Base catalysed rearrangements involving ylide intermediates. Part 9. The rearrangement reactions of cyclic allylic ammonium and sulphonium ylides

The allylic ammonium ylides (22) and (47) and sulphonium ylide (53) are isolable, usually as crystalline solids, because their [3,2] sigmatropic rearrangements are inhibited by the strain associated with the bicyclic transition state of a concerted reaction mechanism. The tetrahydropyridinium (22) and dihydrothiopyranium (29) ylides undergo a thermal [3,2] rearrangement, but, the pyrrolinium ylide (47) rearranges by a [1,2], rather than a [3,2], sigmatropic pathway. The dihydrothiophenium ylide (53) does not undergo either a [1,2] or [3,2] sigmatropic rearrangement but instead reacts in a bimolecular fashion to give eventually buta-1,3-diene, 2,5-dihydrothiophen, and the heterocycle (55). The [3,2] rearrangements of the ylides (22) and (29) follow predominantly an endo-pathway leading to a single diastereoisomer of the products (23) and (31); this strong endo preference is not shown by analogous acyclic ammonium ylides.