Synthesis of the Four Stereoisomers of 6-Acetoxy-19-methylnonacosane, the Most Potent Component of the Female Sex Pheromone of the New World Screwworm Fly, with Special Emphasis on Partial Racemization in the Course of Catalytic Hydrogenation

Starting from the enantiomers of citronellal and 1-octyn-3-ol, all four stereoisomers of 6-acetoxy-19-methylnonacosane were synthesized with sufficiently high stereochemical purities (more than 90% ee at C-6; about 97% ee at C-19) for their biological testing as the female sex pheromone of the screwworm fly (Cochliomyia hominivorax). All four isomers showed strong pheromone activity even at 1 microgram, and no significant difference was observed in their potency. Adams platinum oxide was found to cause partial racemization of enantiomerically pure secondary propargylic alcohols in the course of their catalytic hydrogenation to saturated and secondary alcohols, while palladium-charcoal was less potent in causing partial racemization. A new HPLC-based discrimination of chiral and secondary alcohols proved to be useful in following such a subtle partial racemization.