2-Halogeno-1,3-dithiane 1,3-dioxide: a diastereoselective carbonyl anion equivalent in reactions with aldehydes

Abstract

The metal anions of 2-halogeno-1,3-dithiane trans-1,3-dioxide react diastereoselectively with aldehydes. The scope and limitations of this reaction have been studied through variations in the metal counter-ion, halogen, aldehyde and reaction temperature, with a view to achieving umpolung asymmetric Darzens type reactions. Chlorohydrins were obtained with high diastereoselectivity by reaction of the lithium anion of 2-chloro-1,3-dithiane dioxide 4 with aromatic aldehydes under equilibrating conditions. Aliphatic aldehydes gave poor selectivities. Attempted stereospecific conversion of the chlorohydrins into epoxides resulted only in reversion to 4 and aldehyde. Use of sodium or potassium bases gave lower yields of chlorohydrins; epoxides were not isolated. It was possible to obtain aromatic bromohydrins with high selectivity using the magnesium anion of 2-bromo-1,3-dithiane 1,3-dioxide 5 as the nucleophile, but once again ring closure to form epoxides failed to compete with the facile retro-aldol process. Use of sodium bases with 5 resulted in capricious reactions from which a single isomer of epoxide could be isolated. There is evidence of decomposition of the target epoxides under the reaction conditions to α-substituted carbonyl compounds and 1,2-dithiolane 1-oxide.