A new synthesis of both enantiomers of naturally occurring febrifugine (1) and its analogue halofuginone (3) are reported. This robust route relies on an enzymatic kinetic resolution (EKR)-cross metathesis (CM) sequence, performed on allylic alcohol 8. A diastereoselective Lewis acid-mediated cyclisation of resultant enone 6 affords piperidine 12. In relation to its enantiomeric excess, values of 87 and 88% e.e were obtained for both enantiomers of 12 and for (−)-12 this was increased to 98% e.e by conducting the resolution process twice. A bromination (featuring an in situ temporary alcohol protection), alkylation and amino-deprotection sequence finally afforded the target compounds (+)- and (−)-1, and (+)- and (−)-3. Studies demonstrate that, as their ammonium salts, the compounds are stereochemically stable. However, as their free-bases, particularly in chloroform, C-2 isomerisation occurs. This isomerisation has been taken advantage of synthetically to provide enantioenriched (+)-isofebrifugine (2) and both enantiomers of isohalofuginone (14).Download high-res image (248KB)Download full-size image

•Synthesis of novel cyclic sulfonamides in high yield from simple precursors.•One-pot reductive intramolecular Heck reaction.•Tandem transformation using same palladium catalyst for two reactions.•Selectivity observed for nitro and alkene substitution patterns.A modified method is reported for the conversion of unsaturated sulfonamides into their cyclic saturated counterparts. This method utilises a single palladium catalyst for an intramolecular Heck reaction and subsequent transfer hydrogenation, which is achieved in one-pot following the addition of ammonium formate. Accordingly, a range of fourteen structural variations are reported and under optimal conditions the adducts were generated in typically good to excellent yields. Notably, discrimination of differentially substituted dienes can be accomplished in the case of compounds 28 and 29 and the process was only observed to fail with the more sterically hindered precursor 32.Download high-res image (148KB)Download full-size image

An operationally simple protocol for the synthesis of γ-hydroxy vinyl sulfones has been developed using a proline-based aldehyde aminooxylation, followed by a vinyl sulfone forming Horner–Wadsworth–Emmons olefination. The adducts, formed in high enantiopurity, were subsequently converted to γ-azido vinyl sulfones, and azide–alkyne click chemistry enabled the synthesis of vinyl sulfone-based triazoles as potential nonpeptidic cysteine protease inhibitors.

The combination of ring closing, or enyne metathesis with oxidation in order to prepare N-sulfonyl pyrroles is described. Reasonable to good yields were obtained for a variety of substituents and the procedure may also be conducted in one-pot. 2-Bromo N-sulfonyl adducts prepared in this manner were subjected to an intramolecular Heck-type cyclisation, forming cyclic sulfonamides.

Both enantiomers of deoxyfebrifugine (4) and deoxyhalofuginone (5), analogues of the quinazolinone-containing biologically active compounds febrifugine (1) and halofuginone (3), have been prepared in a six-step reaction sequence featuring an organocatalyzed Mannich reaction as the key stereo-inducing step. The compounds were isolated as their dihydrobromide salts in 29–42% overall yield and in 74–80% enantiomeric excess.

An improved, Weinreb amide-based, synthesis of anti-trypanosomal lysine-containing vinyl sulfones is described incorporating, as a feature, diversity at the ε-lysine amino group. Members of this family demonstrated moderate to good efficacy as anti-trypanosomal agents and a fluorescent dansyl (19) derivative was used to investigate subcellular localisation of the compound class.

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.

(+)-Perillyl alcohol (1) has been synthesised in four steps and 39% overall yield from commercially available limonene oxide (4). The sequence features, as its key step, a palladium(0)-mediated transformation of a secondary allylic acetate (6) into its primary isomer (7). An application of (+)-perillyl alcohol (1) in a formal synthesis of naturally occurring (−)-mesembrine (2) and (−)-mesembranol was demonstrated.

A series of vinyl sulfone-containing peptidomimetics were rationally designed, synthesized, and evaluated against Trypanosoma brucei brucei. These electrophilic compounds are likely to exert their antitrypanosomal activity via inhibition of trypanosomal cysteine proteases, TbCatB and rhodesain, through alkylation of a key cysteine residue within the protease active site. The series was designed to present complementary groups to naturally recognized peptide substrates while probing tolerance to a range of substitutions at the P1, P1′, and P2 positions. The most potent compound, 29 (EC50 = 70 nM, T. b. brucei whole cell assay), displayed minimal toxicity (>785 times selectivity) when assayed for cytotoxicity against the human promyelocytic leukemia (HL-60) cell line. Cells treated with compound 29, as with K777 (2), exhibited an increase in both the number of multinucleated cells and cells with swollen flagellar pockets. Computational analysis revealed a strong correlation between the hypothetical binding mode in TbCatB/rhodesain and trypanocidal activity in vitro.

The synthesis of (+)-mesembrine (1) and (+)-mesembranol (2) has been achieved from the monoterpene (S)-(−)-perillyl alcohol. Key transformations include a diastereo- and regioselective Pd-mediated intramolecular Heck reaction, and a double reduction of the resultant cyclic sulfonamide, to afford the cis-3a-aryloctahydroindole skeleton.

