The balance of steric and conjugative effects in the opening of arylsubstituted oxiranes. Stereochemistry of the ring opening of 1-(1′-naphthyl)-1,2-epoxycyclohexane

The steric course of the ring opening of 1-(1′-naphthyl)-1,2-epoxycyclohexane (3) has been investigated and compared with that of the similar reactions of 1-phenyl-1,2-epoxycyclohexane. Reaction of 3 with KOH in DMSO-H₂O gives small yield of trans diol 4. Reaction of 3 with CCl₃COOH and HCl in low polarity aprotic solvents takes place with complete retention of configuration. However reaction with H₂SO₄ in H₂O is not completely stereospecific and shows a greater tendency than the phenyl substituted epoxide towards cis opening. Explanations of the observed stereochemical results, particularly with respect to the ability of the 1-naphthyl group to stabilize the intermediate carbocation, are discussed.     

Cyclic α, β-epoxysilanes: Acid-catalysed reactions of 1-trimethylsilylcyclooctene oxide

The acid-catalysed reactions of medium ring α,β-epoxysilane 1 are described. The epoxide 1 gives exclusively the bicyclic alcohol 2 with boron trifluoride etherate. With aqueous sulphuric acid the products are due to both transannular ring closure, 2, and transannular hydride migration, 3 and 4. trans-Cyclooctene derivatives 3 and 4 are formed by the trans-elimination of trimethylsilanol from the likely intermediate β-hydroxysilanes 10 and 11. The results with methanolic sulphuric acid are similar. It is also noticed that the hydride migration is facilitated by the nucleophilic strength of the medium.     

Synthesis of carba analogs of 6-O-(benzyl)-d-allal- and -d-galactal-derived allyl epoxides and evaluation of the regio- and stereoselective behavior in nucleophilic addition reactions

The new racemic diastereoisomeric epoxides 6α and 6β, the carba analogs of the corresponding d-galactal- and d-allal-derived allyl epoxides have been synthesized and their regio- and stereoselective behavior examined in addition reactions with model O-, C-, N-, and S-nucleophiles. The results have indicated that epoxide 6β has a pronounced tendency toward anti-1,2-addition, whereas epoxide 6α shows interesting levels of syn- and/or anti-1,4-addition processes. A chiral recognition process found with epoxide 6β, turned out to be consistently reduced in epoxide 6α. All the results have been rationalized on the basis of conformational, steric, and stereoelectronic effects.     

A practical enantioselective synthesis of (S)-3-hydroxytetradecanoic acid

(S)-3-Hydroxytetradecanoic acid 1 has been synthesized in an overall yield of 27% from (S)-epichlorohydrin 2 as follows: (1) regio and chemoselective epoxide opening of 2 with a Grignard reagent under the catalysis by Cu(I) followed by consecutive epoxide formation; (2) regioselective epoxide opening of (S)-1,2-epoxytridecane 4 with cyanide anion under pH controlled conditions followed by consecutive nitrile hydrolysis with alkaline H₂O₂ gave crude 1; (3) its purification via the N,N-dicyclohexylammonium salt 6. The method thus devised is practical and scalable for the industrial production of 1.     

Characterization of the photoisomers from cis- and trans-chlordanes, trans-nonachlor and heptachlor epoxide

Irradiation of acetone solutions of trans-chlordane (4a) and trans-nonachlor (4b) with UV light produces new half-cage photoisomers (5a or 6a and 5b or 6b, respectively) with bridging that differs from that of the photoisomers (2 and 8a) obtained from cis-chlordane (1b) and heptachlor epoxide (7). A new photoisomer (10) obtained from heptachlor epoxide is transformed into photoisomer 8a on further irradiation. Detailed PMR and ¹³C-NMR studies establish the structures of the new photoisomers and permit a decision between alternative formulations for the structures of the half-cage photoisomers of heptachlor epoxide and cis-chlordane,     

Stereoselective synthesis and anti-HCV activity of conformationally restricted 2′-C-substituted carbanucleosides

Conformationally restricted 2′-C-azido-, hydroxy- and fluoromethyl-carbanucleosides 4b–f were efficiently synthesized via the stereoselective conversion of ketone 7 to epoxide 14, followed by the stereoselective opening of the epoxide with nucleophiles (OAc, N₃, and F), while the corresponding 2′-C-methyl-carbanucleoside 4a was synthesized via the stereoselective Grignard reaction of ketone 7 with methylmagnesium iodide as a key step. All the final nucleosides 4a–f were assayed for anti-HCV activity, but showed neither significant anti-HCV activity nor cytotoxicity in a cell-based replicon assay.     

