Studies in sigmatropic rearrangement: synthesis of a [6,6]pyranothiopyran ring system by sequential claisen rearrangement and pyridine hydrotribromide mediated regioselective "6-Endo" cyclization

reaction: see text] 4-(4'-Aryloxybut-2'-ynylthio)1]benzopyran-2-ones are refluxed in chlorobenzene to afford 4-aryloxymethylthiopyrano3,2-c]1]benzopyran-5(2H)-ones which are subsequently subjected to heating in o-dichlorobenzene in the presence of N,N-diethylaniline and then treated with pyridine hydrotribromide to give 6,6]pyranothiopyrans in almost quantitative yield.