ABSTRACT
A crossed aldol reaction has successfully been used as the key step in the total synthesis of lasalocid A, monensin, A23187, and also narasin and salinomycin. Based on an analysis using the anomeric effect well known in carbohydrate chemistry, it was anticipated that the intramolecular ketalization of the corresponding dihydroxy ketone under thermodynamically controlled conditions should result in the spiroketal with the desired stereochemistry. Grieco et al. synthesized the key intermediate of the Evans synthesis from a bicyclo precursor. Nakahara et al. have made considerable progress in the total synthesis of the antibiotic from D-glucose. The methyl ester was hydrolyzed with lithium n-propylmercaptide to complete the total synthesis. Since the sign of the rotation of the synthetic substance was the same as that of natural A23187, the absolute configuration previously assigned to the antibiotic was confirmed by this total synthesis.
