STEREODIVERGENCE IN AN INTRAMOLECULAR HORNER-EMMONS MACROCYCLIZATION - EFFECT OF REACTION CONDITIONS ON PRODUCT DISTRIBUTION

In a macrocyclization by intramolecular Horner-Emmons reaction, it has been shown that employment of K2CO3/18-crown-6 leads to formation of an E-disubstituted olefin as the major isomer whereas the use of LiCl/ DBN affords the Z-isomer as the major product. Changes were made to th e base, solvent and reaction temperature in an attempt to identify the factors which influence the stereochemical outcome of the cyclization . The results of this study suggest that the sterodivergence arises fr om a change in the rate-determining step of the reaction, possibly att ributable to the strength of the base employed. Such an effect has bee n previously invoked in the intermolecular Horner-Emmons reaction to a ccount for Z-selective conditions.