Oxazolidinone/enamine ratios in the reactions of α-silyloxy and α-alkoxy aldehydes with proline

α-Silyloxy and α-alkoxy aldehydes (propanal derivatives 1-4), when treated with proline in DMSO-d6, give oxazolidinones rather than enamines. In fact, the relative trend is much stronger than that of all other carbonyl compounds examined, including simple aldehydes, α-branched aldehydes, α-(alkylthio)aldehydes, and standard ketones. For 1-4, the high prevalence of oxazolidinones prevents or delays the partial racemization of the α-stereocenters.