Chemoenzymatic synthesis of calcilytic agent NPS-2143 employing a lipase-mediated resolution protocol

The kinetic resolution of chlorohydrin (±)-6 has been successfully carried out via a lipase-mediated transesterification with vinyl acetate in organic as well as ionic liquid media to yield (R)-alcohol 6 and (S)-acetate 7 in high enantioselectivity. An enantioconvergent synthesis has also been achieved by a Mitsunobu esterification of a mixture of (R)-alcohol 6 and (S)-acetate 7 in one pot to convert the (R)-alcohol 6 to (S)-acetate 7. (S)-Acetate 7 has been hydrolyzed by LiOH·H₂O to give epoxide (R)-2. This enantiopure epoxide has been used as a chiral precursor for the synthesis of calcilytic agent NPS-2143.     

Bacillus subtilis epoxide hydrolase-catalyzed preparation of enantiopure 2-methylpropane-1,2,3-triol monobenzyl ether and its application to expeditious synthesis of (R)-bicalutamide

Expeditious synthesis of (R)-bicalutamide (1), a synthetic antiandrogen, from enantiopure 2-methylpropane-1,2,3-triol monobenzyl ether (4) was achieved. An engineered Bacillus subtilis epoxide hydrolase worked enantioselectively on the racemic epoxide (7) to provide the above starting material in highly enantiomerically enriched state.     

Use of epoxidation and epoxide opening reactions for the synthesis of highly functionalized 1-oxaspiro[4.5]decan-2-ones and related compounds

Epoxidation of ethyl 3-(6-hydroxycyclohex-1-en-1-yl)propanoate (11) provided the syn epoxide 12. By invoking chelation controlled epoxide opening the triol derivatives 13 and 14 or the spiro lactone 25 could be obtained. Elimination of HBr from the bromides 26 and 27 produced the spiro cyclohexenones 28 and 29. Epoxidation of the double bond occurred in a diastereoselective manner to give epoxides 30 and 31, respectively. Treatment of the epoxide 31 with LiBr/AcOH gave the bromo hydrin 38. In a ‘merry go round’ fashion 38 was further functionalized on the cyclohexane ring by elimination, epoxidation, and epoxide opening resulting in the bromo hydrin 43. Other cyclohexane derivatives that were produced during these studies include the cyclohexenone 19 and the cyclohexanediol 23. In addition, enolate azidation of the spiro lactone 29 proceeded in a diastereoselective manner providing the α-azido lactone 32.     

The catalytic hydrogenolysis of 1-phenylbicyclo[4.1.0]heptane and the corresponding azidirine and epoxide

