BENZYL METHYL (S)-2-(P-TOLYLSULFINYL)MALEATE, AN EFFICIENT DIENOPHILE IN ASYMMETRIC DIELS-ALDER REACTIONS

The enantiomerically pure dienophile 3 [1-benzyl 4-methyl (S)-(p-tolylsulfinyl)maleate] was readily prepared in a three-step sequence from benzyl acetate and (-)-(S)-menthyl p-toluenesulfinate (46% overall yield). This vinyl sulfoxide reacted in high yields and with very high regioselectivities and stereoselectivities with a wide variety of 1,3-dienes (10 examples) at low temperature in the presence of Eu(fod)3 or TiCl4. Whereas cycloadditions catalyzed by TiCl4 (usually carried out at -78-degrees-C) occurred with complete regioselectivity, endo selectivity, and pi-facial selectivity, the cycloaddition catalyzed by Eu(fod)3 (usually performed at 0-degrees-C) also occurred with very high regioselectivity and pi-facial selectivity, but with low endo selectivity. Interestingly, all the adducts (excepting the adducts from cyclopentadiene) are unstable at room temperature, undergoing spontaneous sulfinyl elimination to give 1,3-cyclohexadienes 7 and/or 1,4-cyclohexadienes 11, in excellent yields. Regardless of the catalyst, compounds 7 showed a very high optical purity (ee greater-than-or-equal-to 96%). Finally, some models based on the conformational equilibrium of vinyl sulfoxide 3 have been proposed to explain the observed stereoselectivities.