DIASTEREOSELECTIVE EPISULFIDATION OF STRAINED CYCLIC ALKENES BY A THIOPHENE ENDOPEROXIDE VERSUS EPOXIDATION BY DIMETHYLDIOXIRANE

Thiophene endoperoxide 2, which was prepared by photooxygenation of th iophene 1, transfers a sulfur atom (up to 92%) to strained cycloalkene s to form thiiranes when thermolyzed in their presence. The diastereom eric pair cis/trans-cyclooctene (5b) reacted stereoselectively, which speaks for a concerted process rather than open dipolar and/or diradic al intermediates. The set of chiral cyclooctenols 5c-e was also invest igated, and the relative configurations of the respective thiiranes we re assigned by chemical correlation and NMR spectral and X-ray analysi s. The first-order kinetics of the process clearly shows that the endo peroxide 2 itself is not the sulfur-transferring species, but it is th ermally transformed to the intermediates I and II. Whereas intermediat e II is responsible for the competitive formation of elemental sulfur, intermediate I, presumably an oxathiirane, is the active sulfur-trans ferring species. The episulfidation was compared with the epoxidation by the related dimethyldioxirane, and both show the same qualitative t rends in the diastereoselectivity and the reactivity toward the alkene s 5a-e.