Asymmetric Michael addition reaction organocatalyzed by stereoisomeric pyrrolidine sulfinamides under neat conditions. A brief study of self-disproportionation of enantiomers

This paper describes the synthesis of all four possible stereoisomers of the pyrrolidine sulfinamides derived from (S)-proline or (R)-proline and either enantiomer of t-butylsulfinamide. These bifunctional derivatives were examined as organocatalysts in the asymmetric Michael addition reaction, finding that the pyrrolidine stereocenter, rather than the stereogenic sulfur, is responsible for the observed stereo control in the asymmetric reaction. This result is in accordance with previous reports in the literature (aldol reactions: W. Wan, J. Hao, and coworkers, RSC Adv. 2014, 4, 26563-26568; J. A. Ellman et al., Tetrahedron 2011, 67, 4412-4416). Consequently, no evidence for double stereoinduction is found. The desired Michael adducts were obtained with good yields, high diastereoselectivities, and moderate to good enantioselectivities. Some tests designed to examine the potential manifestation of self-disproportionation of enantiomers (SDE) were carried out by means of chromatographic purification and subsequent evaluation of the enantiopurity of the corresponding Michael adducts. (C) 2017 Elsevier Ltd. All rights reserved.