Chiral, nonracemic epoxides, synthesis

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

Preparation of nonracemic epoxides has been extensively studied in recent years since these compounds represent useful building blocks in stereoselective synthesis, and the epoxide functionality constitutes the essential framework of various namrally occurring and biologically active compounds. The enantiomericaUy enriched a-fluorotropinone was anchored onto amorphous KG-60 silica (Figure 6.6) this supported chiral catalyst (KG-60-FT ) promoted the stereoselective epoxidation of several trans- and trisubstituted alkenes with ees up to 80% and was perfectly reusable with the same performance for at least three catalytic cycles. 

One of the most elegant methods for the selective formation of C—O bonds is the catalytic Jacobsen-Katsuki epoxidation, the enantioselective synthesis of optically active epoxides by oxygen-transfer reactions with chiral, nonracemic manganese 0x0 salen complexes. These complexes have been suggested as the catalytically active species in epoxidations catalyzed by metal-salen and porphyrin complexes [78]. One of these complexes was for the first time isolated and characterized by Feichtinger and Planner through ESI-MS studies [79]. 

A related synthesis used chiral, nonracemic epoxy-alcohol 6.90, and involved oxidation of the alcohol moiety to an acid moiety (to give 6.91). Opening the epoxide with ammonia led to a 66% overall yield of R-GABOB (49%ec). It is noted that allyl oxirane was recently converted to R-GABOB in two steps. ... 

Sulfonium ylides in synthesis of optically active epoxides 93PS(74)215. Syntheses and reactions of chiral acetylenic oxiranes 92BSB415. Syntheses of nonracemic glycidol and related 2,3-epoxy alcohols 91 CRV437. 

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