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 temp erature in methylene chloride (48 h) resulted in two readily separable diastereomeric racemic 4,5-dihydrothiophenes via transient cycloadduc ts that underwent rearrangement under these reaction conditions. Using chiral carbohydrate-derived nitroalkenes, two diastereomeric dihydrot hiophenes were obtained. showing that regiospecificity and facial sele ctivity were involved in these cycloadditions. IH NMR data and trappin g experiments with isolated initial cycloadducts indicated that the cy cloadditions were reversible and accounted for observed adduct and fin al product ratios. Single-crystal X-ray determinations Established the structures of critical intermediates and products, and PM3 semiempiri cal MO calculations provide a rationalization for both the reactivity of the thiazolium-4-olates and the regioselectivity observed in the cy cloadditions.