[2,3]-THIA-WITTIG REARRANGEMENTS PROCEEDING WITH COMPLETE INVERSION OR WITH PARTIAL LOSS OF CONFIGURATION AT THE CARBANIONIC CENTER

Two pairs of diastereomeric gamma-(benzyloxy)-alpha-(tributylstannyl) sulfides (anti- and syn-13, anti- and syn-30) were prepared. nBuLi induced tin/lithium exchange in each of these compounds furnished alpha-lithiated sulfides which underwent 12,31-thia-Wittig rearrangements in THF at -78-degrees-C within 30 min. Competing [1,2]-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 reversible syn-31 prior to the sigmatropic bond shift.