Alkyl- and arylsubstituted ketenedithioacetal tetroxides: Diels-alder reactivity and reductive desulfonylation of the adducts

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The Diels-Alder reactivity of the representative alkyl and aryl-substituted ketenedithioacetal tetroxides of general formula 1is reported. These dienophiles reacted under thermal conditions (refluxing toluene) with cyclopentadiene to afford predominantly the endo adduct. In the case of 1C and 1D the endo and exo adducts were formed with total stereoselectivity with respect to the biphenyl or binaphthyl residue as determined on the basis of n.O.e. experiments. Dienophiles 1A-D failed to react with other dienes under similar reaction conditions but reacted efficiently in ether containing 5 M lithium perchlorate under sonication. Under these reaction conditions, the cycloaddition to quadricyclane, 1-methoxy-, 1- and 2-trimethylsilyloxy-1,3-butadiene and to the Danishefsky diene were investigated. In the cycloaddition to 1-trimethylsilyloxy-1,3-butadiene and to the Danishefsky diene the open chain adducts ( Z)- 14 and 17 were obtained in high yield as primary adducts. Eventually, the former undergoes isomerization to ( E)- 14, while the latter is further hydrolyzed to 18. The open-chain compound 19 was also obtained via base treatment of 12Ac. The generation of a highly stabilized bis(phenylsulfonyl)-substituted carbanion is probably the driving force of these transformations. Reductive desulfonylation of 8Ac unexpectedly afforded the norbormadiene 20, possibly via a carbene intermediate. At variance, adduct exo- 8Dc and 10Ac upon reduction gave the hydrocarbon exo- 21 and 22. Finally, reduction of 12Ac occurred with concomitant reduction of the carbonyl group to give alcohol 23.