Sequential pericyclic reactions of unsaturated xanthates. One-pot synthesis of hydrobenzo[c]thiophenes

Abstract

O-Alka-2,4-dienyl S-(alk-2-ynyl or alk-2-enyl) dithiocarbonates (xanthates) underwent [3,3]sigmatropic rearrangement to give the dithiol esters which, on heating, extruded carbon oxysulfide (COS) to give the allylically rearranged sulfides which then underwent intramolecular Diels–Alder cycloaddition to give hydrobenzo[c]thiophenes. Lewis acids catalysed the extrusion and intramolecular cycloaddition reactions, in which the reactivity and endo-selectivity were remarkably enhanced. The structures of the cycloadducts were determined on the basis of the X-ray structure of a hydrobenzo[c]thiophene.

Based on these findings together with MO calculation data, the observed reaction behaviours are discussed in terms of frontier molecular orbital considerations.