approach.38 These analogues were then evaluated in order to deduce the roles played in terms of activity by both the fused benzene moiety and the N-1 atom of the 4-quinazolinone ring. All analogues exhibited moderate antimalarial activity but were not selective against Plasmodium fal- ciparum, thus indicating the importance of the 4-quinazolinone moiety in antimalarial activity and selectivity.
In 2003, Hirai et al. reported the biochemical synthesis of metabolites of (+)-febrifugine (1) generated by mouse liver S9.39 The metabolites 52 and 53 (Fig. 4) were isolated after incubation of (+)-febrifugine 1 with mouse liver S9 for one hour. They were then extracted and purified by HPLC. Two additional metabolites 54 and 55 were also isolated after identical incubation of the bicyclo analogue 40 (Scheme 9). The structures of these four metabolites were elucidated by NMR and mass spectrometric

analogues 40 (EC50 = 1.6 ti 10ti9 M) and 41 (EC50 = 2.8 ti 10ti9 M) showed antimalarial activity with much higher potency in vitro than chloroquine (EC50 = 1.8 ti 10ti8 M) against the chloroquine- sensitive FCR-3 strain. These analogues proved selective for Plas-

N
H
OH
O

N
N

N
H
O OH

N
N

modium falciparum and showed high antimalarial activity against the chloroquine-resistant K1 strain. However, their cytotoxicity against mouse mammary FM3A was similar to that of the natural alkaloids. In vivo, the antimalarial activity of 40 derived from (+)- febrifugine (1) was observed to be 24 times more potent against

N
H
OH
O

N
H

O OH

the mouse malaria parasite Plasmodium berghei than 41.
Figure 3. Quinazolinone ring analogues of (+)-febrifugine 1.

1
OR
O

(i)

OH
O

(iii)

N N
R O
42: R = R1 = COMe; 43: R = CO2Et; R1 = H
or (ii)
N
H

(+)-1 (iv)
OH
OH
N

(v)
N
H
44 RO H

H
N

N
H

45
N

O
N O
46: R = H; 33%
N

47: R = MOM; 10%

Scheme 10. Kikuchi’s analogues of (+)-febrifugine (1). Reagents: (i) Ac2O; (ii) ClCO2Et; (iii) Boc2O; then DMPI; then HCl; (iv) NaBH4; (v) CH2(OMe)2.

OH
O

H
N

OH
O

N
N
OH N
N
N
N

H
52O
H
53O
H
54O

N
H

OH
O

N
H

O OH

N
N

Figure 4. Metabolites 52–55 of (+)-febrifugine (1) and synthetic isomer 56.

N
H
OH
O

57
OH
O

N
N

F N H
OH
O

58
OH
O

N
N

N
H
OH
O

59
OH
O

N
N

O
N
N
Me

N
N
N
N
S
N
N

H
60O
H
61O
H
62

Figure 5. Racemic quinazolinone analogues of febrifugine.

