Studies in sigmatropic rearrangement: Synthesis of a [6,6]pyranothiopyran ring system by sequential Claisen rearrangement and pyridine hydrotribromide mediated regioselective 6-endo cyclization

[GRAPHICS] 4-(4'-Aryloxybut-2'-ynylthio)[1]benzopyran-2-ones are refluxed in chlorobenzene to afford 4-aryloxymethylthlopyrano[3,2-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.