The hydrogenolysisof 1-phenylbicyclo[4.1.0]heptane (1a), cis-1-phenyl-2-methylbicyclo[4.1.0]heptane (1b), 1-phenyl-7-azabicyclo[4.1.0]heptane (2) and 1-phenyl-7-oxabicyclo[4.1.0]heptane (3) was studied using Ni, Pd, Rh and Pt as catalysts. The hydrogenolysis of the C₁C₇ bond of 1a and 1b led to the selective formation of trans-1-phenyl-2-methylcyclohexane (4a) with retention of configuration. Compound 1a gave not only 4a but also phenylcycloheptane (6a), which is the product of C₁C₆ bond fission, and the ratio of 6a to 4a increased in the sequence: Ni ? Pd, Rh < Pt. No C₁C₆ bond fission was observed in the hydrogenolysis of 1b. These results can be explained by a mechanism involving the formation of the π-benzyl complex.trans-2-Phenylcyclohexylamine (8) was obtained stereoselectively in the hydrogenolysis of 2 over Raney Ni. This selective formation can be ascribed to the competition of “SN i” and “radical” processes. The Pd catalysed hydrogenolysis gave cis-2-phenylcyclohexylamine (9) as the main product, while the presence of sodium hydroxide promoted the formation of 8.Raney Ni catalysed hydrogenolysis of 3 yielded a mixture of phenylcyclohexane (13) and 2-phenylcyclohexanols (10 and 11). trans-2-Phenylcyclohexanol (10) was the dominant isomer; the hydrogenolysis resulted in the predominant configurational retention. Compound 13 was confirmed to be produced via 1-phenylcyclohexene (12). This deoxygenation may be explained by a mechanism involving the radical cleavage reaction of 3. The presence of sodium hydroxide led to the formation of cis-2-phenylcyclohexanol (11). The Pd catalysed hydrogenolysis also gave mainly 11.The difference in behaviour of cyclopropane, azidirine and epoxide we ascribe to the differences in the affinity for the catalyst and differences in the electronegativity between C, N and O atoms.     

Chemoenzymatic access to all four enantiopure stereoisomers of 1-ferrocenyl-1,3-butanediol

The kinetic resolution of ferrocenyl aldol 2 was achieved by a lipase-catalyzed esterification in organic solvent. Lipase from Candida antarctica was also found effective in promoting the enantioselective alcoholysis of acetate (±)-3 with n-BuOH. Both enantiomers of 2 were obtained in enantiopure form and subjected to chemical reduction to afford the corresponding syn- and anti-diols. These diols serve as starting materials for the preparation of new ferrocenyl amino alcohols bearing two stereocenters in the side chain.     

A biomimetic approach to sporidesmins. synthesis of an α,β-epoxytryptophan derivative

The reaction of 19 - prepared efficiently from 16 - with DDQ affords 22. This epoxide may serve as a model compound to investigate the biosynthesis of sporidesmins, e.g. 1 and 2.     

Enzyme-catalysed approach to the preparation of triazole antifungals: synthesis of (−)-genaconazole

The work describes a new enzyme-mediated approach to optically active epoxide (2R,3S)-6, which is an important key intermediate in the preparation of single enantiomers of chiral azole antifungals. The conversion of (2R,3S)-6 into (?)-genaconazole is reported as an example of its synthetic relevance.     

Flavanoid epoxides—XIII : Acid and base catalysed reactions of 2′-tosyloxychalcone epoxides. Mechanism of the algar-flynn-oyamada reaction

2′-Tosyloxychalcone epoxide (6a) on reaction with alkali gave flavonol (4a) while similar treatment of 6′-methoxy-2′-tosyloxychalcone epoxide (6b), both at room temperature and at the boiling point of the reaction medium, afforded 5-methoxyaurone (5b). The latter result indicates that an epoxide is not an intermediate in the production of flavanols from 2′-hydroxy-6′-methoxychalcone epoxides on treatment with alkaline hydrogen peroxide (AFO Reaction) at the higher temperature. Epoxide 6a on treatment with boron trifluoride etherate gave a mixture of flavanonol and flavonol while epoxide 6b gave formyldesoxybenzoin (9) under similar conditions.     

Analyse conformationnelle de derives du thiophane,des sulfoxydes et sulfones correspondants

The conformations of biotin 1, cis 3,4-isopropylidenedioxythiophane 2, its 2,5-methylated homologs, 3, 4 and 5 and those of the corresponding sulfoxides and sulfones have been studied by NMR. Coupling constants, temperature and solvent effects show that all these compounds exist in a highly preferred conformation, the thiophane being in the envelope form with the sulphur atom out of the plane. In series 1, 2, 3 and 5 the arrangement is of the “boat-chair” type, in series 4 of the “crown” type, the sulphur being respectively cis and trans with respect to the other ring.     

