Benzyl Methyl (S)-2-(p-Tolylsulfinyl)maleate, an Efficient Dienophile in Asymmetric Diels-Alder Reactions

Abstract

The enantiomerically pure dienophile (3) [1-benzyl 4-methyl (S)-(p-to- lylsulfinyl)maleate] was readily prepared in a three-step sequence from benzyl acetate and (-)-(S)-menthyl p-toluenesulfinate (46% overall yie- ld). This vinyl sulfoxide reacted in high yields and with very high re- gioselectivities and stereoselectivities with a wide variety of 1,3-di- enes (10 examples) at low temperature in the presence of Eu(fod) 3 or TiCl 4 . Whereas cycloadditions catalyzed by TiCl 4 (usually carried out at -78 o C) occurred with complete regioselectivity, endo selectivity, and π-facial selectivity, the cycloaddition catalyzed by Eu(fod) 3 (usu- ally performed at 0 o C) also occurred with very high regioselectivity and π-facial selectivity, but with low endo selectivity. Interestingly, all the adducts (excepting the adducts from cycloentadiene) are unstable at room temperature, undergoi,g spontaneous sulfinyl elimination to give 1,3-cyclohexadienes (7) and/or 1,4-cyclohexadienes (11), in excel- lent yields