CHIRAL CROTYLBORONATIES; METHODOLOGY AND SYNTHESIS

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The efficient stereo- and enantioselective construction of complex arrays of acyclic stereocenters constitutes one of the most intriguing challenges in modern synthetic organic chemistry. The multitude of these acyclic stereocenters in macrolides, polyether antibiotics and numerous other biologically active natural products behooves the organic chemist to devise methodology that is practical, efficient and applicable to a board spectrum of problems. The studies we propose are directed towards the development of a general solution to the polypropionate (-CHMe-CHOH-CHMe-) and polyacetate (-CHOH-CH2-CHOH) acyclic stereochemical problems. In preliminary work we have developed a class of tartrate ester modified allyl and crotylboronic esters that undergo highly enantioselective and diastereoselective reactions with achiral and chiral aldehydes. In studies planned for the next four year period, we intend to explore the scope and generality of these reagents, probe the origin of asymmetry and develop second generation reagents with increased, near perfect levels of enantioselectivity. We also intend to apply this methodology in the synthesis of natural products of propiogenic biosynthetic origin. Suggested targets include the ansa chain of streptovaricin D (an ansamycin antibiotic with significant anti-viral activity) and bafilomycin A1 (a member of the novel hygrolide family of macrolides that have a range of significant biological properties including antiparasitic and antifungal activity). The synthetic objectives will receive lower priority than the methodological investigations. If our goals are met, we will have developed a family of chiral allylboronates that function as highly enantioselective acetate and propionate enolate equivalents, and will have greatly expanded the scope of reagents available to the organic chemist for the enantio- and diastereoselective construction of complex, biologically active molecules.