analysis and were then prepared by total synthesis. The antipode of (+)-febrifugine (1) was reported by Koybashi et al. to have 1/2632 of the antimalarial activity of the natural alkaloid (Section 1.2.1)12 which allowed inferences to be made regarding the relative EC50 values of these racemic metabolites.
The antimalarial activity was measured against Plasmodium fal- ciparum (FCR-3 strain). In vitro metabolite 52 (EC50 = 2.2 ti 10ti9 M) showed high antimalarial activity with good selectivity whereas the metabolite 53 (EC50 = 6.6 ti 10ti6 M) showed low antimalarial activity and little selectivity. In vivo, 52 showed low toxicity whilst maintaining antimalarial activity (ED50 = 0.6 mg/Kg/day) compara- ble with that of (+)-1 itself (ED50 = 0.3 mg/kg/day). During the syn- thesis of metabolite 52, its ‘isofebrifugine’ isomer 56 was also isolated and its antimalarial activity against Plasmodium falciparum (FCR-3 strain) was evaluated. In vitro, 56 showed high antimalarial activity (EC50 = 2.7 ti 10ti10 M) which was comparable with that of (+)-febrifugine (1). It also showed low toxicity (EC50 = 2.9 ti 10ti6 M) and excellent Plasmodium falciparum activity. The excellent in vitro antimalarial activity displayed by both 52 and 56 indicates that a hydroxyl group in the C-6 position of the 4-quinazolinone ring lowers toxicity against mammalian cells whilst maintaining high antimalarial activity.
Metabolite 54 was synthesized from 36 (Scheme 8), an interme- diate in Takeuchi and co-workers synthesis of (±)-febrifugine (1). In vitro, 54 showed excellent selectivity and good antimalarial
cytotoxicity against mouse L929 cells of each analogue was also evaluated.
The fluorinated analogue 57, which was designed to prevent metabolic oxidation, actually showed higher antimalarial activity than that of (+)-febrifugine (1) but it also proved more toxic. Ana- logue 61 showed high in vitro and in vivo antimalarial activity however, the other analogues, 58–60, showed little antimalarial activity with total loss of antimalarial activity being observed for 62. The complete loss of antimalarial activity in analogue 62 clearly indicates that a basic nitrogen group within the heteroaromatic portion is essential for antimalarial activity.
Kikuchi et al. then set about synthesizing analogues of febrifu- gine with modifications of the linker and the piperidine moiety (Fig. 6).40 Initially analogues 63–65 were synthesized in which the length of the deoxygenated linker was varied, which resulted in a total loss of antimalarial activity. Analogues 66–68 were then synthesized which modified the piperidine ring of febrifugine. No antimalarial activity was reported for these analogues, suggesting that any modifications to piperidine moiety would result in a total loss of activity. Kikuchi et al. concluded that the piperidine moiety and linker of febrifugine were essential to maintain potent antima- larial activity. This important study revealed that both these com- ponents are required and demonstrated that good antimalarial

activity (EC50 = 2.2 ti 10ti8 M) against Plasmodium falciparum (FCR-3 strain). Low toxicity against mammalian cells (EC50 = 3.6 – ti 10ti5 M) was also observed. In vivo, this metabolite showed mod- erate antimalarial activity (ED50 = 2.4 mg/kg/day) with little toxicity. Metabolite 55 was synthesized from D-aspartic acid in 19 steps. However, in vitro, 55 showed little antimalarial activity against Plasmodium falciparum (FCR-3 strain).
In 2006, Kikuchi et al. reported the synthesis and biological eval- uation of a series of analogues based on modifications carried out on the 4-quinazolinone, linker and piperidine ring.40 Several modi-
OH
N N n
H
63-65: (n = 0-2) OH
O
NN
67
N

N
H

OOH

66
O
O

N
N

fications were carried out on the 4-quinazolinone ring, resulting in the isolation of the analogues 57–62 (Fig. 5). These analogues were synthesized as racemic mixtures via Takeuchi et al.’s protocol.27f The antimalarial activities of each of these analogues against both the chloroquine-sensitive FCR-3 and chloroquine-resistant

N
H
O

N
N

K1 Plasmodium falciparum strains were evaluated in vitro. The Figure 6. Analogues of febrifugine.

OH
O

[O]

OH
O

NIH

OH
O

N
H

1
N

O
N
H

70
N

O
shift
N
H

52
N

O
OH

Scheme 11. Proposed oxidative metabolism of febrifugine via arene oxide 70 and its NIH shift.

N
H
OH
O

N
N

N
H
OH
O

N
N

CF3

N
H
OH
O

N
N

N
H
OH
O

N
N

Figure 7. Quinazolinone febrifugine analogues 71 and 72.

candidates, based on 1, could only be accessed through modifica- tions carried out on the 4-quinazolinone ring.

