Progressive Studies on the Novel Samarium-Catalyzed Diastereoselective Tandem Semipinacol Rearrangement/Tishchenko Reduction of Secondary α-Hydroxy Epoxides

A novel and highly diastereoselective samarium‐catalyzed tandem rearrangement/reduction of secondary α‐hydroxy epoxides, which involves a C1 to C3 carbon migration rearrangement and a very interesting hetero‐Tishchenko reduction of the intermediate aldehyde and the reductant aldehyde, has been reported. This reaction could be developed to provide a facile and stereoselective construction of 2‐quarternary 1,3‐diol units with an hydroxymethyl moiety attached to the diastereogenic quaternary carbon center. Detailed investigations have been carried out concerning the screening of the aldehydes as a reductant, the optimization of reaction conditions, and the substrate scope of this tandem reaction. A catalytic cycle for this reaction, the electronic and steric effects of the reductant aldehydes, and the mechanism for the acyl migration of 1,3‐diol monoesters are proposed. A novel tandem reaction of secondary α‐hydroxy epoxides is promoted by substoichiometric amounts of SmI2 (see scheme). Interestingly, the reaction is influenced by significant steric and electronic effects of the reductant aldehydes. The sequence could also be developed to efficiently construct 2‐quaternary 1,3‐diol units with an hydroxymethyl group attached to the diastereogenic quaternary carbon center.