[2,3]1‐Thia‐Wittig Rearrangements Proceeding with Complete Inversion or with Partial Loss of Configuration at the Carbanionic Center

Two pairs of diastereomeric γ1‐(benzyloxy)1‐α1‐(tributylstannyl) sulfides ( anti ‐ and syn ‐ 13 , anti ‐ and syn ‐ 30 ) were prepared, n BuLi induced tin/lithium exchange in each of these compounds furnished α1‐lithiated sulfides which underwent [2,3]1‐thia‐Wittig rearrangements in THF at – 78°C within 30 min. Competing [1,2]1‐thia‐Wittig rearrangements were not observed, not even starting from stannyl sulfides 30 where the [2,3] shift requires the intermediacy of lithium thiolates 32 in which the aromaticity of a benzene ring is given up. The homoallyl thiols syn ‐ and anti ‐ 19 were formed stereoselectively starting from the allylthio stannanes anti ‐ and syn ‐ 13 , respectively. This implies that the underlying thia‐Wittig rearrangements are stereospecific and proceed with inversion of configuration at the carbanionic center, and that the lithio sulfides anti ‐ and syn ‐ 18 formed in the course of these reactions are configurationally stable until they rearrange. The benzylthio stannanes anti ‐ and syn ‐ 30 rearranged with low stereoselectivity. They furnished the benzyl thiols 33 as 71:29 and 28:72 mixtures of diastereomers, respectively. This means that the lithio sulfide intermediates involved in the latter rearrangements suffer some epimerization anti ‐ 31 ⇄ syn ‐ 31 prior to the sigmatropic bond shift.