Stereoselective Chelate-Controlled Addition of Grignard Reagents to Unsaturated Medium-Ring Heterocycles.

Various medium-ring heterocycles, bearing a C2-substituent that contains an accessible Lewis basic heteroatom, react with Grignard reagents with high levels of regio- and stereochemical control. The substrates can be prepared in the optically pure form by the Zr-catalyzed kinetic resolution; subsequent reaction with alkylmagnesium halides leads to the formation of optically pure alkylation products. The studies outlined herein probe the influence of the length and position of the heteroatom side chain on the facility and regio- and stereoselective outcome of the allylic substitution process. A catalytic procedure for the subsequent removal of the requisite heteroatom chelating group is presented.