Ring Contraction of 3,6‐Dihydro‐2 H ‐thiopyrans to Thiolanes by an Iodo‐Oxyacylation Reaction

Reaction of functionalized 3,6‐dihydro‐2 H ‐thiopyrans with N ‐iodosuccinimide in the presence of carboxylic acids results in the stereospecific formation of poly‐functionalised thiolanes in good yield. The formation of these thiolanes is believed to proceed through either a nucleophilic or an electrophilic pathway leading to 4,5‐ cis ‐substituted derivatives. The use of unsymmetrical 2,2‐substituted 3,6‐dihydro‐2 H ‐thiopyrans gave mixtures of isomers that could be separated in several cases. From a 3‐substituted thiopyran a 2,2,3,4,5‐pentasubstituted thiolane was obtained. Attempts to use alcohols as external nucleophiles were unsuccessful with NIS, NBS, and N ‐bromoacetamide. Iodo‐azidination with in situ generated IN 3 was also unsuccessful. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)