2.4- Disubstituted 1,3-oxathianes

Disubstituted 1,3-oxathianes 316, prepared from benzaldehyde and the corresponding 3-mercaptoalkanol, have been stereoselectively deprotonated at the equatorial position to yield, after reaction with electrophiles, 2,2-disubstituted products. t.S j-Benzoin was obtained in 75% ee in the case of using compound 316 (R = OTBS), after reaction with benzaldehyde and final deprotection of the major product 317 with NCS and silver nitrate (Scheme 83)490. 

Similarly, using either sulfuric acid, the SOs/dioxane complex, or a solution of SO3 in chloroform/dioxane, 4,6-diphenyl-l,2-oxathiin 2,2-dioxide was obtained from phenyl acetylene <1999RJ0415>, 3,6-disubstituted-l,2-oxathiane 2,2-dioxides were obtained from allylphenol <2002RJ01210>, and 3,4-dihydro-6-phenyl-l,2-oxathiin-4-one 2,2-oxide was obtained from Ph-CO-CH2-COMe <1991CIL253>. 

Routes to benzo-fused derivatives of 1,4-dioxanes, 1,4-oxathianes and 1,4-dithianes make use of anions or dianions of the appropriate 1,2-disubstituted benzene. An alternative approach to the synthesis of 1,4-benzodioxanes involves Diels-Alder addition reactions of alkenes across the quinone function of 1,2-benzoquinones, e.g. (352) — (353). 

The Cu-catalysed ring expansion of 2-substituted 1,3-oxathiolanes with ethyl (triethylsilyl)diazoacetate affords 2,3-disubstituted 1,4-oxathianes with variable diastereoselectivity the intermediacy of a Cu carbene and of a S-ylide is proposed <02CC346>. 

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