Cycloaddition Chemistry of 1,3-Thiazolium-4-olate Systems. Reaction with Nitroalkenes and Interpretation of Results Using PM3 Calculations.

1,3-Dipolar cycloaddition of 1,3-thiazolium-4-olates, readily prepared from thioureido derivatives, and trans-beta-nitrostyrene at room temperature in methylene chloride (48 h) resulted in two readily separable diastereomeric racemic 4,5-dihydrothiophenes via transient cycloadducts that underwent rearrangement under these reaction conditions. Using chiral carbohydrate-derived nitroalkenes, two diastereomeric dihydrothiophenes were obtained, showing that regiospecificity and facial selectivity were involved in these cycloadditions. (1)H NMR data and trapping experiments with isolated initial cycloadducts indicated that the cycloadditions were reversible and accounted for observed adduct and final product ratios. Single-crystal X-ray determinations established the structures of critical intermediates and products, and PM3 semiempirical MO calculations provide a rationalization for both the reactivity of the thiazolium-4-olates and the regioselectivity observed in the cycloadditions.