N
H
OBn
O

N
H

OBn
O

CF3

A short synthesis of racemic deoxyfebrifugine (69) was reported by Michael and co-workers in 2006 (Fig. 6).41 However, to date no activity has been reported for this analogue.
In 2006, Zhu et al. synthesized and evaluated a range of novel febrifugine analogues based on the metabolite 52 (Fig. 4) first iso- lated by Hirai and co-workers.42 Many of the analogues synthe- sized by Zhu et al. possessed lower toxicity than the parent alkaloid. Based on these findings it was proposed that febrifugine could undergo metabolism to arene oxide 70 by the action of cyto- chrome P-450 enzymes (Scheme 11). This highly electrophilic intermediate (70) could then potentially react in an indiscriminate fashion with DNA, RNA, proteins or other biomolecules in the host. It is probable that the arene oxide (70) is a short lived intermediate of the metabolite 52.
Zhu et al. set about designing and synthesizing analogues of febrifugine with inherent structural modifications. The aim of this investigation was to make the above process unfavourable, or impossible. This was addressed in two ways: firstly, the metabo- lism was blocked by replacing the the C-5, or C-6 positions in the 4-quinazolinone ring with an atom other than hydrogen. Alterna- tively the oxidation potential of the molecule was increased by adapting the aromatic ring electronically. Utilizing Taniguchi et al.’s27m synthetic route, a range of analogues were synthesized which fulfilled the above criteria.
In vitro, two analogues 71 and 72 (Fig. 7) were observed to be over four times more potent than (+)-febrifugine (1) against both Plasmodium falciparum parasite clones (chloroquine-sensitive) D6 and (chloroquine-resistant) W2. Remarkably both analogues were also over 100 times less toxic against rat hepatocytes than (+)-1 thus making them promising leads in the discovery and develop- ment of new antimalarial drugs.
In 2009, Zhu et al. designed and synthesized a second set of feb- rifugine analogues in order to address the isomerization of febrifu- gine (1) to isofebrifugine (2) (see Schemes 1 and 6).43 The electrophilicity of the likely a,b-unsaturated intermediate (of the type 21, Scheme 6) implicit in this pathway was recognized as a potential biomolecule alkylating agent. In this context the forma- tion of 5-membered rings is entropically more favourable than that of 6-membered rings. Therefore, it was anticipated that the substi- tution of the piperidine moiety of febrifugine for a pyrrolidine moiety would result in an analogue for which this type of isomer- ization was less favourable. Pyrrolidine analogues of febrifugine were designed and synthesized and the antimalarial activity of each was evaluated in vitro against the chloroquine-resistant W2 Plasmodium falciparum strain and in vivo against Plasmodium berg- hei in mice.
Both analogues 73 and 74 showed higher antimalarial activity than (+)-febrifugine (1) whilst maintaining low toxicity. They showed antimalarial activity over six times higher than febrifugine
Figure 8. Pyrrolidine-based febrifugine analogues.

(1) in vitro and were over four times more potent in vivo. Both were also over nine times less toxic than febrifugine and 1.3 times less toxic than the existing antimalarial drug chloroquine. The analogues 75 and 76, also shown in Figure 8, containing a benzyl- oxy group on the pyrrolidine ring similarly displayed excellent antimalarial activity in vitro and were observed to be over 16 times more potent than (+)-febrifugine (1) in vivo. However, both these analogues were observed to be over three times more toxic than their hydroxyl analogues 73 and 74.

