Asymmetric Nucleophilic Epoxidation
Asymmetric epoxidation of a prochiral alkene is an appealing process because two stereogenic centers are established in the course of the reaction. Often, the starting alkene is inexpensive. There have been several interesting recent advances in the asymmetric nucleophilic epoxidation.
Keiji Maruoka of Kyoyo University reports (J. Am. Chem. Soc. 2004, 126, 6844. ) the development of enantiomerically-pure quaternary ammonium salts (2) that catalyze the epoxidation of enones. The epoxidation of the t-butyl ketone 1 is particularly interesting, as Baeyer-Villiger oxidation would be expected to convert 3 into the ester 4.
Masakatsu Shibasaki of the University of Tokyo reports (J. Am. Chem. Soc. 2004, 126, 7559. ) that use of a BINOL-derived catalyst with cumyl hydroperoxide enables the enantioselective epoxidation of unsaturated N-acyl pyrroles such as 7. The pyrroles 7, prepared from the precursor aldehydes such as 5 with the reagent 6, can be used directly, without further purification.
The product epoxy pyrroles such as 8 can be efficiently converted to the alcohol 9, the homologated ester 10, and the homologated ketone 11.
The Juliá-Colonna epoxidation uses poly-L-leucine and hydrogen peroxide to effect enantioselective epoxidation of chalcone derivatives such as 12. In a pair of back-to-back papers (Tetrahedron Lett. 2004, 45, 5065, and 5069, ) , H.-Christian Militzer of Bayer HealthCare AG, Wuppertal, reports a detailed optimization of this procedure. In the following paper (Tetrahedron Lett. 2004, 45, 5073. ) , Stanley Roberts of the University of Liverpool reports the extension of this procedure to unsaturated sulfones such as 14.