Thermal fragmentation of a 1,5,2-oxathiazole 5-oxide system via two parallel pathways: interception of a vinyl nitrene and of a sulphene

Heating of 1,5,2-oxathiazole 5-oxide (2) (obtained by cycloaddition of fluorene-9-thione S-oxide and benzonitrile oxide) in benzene resulted in the formation of the dispiro{fluorene-9,4′-[1,2]thiazole-5′,9″- fluorene} 1 ′,1 ′-dioxide (3) (84%). The formation of product (3) is explained by assuming thermal fragmentation of (2) via two parallel pathways, one involving the loss of benzonitrile to give fiuorene-9-thione S,S-dioxide (5) and the other the elimination of sulphur dioxide to produce the vinyl nitrene (7). Cycloaddition of the sulphene (5) and the nitrene (7) then leads to the heterocycle (3). This mechanism is supported by the fact that the vinyl nitrene (7) could be trapped by reaction with 4,4′-dimethylthiobenzophenone, and the sulphene (5) by reaction with methanol. The alternative rationale for the formation of (3), viz. reaction of an azirine (4) with the sulphene (5), could be ruled out.