Epoxides, vicinal coupling

A study of a series of carbohydrate 2,3-epoxides and related compounds of the type shown (26) has provided information concerning the stereodependence of the 2 and 4 vicinal couplings where the precise hybridization of C-2 and C-3 is uncertain. It has been shown that the coupling from the epi ring hydrogens at C-2 (and C-3) to... 

The oxidative cleavage of carbon-carbon bonds in vicinal diols [756, 759] is a reaction widely used in saccharide chemistry. Besides its application in this reaction, periodic acid achieves the oxidative coupling [757] or oxidation to quinones [758] of polynuclear aromatic hydrocarbons, the oxidation of methyl groups in aromatic compounds to carbonyl groups [760], the conversion of epoxides into dicarbonyl compounds [761], and the oxidative cleavage of trimethylsilyl ethers of acyloins to carboxylic acids [755]. 

The alternative synthesis is shown in Scheme 31. The epoxide 151, prepared from 145, and the trienoic acid 152 were coupled with Ti(Ot-Bu)4 to provide 153. After protection of the vicinal diol, the ring closing metathesis was carried out using Grubbs catalyst to provide the macrolactone 150 with a single C8-C9 alkene of -geometry. Finally, (-)-dactylolide was converted into zampanolide by the aza-aldol addition of 154. 

In addition to direct oxygenation, e.g. by aryl hydrocarbon hydroxylase, oxidative N- or 0-dealkylation is another process catalyzed by components of the Cytochrome P-450 System (mixed-function oxidases). Reduction also occurs in this system NADPH-cytochrome P-450 reductase has an activity similar to microsomal nitroreductase, i.e. transformation of aronaatic nitro compounds into the corresponding arylamines takes place. The oxidation may be followed by other enzymic reactions, e.g. epoxides are hydrated to vicinal diols by microsomal epoxide hydratase or they are coupled with glutathione by glutathione-S-epoxide transferase. 

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