Predicting the Diastereoselectivity of Intramolecular Diene Cyclozirconation: Applications to Natural Product Synthesis

D      F. Taber; C      L. Campbell; J      P. Louey; Y      Wang; W      Zhang

doi:10.2174/1385272003376003

Abstract

Under the conditions originally developed by Taber and Nugent, a 1,6- or a 1,7-diene will smoothly condense with zirconocene to give the intermediate metallacycle. While these zirconacycles can be isolated and characterized, they usually are carried on directly, by reaction with a suitable electrophile (e.g. O2, or CO), to give the organic product. As zirconium is inexpensive, the procedure is operationally simple, and the byproduct zirconium sulfate is nontoxic, intramolecular diene cyclozirconation has become a useful tool for carbocyclic and heterocyclic ring construction From the inception of this reaction, it was anticipated that the central Zr could serve as a scaffold to direct the formation of two, three or even four new stereogenic carbon centers. Subsequent work has shown that the cyclometallations are reversible, and that the reaction can be carried out under either kinetic or thermodynamic control. It is particularly exciting that the semiempirical computational package ZINDO can serve effectively to predict the relative stability of the intermediate diastereomeric zirconacycles. This has allowed ZINDO to be used in a predictive sense, to guide the design of dienes that cyclize with high diastereoslectivity. The use of this approach in conjunction with total syntheses of the natural products elemol, dendrobine, haliclonadiamine and androstenedione is described in this article.