Enantio- and diastereo-convergent synthesis of (2R,5R)- and (2R,5S)-Pityol through enzyme-triggered ring closure

A short chemoenzymatic synthesis of the (2R,5S)- and (2R,5R)-stereoisomer of the bark beetle pheromone Pityol 1 was achieved from (±)-Sulcatol 2 in an enantio- and diastereo-convergent fashion without the formation of any ‘unwanted’ stereoisomer. The key steps include: (i) lipase-catalyzed deracemization of (±)-2 using kinetic resolution coupled to an in-situ inversion or, alternatively, dynamic resolution using combined lipase- and Ru-catalysis; and (ii) creation of the second stereogenic center by an epoxide hydrolase-catalyzed diastereo-convergent hydrolysis of a haloalkyl oxirane, followed by spontaneous ring closure to form 1 in a stereoselective fashion.     

C-Nitroso compounds—XXXV : Reaction of organometallic compounds with 1-chloro-1-nitroso-2,2,6,6-tetramethylcyclohexane

Reaction of Grignard reagents and organolithium compounds (RM) with the congested 1-chloro-1-nitroso-2,2,6,6-tetramethylcyclohexane 1 leads to the formation of significant amounts of the reduction product 2,2,6,6-tetramethylcyclohexanone oxime 3 (61–90%) together with the corresponding oxime O-R ether 4 (0–11%). Attack on nitrogen is unimportant as shown by very low yields of nitrone. Formation of the products is rationalised with a pathway involving transfer of an electron from RM to 1. This leads—after separation of MCI—to a radical pair consisting of R and the relatively stable iminoxy radical 2 (Schemes 1 and 2). Combination of these radicals explains formation of oxime ether 4 and nitrone 5, while reaction of iminoxy radical 2 with excess of RM can give oxime 3. Reactive radicals R (i.e. Me, Ph, and to a minor extent n-Bu) are furthermore capable of abstracting hydrogen from the solvent (diethyl ether, toluene, or cumene), and the solvent derived radicals can also combine with 2 on oxygen, under formation of oxime ether (26% of 6a). The corresponding benzyl- and cumyl ethers 6b and 6c are only formed in trace amounts because dimerisation of benzyl radicals (7%) and cumyl radicals (22%) is favoured.     

Enantioselective total synthesis of cis-(+)- and trans-(+)-disparlure

An expedient enantioselective synthetic approach for the gypsy moth sex-attractant pheromone cis-(+)-1 and trans-(+)-disparlure 2 is described employing the optimized combination of organocatalytic MacMillan’s self aldol reaction, Wittig olefination, regioselective ring opening of an epoxide and Mitsunobu esterification reactions as key steps.     

Transformation of (+)-thiomicamine into the p-methylthio analogue of (+)-5-epi-cytoxazone

(2S,3S)-(+)-Thiomicamine 3, a commercially available aminodiol, was transformed into (4R,5S)-5-hydroxymethyl-4-(p-methylthiophenyl)-2-oxazolidinone 10, a compound related to cytoxazone-type biologically active natural products. The synthetic strategy of the highly regio- and stereoselective synthesis was based upon the reversal of the position of the hydroxyl and amine functionalities in 3, accomplished via azidolysis of the key intermediate, epoxide 6.     

A novel azide-free asymmetric synthesis of oseltamivir phosphate (Tamiflu) starting from Roche’s epoxide

A novel azide-free asymmetric synthesis of oseltamivir phosphate 1 (Tamiflu®) starting from Roche’s epoxide is described. Roche epoxide 2 was converted into N-acetyl aminoalcohol 3 in 95% yield via a BF₃·OEt₂-catalyzed epoxide-opening with acetonitrile as a nucleophile. Compound 3 was then transformed into a methanesulfonate 4 in 98% yield. Compound 4 was converted into aziridine 5 in 91% yield. Aziridine 5 was subsequently converted into oseltamivir phosphate 1 via two paths (a and b). In the path a, compound 5 underwent aziridine-opening with diallylamine as a nucleophile to afford compound 7 in 93% yield; compound 7 could then be converted into oseltamivir phosphate 1 in 88% yield. In path b, compound 5 underwent aziridine-opening with isopropyl 2,2,2-trichloroacetimidate as a nucleophile to afford compound 8 in 94% yield, which was then converted into oseltamivir phosphate 1 in 82% yield.