Pyrolysis of tricyclic cyclobutane-fused sulfolanes as a route to cis-1,2-divinyl compounds and their Cope-derived products

Functionalisation of the double bond of 3-thiabicyclo[3.2.0]hept-6-ene 3, readily formed by hydrolysis of the [2+2] cycloadduct I of 3-sulfolene and maleic anhydride followed by oxidative bis-decarboxylation, gives tricyclic sulfones 5-7 and 9 with the bicyclo[3.3.0.0(2,4)] skeleton. FVP of 3 results in stereospecific extrusion of SO2 to give Z-hexa-1,3,5-triene which undergoes electrocylisation to give cyclohexa-1,3-diene while reaction of 3 with LiAlH4 results in non-stereospecific extrusion to give Z- and E-hexa-l,3,5-triene. Upon FVP the tricyclic sulfones 5-7 and 9 lose SO2 to give 7-membered ring products 16-19 by Cope rearrangement of the initially formed cis-1,2-divinyl intermediates 15. The 1,3-dipolar cycloaddition of nitrile oxides and a nitrone to the double bond of 3 gives tricyclic sulfones with the tricyclo[5.3.0.0(2,6)] skeleton and a wider variety of these can be prepared by conventional reactions of 1. Upon FVP these lose SO2 to give stable cis-l,2-divinyl compounds 23, 24, 37-40 and 41-44. The Diels-Alder adducts 48 and 49 have been prepared from 3 and these behave differently upon FVP, losing SO2 and butadiene to give tetrasubstituted benzenes, in the latter case by way of an unexpected tetracyclic intermediate.