[4 + 2] Cycloaddition
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
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.
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