An Unexpected Rearrangement in SO-substituted Acetal-allenes

During the last few years our research group has been involved in the investigation of the hydride donor ability of acetalic functions in a series of intramolecular processes. In these reactions the released hydride is transferred to the electrophilic central carbon atom of some heterocumulenic fragments (ketenimines, carbodiimides) and other electrophilic functions. Gratifyingly, we have shown that several of such hydride-like [1,4] and [1,5]-H shifts occur under mild reaction conditions, this migration step being habitually followed by a subsequent pericyclic transformation, most usually a 6π electrocyclic ring-closure (6π-ERC). Following with our efforts in this area we next checked the replacing of the heterocumulenic hydride-accepting function by an allene group, also possessing an electrophilic, sp-hybridized central carbon atom. Surprisingly, when we carried out the thermal activation of a particular class of sulphoxide-substituted acetal-allenes (generated in situ from the respective propargylic sulphoxides) built on an ortho-phenylene scaffold, an unexpected rearrangement, involving the isomeric O-allenyl sulphenate, took place instead of the presumed [1,5]-H/6π-ERC tandem process. In this communication we will disclose the characterization of the reaction products, as well as a mechanistic rationale of the rearrangement.