Trapping reactions of sulfenes generated from sulfonic acid derivatives and bases, 2. [4 + 2] cycloadditions of 1,2,3,4,5‐pentamethylcyclopentadiene and 1,3‐diphenylbenzo[c]furan with sulfenes

Sulfenes 2a‐o, generated in situ from sulfonyl chlorides and Et3N, are trapped by 1,2,3,4,5‐pentamethylcyclopentadiene (6) to give [4 + 2] cycloadducts 8a‐o containing the 7‐methyl group in anti position. With mesylsulfene (7), formed in situ from mesyl chloride/Et3N in MeCN solution, the [4 + 2] cycloaddition to endo/exo‐ 9 (X‐ray analyses) is accompanied by a [2 + 2] cycloaddition to 10 (X‐ray analysis). (+)‐10‐Camphorsulfonyl chloride/Et3N afforded only two (and no more) diastereomeric [4 + 2] cycloadducts (endo/exo‐8d). Et3N can be replaced by Me3N, N,N,N′,N′‐tetramethylnaphthalene‐ 1,8‐diamine or NaH, the sulfonyl chlorides to a limited extent by sulfonyl fluorides, sulfonic acid anhydrides or aryl esters. Formation of 8p from 6 and dichloromethanesulfonyl chloride requires NaH or AgNO3 or tertiary amine/AgNO3. N‐(2‐Methyl‐1‐propenyl)pyrrolidine is more reactive than 6 in trapping sulfene 2a. 6, however, is a more efficient sulfene scavenger than 1,3‐diphenylbenzo[c]furan (19), which reacts with sulfene 2d by [4 + 2] cycloaddition and SO2 extrusion to give ketone 21d in only 18% yield.