Abstract
Ti(IV)-salan 4 catalyzes the diastereo- and enantioselective monoepoxidation of conjugated dienes using 30% H2O2 at rt or below even in the presence of other olefins and adjacent stereocenters. Its enantiomer, ent-4, provides access to the opposite diastereomer or enantiomer. The resultant chiral allylic epoxides, and the triols derived from them, are versatile synthetic intermediates as well as substructures present in many bioactive natural products. The epoxidation is highly specific for Z-olefins. For 1-acyl(silyl)oxypenta-2,4-dienes, epoxidation of the distal olefin is generally favored in contrast to the adjacent regioselectivity characteristic of Sharpless, peracid, and other directed epoxidations of hydroxylated dienes.
Highlights
Ti(IV)-salan 4 catalyzes the diastereo- and enantioselective monoepoxidation of conjugated dienes using 30% H2O2 at rt or below even in the presence of other olefins and adjacent stereocenters
Allylic epoxides display a facile and diverse reaction manifold that arises from the juxtaposition of the inherently strained three-membered epoxide with an olefinic π-system.[1]
The rate of nucleophilic addition, for instance, can be up to 104 times faster than that for an isolated epoxide, proceeding via SN2 or SN2′ pathways.[2]. Due to their considerable synthetic appeal,[1,3] a variety of procedures have been introduced for the preparation of allylic epoxides.[1,4]
Summary
The second generation oxazolidinone reagent (entry l),[9] developed by Shi for diene applications, performed much better when applied to 1 (R = Ac) and afforded only 2 and 3 (4:1 ratio) This is likely due, in part, to the inductive deactivation of the adjacent olefin by the acetyloxy group; inductive deactivation with these reagents has been observed previously.[6,9] Titanium, sequestered within the salan-type ligands pioneered by Katsuki and colleagues,34a afforded both high yield and excellent control of diastereoselectivity (entry m). The chiral hydroxyl centers in the examples are too distant from the site of epoxidation to exert any influence This is not to say that stereocenters in other locations or conformational effects, especially in cyclic dienes, will be without effect; the epoxidation of steroid 34 by ent-4, but the total lack of reactivity with the enantiomeric catalyst 4, is strong testament in favor of steric factors. This material is available free of charge via the Internet at http://pubs.acs.org
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