Evaluation of ethyl 2-carbomethoxyethenesulfinates as 2-hydroxymethyl enethiol equivalents in the Diels-Alder reaction

Z- and E-ethyl 2-carbomethoxyethenesulfinates 1 and 2 hold the potential to be enethiol equivalents by way of Diels-Alder cycloaddition chemistry followed by reduction. To pursue this, both dieneophiles were subjected to thermal and Lewis acid mediated [4+2] cycloadditions with a number of dienes. Yields of cycloadduct ranged from 34 to 94%, with cycloadditions of the aromatic dienes proceeding in moderate yields, presumably due to the reversibility of the reactions. Many cycloadducts were obtained as a mixture of sulfur epimers, but these isomers were unified in the subsequent reduction step. Using either LAH or DIBAL, the cycloadducts were reduced providing β-mercapto (or sulfanyl) carbinols in 32-97%. The sequence of two reactions places sulfur and hydroxy functional groups in set relative stereochemistry within a 6-membered ring.