6.Halofuginone

Halofuginone (3) is an analogue of febrifugine which was orig- inally synthesized in the late 1960s as a potential antimalarial agent in which one mechanism for the metabolism of this pharma- ceutical is blocked (Scheme 11). The hydrobromide salt of racemic halofuginone has been used as anticoccidial feed additive for broil- ers and turkeys6 and this ultimately led to the discovery that in addition to antiprotozoal activity, 3 also possesses mammalian bio- logical properties. In 2005, Jiang et al. assessed the antimalarial activity of halofuginone.44 In vitro halofuginone showed antimalar- ial activity of IC50 = 0.145 ng/ml against the W2 Plasmodium falci- parum strain. This was higher than the activity observed for febrifugine (IC50 = 0.53 ng/ml) against the same Plasmodium falci- parum clone. In vivo, halofuginone was observed to be 10 times more active against Plasmodium berghei in mice than febrifugine. At 0.5 mg/kg/day halofuginone was observed to be lethal to mice, whereas febrifugine was described as toxic.
Racemic halofuginone lactate has also been used in the preven- tion and treatment of diarrhoea due to Cryptosporidium parvum in non-ruminating calves. Because the initial activities were reported on racemic material, most subsequent biological work appears to have been performed on this mixture of optical isomers (see be- low). Addressing this question, in 2007 Linder et al. reported the resolution of (±)-halofuginone (racemic) with (+)-Noe lactolti which gave a separable mixture of diastereoisomers.45 The activity of each enantiomer was compared against Cryptosporidium parvum and it was found that both the activity of (+)-halofuginone (3) and its toxicity were much higher than that of (ti )-halofuginone (3). Based on these findings, Linder et al. concluded that only (+)-halo- fuginone (3) contributed to the toxicity of (±)-halofuginone (3).
Bearing in mind the current interest surrounding 3 it is some- what surprising that only one total synthesis has been reported in detail in the general literature.27s This method (Scheme 12) relies on an asymmetric dihydroxylation of vinyl sulfone 77, which, following Horner–Wadsworth–Emmons olefination of intermedi-

(ii)

OHO

Steps

OH
O

(i)
N OH N N
N
Cl

NH SO2Ph Cbz
77
(iii)
Cbz Cbz
78 (-)-79 OH
(ii)

OHO

Steps
H
(+)-3

OH
O
O

N OH N N
N
Cl

Cbz Cbz H (-)-3 O

(+)-79

O
N Br

(EtO)2OP
Na

80
HN

Scheme 12. The structures of (+)-halofuginone and (ti )-halofuginone (3). Reagents (i) AD mix-a; (ii) 80 then BF3ti OEt2; (iii) AD mix-b.

ate 78 and a stereoselective intramolecular conjugate addition, generates trans-2,3-disubstituted piperidine 79. Employing either AD mix-a, or AD mix-b enabled the isolation of either enantiomeric form of 79. Functionalization of the methyl group according to Takeuchi’s protocol,27j,k ultimately enabled alkylation with 7-bromo-6-chloroquinazolin-4-one 81 in order to form both (+)- and (ti)-3.
6.1.Halofuginone and fibrosis

Fibrosis is the formation of excess fibrous connective tissue in an organ or tissue as a result of injury or long-term inflammation leading to a persistent and excessive scarring and, ultimately, organ failure. In animal models of fibrosis, in which excess collagen is the hallmark of the disease, halofuginone prevented differentia- tion of the fibroblasts to myofibroblasts and thereby inhibited col- lagen synthesis.46 These models included abdominal, uterine horn and urethral adhesions,47–49 chemically induced fibrosis of the liver and pancreas,50,51 pulmonary fibrosis,52 muscle fibrosis in various dystrophies,53–55 and skin fibrosis in chronic graft-versus- host disease (cGvHD)-afflicted mice and the tight skin (Tsk+) mouse model of scleroderma.56,57 The reduction of fibrosis also im- proved physiological parameters such as regeneration in cirrhotic liver,58 cardiac and respiratory performance in muscle dystrophy53 and reduction in the levels of autoantibodies specific for human target antigens in the Tsk+ mouse.59 Halofuginone also promoted the resolution of established fibrosis60 by affecting fibrotic tissue compensatory pathways, such as the matrix metalloproteinase,61 and the collagen triple helix repeat containing 1 systems62 which distinguishes it from all other antifibrotic agents.

