Configuration boat-shaped 15

Dithiins and It4-Di elenins.—1,4-Dithiins (36) are among the photolysis products of biaryl- and benzo-l,2,3-thiadiazoles. The postulated intermediate biradicals (35) dimerize in the direction shown. Thermolysis of cycloalkeno-l,2,3-selenadiazoles produces 1,4-diselenins (37) together with cycloalkynes. A structure determination of 2,5-diphenyl-l,4-dithiin 1-oxide reveals a boat-shaped configuration (Figure) with the sulphur... 

Another explanation takes into account that boat- and twist-shaped six-membered, closed transition states can successfully compete with the chair model. " Evans et al. pointed out that in a-unsubstituted enolate reactions, missing allyl strain interactions lead to lower selectivity in diastereoselective aldol reactions.Calculations indicate that a twist-boat can easily be formed from the U-configuration of a-unsubstituted enolates. The possible transition state in this case has a geometry like 34 and is favored by the chelating character of the complexation mode for the zinc cation and the outward-pointing substituents of the oxazolidinone moiety. This twist-boat transition state correctly predicts the stereochemical outcome of the reaction. 

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