The halonium ion mediated 1,2-Wagner–Meerwein-type rearrangement of a series of benzo-fused bi- and tricyclic sulfonamides is reported. During this rearrangement the carbon–carbon bond that migrates was selectively set in the intramolecular Mizoroki–Heck (IHR) synthesis of the starting materials. Consequently, this method constitutes a means to access the regioisomeric series of cyclic sulfonamides not observed during the Mizoroki–Heck reaction.

Herein is described an operationally simple process concerning the observation that, following either inter-, or intramolecular Heck olefination, stirring of the so formed substituted alkenyl product under an atmosphere of hydrogen efficiently effects alkene hydrogenation. Overall this two-operation, one-pot “reductive Heck” sequence is notable since direct reductive Heck processes, using additives such as formate salts, are restricted to a limited range of substrates. In total 25 examples are reported (yields ranging from 0 to 95%), which were selected in order to probe the scope and limitations of this method. Finally, the utility of this sequence was demonstrated in a short synthesis of the calcimimetic agent, cinacalcet.

Described is an observation that the intramolecular Heck reaction of a trisubstituted alkene proceeds with high regioselectivity and leads to the preferential formation of a quaternary all-carbon-centre. This observation was subsequently applied in a short synthesis of (±)-mesembrane.

The addition of 3,4-dimethoxybenzyl thiol 8, as a benzyl thiol surrogate, to racemic 4-hydroxycyclopent-2-enone 2 and 4-hydroxycyclohex-2-enone 15 gave the corresponding cis-adducts (±)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclopentanone 4b and (±)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclohexanone 16 with good diastereocontrol. In both cases, subsequent treatment with vinyl acetate, in the presence of a lipase enabled enantiomer resolution. Thus, (+)-16 and the acetate of its enantiomer, (–)-(1R,2S)-2-(3,4-dimethoxybenzylthio)-4-oxocyclohexyl acetate, (–)-17 were isolated in 98% enantiomeric excess. Based on the 1,4-dioxygenation pattern, (–)-17 can be used to prepare both enantiomers of 4-(tert-butyldimethylsilyloxy)cyclohex-2-enone 19. Firstly, saponification, with a sub-stoichiometric amount of NaOMe, followed by a one-pot silyl ether formation–sulfide elimination sequence gave (+)-19. Then using the same starting material a 6-step sequence, featuring a diastereoselective NaBH4 reduction and a Cope-type sulfoxide elimination, gave (–)-19.

The naturally occurring PPAR-γ ligand 10-nitrooctadeca-9(E),12(Z)-dienoic acid (10-nitrolinoleic acid) (2a) was prepared as a single regio- and geometrical isomer in a practical eight-step, convergent sequence. The synthetic route featured a nitro aldol reaction between 9-oxononanoic acid methyl ester (3) and 1-nitronon-3(Z)-ene (4) in the key carbon−carbon bond forming step. The ability of 2a (and its methyl ester 9) to bind to PPAR-γ in a ligand-binding assay is reported.

The bromination of a series of cyclohexenyl substituted secondary amines 1a-i has been investigated using Br2, PHT and NBS. In the case of Br2 and NBS the secondary amines preferentially undergo N-bromination. In contrast, PHT cleanly affords the products of alkene dibromination. In the case of Br2 the N-bromo species then give the products of alkene dibromination, albeit less efficiently. On subsequent treatment with K2CO3 these dibromides form the corresponding hexahydroindoles 2a-h and octahydroquinoline 2i. The presence of an N-substituent bearing a stereogenic centre (1h and 1i) was studied and the products 2h and 2i were isolated with no diastereoselectivity. When NBS was used a novel cyclisation, forming bromo-substituted octahydroindoles 9a,b and d, was observed. In relation to this sequence it was shown that these products were not intermediates in the former Br2/PHT processes and that the reaction only proceeded in the presence of the succinimide by-product of N-bromination.

The hexacarbonyldicobalt complexes of a range of 5-alkynyl-5H-dibenzo[a,d]cycloheptenes, 11a−e, readily undergo loss of a carbonyl ligand with concomitant formation of pentacarbonyldicobalt clusters, 12a−e, in which the vacant coordination site on cobalt is now occupied by the C(10)−C(11) double bond of the central seven-membered ring. These (η2-alkene)(μ-alkyne)pentacarbonyldicobalt complexes provide structural models of the first step of the proposed mechanism of the Pauson−Khand process for the formation of cyclopentenones via the coupling of an alkene, an alkyne, and a source of carbon monoxide. X-ray crystallographic data reveal that the distance between the alkene carbons and the nearest alkyne carbon is approximately 2.85 Å, slightly shorter than the theoretically predicted value of ∼2.95 Å. VT NMR data for the interconversion of 11b and 12b yielded activation energies for the forward and reverse processes, 29 and 15 kcal mol−1, respectively, and the enthalpy change for the endothermic process (14 kcal mol−1); these match very well the earlier predictions from DFT calculations. Although the ethynyl-hexacarbonyldicobalt complex, 11f, does not suffer facile elimination of CO, it does participate in an intermolecular PKR with norbornadiene. Likewise, attempts to extend the reach of the alkynyl moiety through incorporation of an additional methylene group yield only an intermolecular PKR product. It is suggested that the pentacarbonyl complexes, 12a−e, do not continue along the PKR pathway because of the unfavorable relative orientation of the cobalt-coordinated alkyne and alkene.