6.2.Halofuginone and cancer

Halofuginone has been shown to inhibit tumor progression and development in a variety of animal models, such as chemically induced bladder carcinoma and metastasis of hepato-cellular carci- noma,63,64 glioma,65 mammary tumor in polyoma middle T antigen transgenic mice,66 fibrous histiocytoma metastasis of the brain,67 xenografts of von Hippel–Lindau pheochromocytoma, hepato- cellular carcinoma, Wilms’ tumor, and prostate and pancreatic cancer.68–72 These results were also achieved by inhibition of the fibroblast-to-myofibroblast transition that forms part of the tumor stroma, and of the level of stroma extracellular matrix (ECM) required for tumor growth. Moreover, halofuginone inhibited the angiogenesis — formation of new vasculature — that is essential for the progression of solid tumor development and serves as a superhighway for metastasis. In a variety of experimental systems that represented sequential events in the angiogenic cascade, halofuginone-dependent inhibition of ECM synthesis resulted in
profound inhibitory effects on basement membrane invasion, cap- illary tube formation, vascular sprouting and also on deposition of sub-endothelial ECM.73 Thanks to its unique mode of action and the fundamental mode of action differences from alternative, conventional chemotherapies, halofuginone represents an ideal candidate for combination therapy, as was demonstrated in treat- ment of pancreatic cancer, prostate cancer and Wilms’ tumor xeno- grafts.74,75 Halofuginone, when used in combination therapies with other cancer-combating drugs that exhibit different modalities, enabled successful reduction of the dosages of chemotherapeutic drugs, and thus offers reductions in the treatment burden imposed on cancer patients.

6.3.Mode of action

Two different pathways have been suggested for the mamma- lian biological activity of halofuginone which may be connected. The most studied mode of action, and probably the canonical pro- cess, concerns inhibition of Smad3 phosphorylation downstream of the TGFb signaling pathway. In most animal models of fibrosis, regardless of tissue type, halofuginone had a minimal effect on the ECM protein content in the non-fibrotic control animals. In contrast, it exhibited a profound inhibitory effect in fibrotic organs. These results suggest that there was differing regulation, on the one hand, of the housekeeping expression — usually low level — of ECM genes and, on the other hand, on the TGFb-driven over- expression induced by the fibrogenic stimulus, which is usually an aggressive and rapid process. Halofuginone was found to over- come collagen synthesis activated by TGFb in human skin fibro- blasts, and by activated fibroblasts derived from scleroderma and cGvHD patients.76 It also inhibited both Smad3 phosphorylation and collagen synthesis in TGFb-activated tight-skin (Tsk+)- mouse-activated fibroblasts, but not in control cells.76,77 No effect of halofuginone was observed on the expression of the TGFb recep- tors gene in the Tsk+ fibroblasts, on TGFb levels in the diaphragm of the mdx mouse model of Duchenne muscular dystrophy, or in the fibrotic pancreas51,52 — a finding that supports the hypothesis that the halofuginone target is downstream in the TGFb pathway. In chemically induced liver fibrosis, halofuginone affected TGFb-regulated genes through the inhibition of Smad3 phosphory- lation of activated liver stellate cells.78 Halofuginone inhibited TGFb-dependent Smad3 phosphorylation and elevated the expres- sion of the inhibitory Smad7 in a variety of cell types, such as fibro- blasts, liver and pancreas stellate cells, tumor cells and myoblasts.51,52,66,78,79 The inhibition of Smad3 phosphorylation was due, at least in part, to halofuginone-dependent activation of Akt MAPK/ERK and p38 MAPK phosphorylation.80
Recently, a second mechanism has been suggested for the halo- fuginone-dependent inhibition of autoimmune diseases which in-

