The increased consideration given to the role of absolute stereochemistry in the function of biologically active molecules has intensified the quest for the development of truly useful catalytic processes to effect highly efficient synthesis of optically pure compounds. Because enantiomerically pure epoxides are among the most versatile chiral building blocks in organic synthesis, development of synthetic catalysts that provide practical access to them is highly desirable. The Jacobsen epoxidation, utilizing (salen)Mn-oxo transfer catalysts, has afforded easy access to a variety of valuable enantiomerically pure epoxides from inexpensive conjugated olefin precursors. However, a satisfactory mechanistic explanation for this catalysts' success remains elusive, precluding the rational design of analogous catalysts for specific asymmetric processes. The proposal herein introduces a novel """"""""shifted top-on"""""""" cation radical (STO+) mechanistic model for the Jacobsen epoxidation that provides a simple and clear explanation for the high enantioselectivities observed experimentally for a wide range of conjugated olefin classes. The STO+ model is then utilized as a heuristic tool for the design of a new series of catalysts to specifically effect the asymmetric epoxidation of the highly elusive conjugated terminal olefin substrate class. To illustrate the synthetic utility of conjugated terminal epoxides, an efficient, highly general, three-step synthetic route to enantiopure beta-aryl amino alcohol adrenergic agents, important in the treatment of such diverse disorders as hypertension and asthma, is proposed.
| White, M C; Doyle, A G; Jacobsen, E N (2001) A synthetically useful, self-assembling MMO mimic system for catalytic alkene epoxidation with aqueous H2O2. J Am Chem Soc 123:7194-5 |