5-Alkylidenecyclopent-2-enones 15a–q may be prepared via a conjugate addition–Peterson olefination sequence, best achieved in one-pot, using exo-2-trimethylsilyl-3a,4,7,7a-tetrahydro-4,7-methanoinden-1-one 12, followed by a retro-Diels–Alder reaction. The geometry of the exocyclic alkene may be controlled according to the use of organometallic species in the conjugate addition step; organocuprate reagents are found to selectively lead to the formation of E-exocyclic alkene adducts, whereas Grignard reagents favour the formation of Z-alkenyl isomers. The use of enantiomerically enriched 12, accessed from an asymmetric Pauson–Khand reaction, affords the corresponding enantioenriched 5-alkylidenecyclopent-2-enones and this approach is exemplified by the short, stereoselective total syntheses of two cyclopentenone phytoprostanes 51 and 13,14-dehydrophytoprostane J165. The ability of this family of synthetic compounds to activate the peroxisome proliferator activated receptor-γ is reported.

Described is an observation that the intramolecular Heck reaction of a trisubstituted alkene proceeds with high regioselectivity and leads to the preferential formation of a quaternary all-carbon-centre. This observation was subsequently applied in a short synthesis of (±)-mesembrane.

An improved, Weinreb amide-based, synthesis of anti-trypanosomal lysine-containing vinyl sulfones is described incorporating, as a feature, diversity at the ε-lysine amino group. Members of this family demonstrated moderate to good efficacy as anti-trypanosomal agents and a fluorescent dansyl (19) derivative was used to investigate subcellular localisation of the compound class.

The addition of 3,4-dimethoxybenzyl thiol 8, as a benzyl thiol surrogate, to racemic 4-hydroxycyclopent-2-enone 2 and 4-hydroxycyclohex-2-enone 15 gave the corresponding cis-adducts (±)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclopentanone 4b and (±)-3-(3,4-dimethoxybenzylthio)-4-hydroxycyclohexanone 16 with good diastereocontrol. In both cases, subsequent treatment with vinyl acetate, in the presence of a lipase enabled enantiomer resolution. Thus, (+)-16 and the acetate of its enantiomer, (–)-(1R,2S)-2-(3,4-dimethoxybenzylthio)-4-oxocyclohexyl acetate, (–)-17 were isolated in 98% enantiomeric excess. Based on the 1,4-dioxygenation pattern, (–)-17 can be used to prepare both enantiomers of 4-(tert-butyldimethylsilyloxy)cyclohex-2-enone 19. Firstly, saponification, with a sub-stoichiometric amount of NaOMe, followed by a one-pot silyl ether formation–sulfide elimination sequence gave (+)-19. Then using the same starting material a 6-step sequence, featuring a diastereoselective NaBH4 reduction and a Cope-type sulfoxide elimination, gave (–)-19.

The bromination of a series of cyclohexenyl substituted secondary amines 1a-i has been investigated using Br2, PHT and NBS. In the case of Br2 and NBS the secondary amines preferentially undergo N-bromination. In contrast, PHT cleanly affords the products of alkene dibromination. In the case of Br2 the N-bromo species then give the products of alkene dibromination, albeit less efficiently. On subsequent treatment with K2CO3 these dibromides form the corresponding hexahydroindoles 2a-h and octahydroquinoline 2i. The presence of an N-substituent bearing a stereogenic centre (1h and 1i) was studied and the products 2h and 2i were isolated with no diastereoselectivity. When NBS was used a novel cyclisation, forming bromo-substituted octahydroindoles 9a,b and d, was observed. In relation to this sequence it was shown that these products were not intermediates in the former Br2/PHT processes and that the reaction only proceeded in the presence of the succinimide by-product of N-bromination.

5-Alkylidenecyclopent-2-enones 15a–q may be prepared via a conjugate addition–Peterson olefination sequence, best achieved in one-pot, using exo-2-trimethylsilyl-3a,4,7,7a-tetrahydro-4,7-methanoinden-1-one 12, followed by a retro-Diels–Alder reaction. The geometry of the exocyclic alkene may be controlled according to the use of organometallic species in the conjugate addition step; organocuprate reagents are found to selectively lead to the formation of E-exocyclic alkene adducts, whereas Grignard reagents favour the formation of Z-alkenyl isomers. The use of enantiomerically enriched 12, accessed from an asymmetric Pauson–Khand reaction, affords the corresponding enantioenriched 5-alkylidenecyclopent-2-enones and this approach is exemplified by the short, stereoselective total syntheses of two cyclopentenone phytoprostanes 51 and 13,14-dehydrophytoprostane J165. The ability of this family of synthetic compounds to activate the peroxisome proliferator activated receptor-γ is reported.