volves selective prevention of the development of Th17 cells by activating the amino acid starvation response.81 Halofuginone treatment reduced the severity and incidence of autoimmune encephalomyelitis associated with Th17 cells that are character- ized by production of interleukin-17 (IL-17). The mechanism by which halofuginone, and indeed other febrifugine analogues, inhibits the amino acid starvation response results from its interac- tion with glutamyl-prolyl-tRNA synthetase (EPRS) which causes inhibition of prolyl-tRNA synthetase activity.82 A crystal structure of the human prolyl-tRNA synthetase with ATP and proline has been reported which demonstrates the key roles of Phe1097 and Arg1152 in the halofuginone inhibition mechanism.83 Thus, it was suggested that halofuginone is a new type of ATP-dependent inhibitor that simultaneously occupies two different substrate binding sites on human prolyl-transfer RNA synthetase.84 These results indicate a possible similar mechanism of action for febrifu- gine in the context of its antimalarial activity.85 Notably, only the 2R,3S isomer of halofuginone, which matches the absolute configuration of febrifugine, exhibits biological activity. Thus, halo- fuginone could potentially be used to address any autoimmune or inflammatory disease related to Th17 cells, by activating the amino acid starvation response.86
It should also be noted that TGFb is required for facilitation of differentiation of the inflammatory Th17 cell subset,87,88 which suggests the existence of a link between inhibition of TGFb signal- ing and the amino acid starvation response.

6.4.Clinical status

Topical application of halofuginone in healthy volunteers dem- onstrated a satisfactory safety and tolerance profile, following re- peated topical applications. Topical application of halofuginone in a promyelocytic leukemia patient who had developed a severe cGvHD after successful bone marrow transplantation, resulted in a marked reduction in skin collagen synthesis, accompanied by improvement in physiological parameters.7 In scleroderma pa- tients with local skin involvement topical application of halofugi- none resulted in significant reduction in the mean total Rodnan skin score after 3 months of treatment.89 The antifibrotic effect of halofuginone was also demonstrated in AIDS-related Kaposi sar- coma patients.90 The recommended dose for chronic oral adminis- tration in patients with advanced and progressive cancer was specified as 0.5 mg/day.8 In a Phase IIa study, oral administration of halofuginone has been evaluated for systemic treatment of patients with recurrent superficial transitional cell carcinoma of the bladder: halofuginone demonstrated a good safety and tolera- bility profile, and promising responses. At present, halofuginone in a slow release formulation is being evaluated in a Phase Ib trial for Duchenne muscle dystrophy (DMD) patients for safety, tolerability, and pharmacokinetics.

7.Concluding remarks

In summary, in this review we have aimed to illustrate histori- cal aspects which led to difficulties concerning the determination

of the structure of naturally occurring febrifugine 1 and isofebrifu- gine 2 and how these issues were ultimately resolved by chemical synthesis. In addition, the range of analogues of 1 prepared for malarial programmes has been highlighted. It is notable that despite the substantial body of literature surrounding febrifugine and isofebrifugine, the detailed biological mechanism, or mecha- nisms for their antiprotozoal activity are not completely known. Exciting new developments in the areas of fibrosis and angiogene- sis concerning halofuginone 3, a compound originally prepared as an antiprotozoal, are presented. Again, it is significant to note that whilst the febrifugine skeleton has been systematically altered during malaria programmes, similar studies with respect to human biology have not been performed. The possibility that the human biology exhibited by this class of compound and their antiparasitic activity may not be unrelated remains speculative and, to the best of our knowledge, has never been investigated. Furthermore, many of the more recent biological studies involving halofuginone have been performed using racemic material, as opposed to single opti- cal isomers.
Finally, the discovery of hydrachine A91 (82) from the roots of Hydrangea chinensis (Fig. 9) and the lack of information concerning the biosynthesis of febrifugine/isofebrifugine also demonstrates that in the future, further discoveries in the area of this natural product class are likely to be made.

Acknowledgements

We thank University College Dublin (P.E.), Donegal County Council (N. P. M.) and the Legacy Heritage Bio-Medical Program of the Israel Science Foundation grant (No. 1315/10) for financial support.

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