1.2- Disubstituted oxiranes, ring

H and 13C chemical shifts have been reported for many 1,2-disubstituted oxiranes432 436-439. Similar to cyclopropanes, oxirane ring carbon chemical shifts are smaller in the cis- than in the frans-isomers, and an increment rule using pairwise substituent interaction cross-terms has also been developed439. 

The amount of catalyst (scandium trisdodecyl sulfate) and ligand, (14), in the ring opening of czT-1,2-disubstituted oxiranes with primary and secondary aromatic amines in water were varied to maximize the yields.33 In all but two cases, the reaction, under ideal conditions, yielded the j3-amino alcohols in >81% yield with enantioselectivity ... 

The enthalpy of polymerization of 3- and 4-membered rings is so much higher than the entropy factor that substitution does not significantly reduce their polymerizability. Disubstituted oxiranes (e.g., isobutylene oxide) or oxetanes (3,3-dimethyloxetane) still polymerize practically irreversibly. Substitution may prohibit polymerization of 5-membered monomers, however. 

A flexible entry into 2,4-disubstituted furan derivatives through condensation of the sulfur ylide derived from 20 with aldehydes, Pd-catalyzed opening of the vinyl oxirane thus formed, and a final oxidative cyclization of the furan ring was reported. Its utility was exemplified by the first total synthesis of the marine natural product ircinin-4 (21) <99SL29>. 

From epoxides. A very efficient access to tetrahydrofuran derivatives has been developed based on ring opening of oxiranes with selenolates. For instance, preparation of 2,4-disubstituted tetrahydrofurans from epoxides is shown in Eq. (3) [ 11 ]. Opening of the epoxide 19 with diphenyldiselenide/NaBH4 followed by prenylation gives the radical precursor 20 in excellent yield. Cyclization furnishes the tetrahydrofuran 21 in almost quantitative yield but with a modest stereocontrol. The oxirane opening approach described here competes with the electrophilic alkoxyselenenylation reported below (Sect. 2.1.2). 

Disubstituted norbornene derivatives 14 give 2,3-e3to-eis-oxiranes with MCPBA. With the cyclopentane ring fixed in the envelope form, there is total stereoselectivity (Eq. 5). ... 

Substituted oxetanes (4-membered rings) polymerize as readily as oxiranes, particularly those disubstituted at the 3-position. Thus, even oxetanes with large substitu-tents like benzoxymethyl or iodomethyl ... 

Mild and stereoselective formation of tri- and tetrasubstituted olefins (17) by regioselective ring opening of 1,1-disubstituted vinyl oxiranes (15) with diethyl dithiophosphates (16) have been described by Njardarson et al (Scheme 6). ... 

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