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

Z- and E-ethyl 2-carbomethoxyethenesulfinates and 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.