Oxiranes enantioselective synthesis

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

Enantioselective synthesis and transformations of oxirane and aziridine derivatives 99PAC423. 

The second system studied was the separation of the chiral epoxide enantiomers (la,2,7,7a-tetrahydro-3-methoxynaphth-(2,3b)-oxirane Sandoz Pharma) used as an intermediate in the enantioselective synthesis of optically active drugs. The SMB has been used to carry out this chiral separation [27, 34, 35]. The separation can be performed using microcrystalline cellulose triacetate as stationary phase with an average particle diameter greater than 45 )tm. The eluent used was pure methanol. A... 

Biochemical methods in the enantioselective synthesis of natural products possessing oxirane and lactone fragments 92BSB393. 

Alcohols can be obtained from many other classes of compounds such as alkyl halides, amines, al-kenes, epoxides and carbonyl compounds. The addition of nucleophiles to carbonyl compounds is a versatile and convenient methc for the the preparation of alcohols. Regioselective oxirane ring opening of epoxides by nucleophiles is another important route for the synthesis of alcohols. However, stereospe-cific oxirane ring formation is prerequisite to the use of epoxides in organic synthesis. The chemistry of epoxides has been extensively studied in this decade and the development of the diastereoselective oxidations of alkenic alcohols makes epoxy alcohols with definite configurations readily available. Recently developed asymmetric epoxidation of prochiral allylic alcohols allows the enantioselective synthesis of 2,3-epoxy alcohols. 

Otera, J., Niiho, Y. and Nozaki, H. (1991) Oxirane ring-opening with alcohol catalysed by organotin phosphate condensates. Complete inversion at tertiary and henzylic centers. Tetrahedron, 47, IClS-lCiA, Corey, E.J. and Helal, C.J. (1993) A catalytic enantioselective synthesis of chiral monosuhstituted oxiranes. Tetrahedron Letters, 34, 5227-5230. 

A concise enantioselective synthesis of (-l-)-muscarine from (R)-benzyloxymethyloxirane was recently reported by Takano et al. (36) (Scheme 10). The starting oxirane was coupled with lithium acetylide-ethylenediamine, forming the hydroxyalkyne, which was converted to the acid-sensitive 2,3-dihydrofuran by methylation and subsequent treatment with base. The dihydrofuran derivative was hydroborated and oxidized in... 

Scheme 9. Flexible, diastereo- and enantioselective synthesis of novel polyhydroxylated piperidines (oxirane azide ring opening variant) [15]...

In addition to the enantioselective synthesis of oxiranes through the use of electrophilic oxidants as described above, there have also been significant advances in the development of nucleophilic oxidation catalysts for the epoxi-dation of electron-deficient substrates. Shibasaki has reported an effective chiral catalyst system readily prepared from equimolar amounts of BINOL (102), Ph3As = 0, and La(Oi-Pr)3 (Scheme 9.12) [101]. Use of 1-5 mol % of the chiral lanthanide catalyst permits the epoxidation of a,/3-unsaturated ketones. The method has also been extended to include a,/funsaturated imi-dazolides as substrates to provide convenient access to chiral carboxylic acids [102]. As an example, asymmetric epoxidation of enone 101 proceeded in excellent selectivity and yield (96% ee, 94%) [103], The resulting epoxide 103 was subsequently converted into (-i-)-decursin (104), a potent cytotoxic agent associated with protein kinase C activation. 

This enantioselective reduction can be used for synthesis of chiral 1-substituted oxiranes.1 2 3 Thus reduction of 2-chloroacetophenone with B2H6 catalyzed by 1 (1 mole %) results in (S)-( + )-(chloromethyl)benzenemethanol, which in the presence of base converts to (S)-( - )-phenyloxirane (styrene oxide). 

Schmid, A., Hofstetter, K., Feiten, H.J., Hollmann, F., and Witholt, B., Integrated hiocatalytic synthesis on gram scale the highly enantioselective preparation of chiral oxiranes with styrene monooxygenase. Adv. Synth. Catal., 2001, 343, 732-737. 

Epoxide formation using biocatalysis is a useful process for the formation of chiral oxiranes (Scheme 29). The synthesis of enantioenriched epoxides using enzymes has been reviewed <1995BCSF769>. Chloroperoxidase has been examined for the oxidation of 2-methyl-l-alkenes, among other alkenes. The yields in some cases can be low, but the enantioselectivities can be high <1995JA6412, 1997JA443>. This enzyme has been used in a synthesis of... 

Dialkylamino-aryloxosulfonium alkylides may be employed for enantioselective epoxidation if the ylide with its chiral sulfur center is resolved into its enantiomeric form, " An enantioselective oxirane is obtained by means of a chiral phase-transfer catalyzed procedure with dimethylsulfonium methylide. The utilization of arsonium ylides was reported some time ago. ° A highly stereoselective synthesis of trans-epoxides with triphenylarsonium ethylide has recently been described.Optically active arsonium ylide has been used in the asymmetric synthesis of diaryloxiranes. ... 

The enantioselective total synthesis of triterpenes of the jS-amyrin series has been described by Corey and co-workers who utilized a MeAlCl2-catalyzed tricarbocyclization of a chiral oxirane <93JA8873>, see also <91TL7005>. 

Enantioselective total syntheses based on asymmetric catalysis including total synthesis of strychnine, asymmetric epoxidation of a,/l-unsaturated acid derivatives, and ring opening of oxiranes 04CPB1031. 

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