The highly strained ?-sultine 4, resulting from the addition of SO2 to benzobenzvalene, is shown to undergo a thermal cycloreversion which gives 1H-indene-1-thiocarbaldehyde dioxide (6) as an intermediate and subsequently 1H-indene-1-carbaldehyde (5). The sulfene can be intercepted with electron poor C=C bonds in a cycloaddition process leading to a five-membered ring. The structure of the fused heterocyclic product 8b resulting from addition of the sulfene 6 to N-phenyl-maleimide was ascertained by X-ray analysis. Pent-4-enethial dioxide (12) produced by a sulfo-Cope rearrangement from allyl vinyl sulfone (11) is found to add to N-phenylmaleimide in the same 1,3-dipolar fashion as the sulfene 6. The cycloadditions need not be via concerted pathways.