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 Et 3 N, 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/Et 3 N 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/Et 3 N afforded only two (and no more) diastereomeric [4 + 2] cycloadducts ( endo/exo ‐ 8d ). Et 3 N can be replaced by Me 3 N, 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 AgNO 3 or tertiary amine/AgNO 3 . 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 SO 2 extrusion to give ketone 21d in only 18% yield.