Stereoselective synthesis of delta-lactones from 5-oxoalkanals via one-potsequential acetalization, tishchenko reaction, and lactonization by cooperative catalysis of samarium ion and mercaptan

By the synergistic catalysis of samarium ion and mercaptan, a series of 5-o xoalkanals was converted to (substituted) delta -lactones in efficient and stereoselective manners. This one-pot procedure comprises a sequence of ace talization, Tishchenko reaction and lactonization. The deliberative use of mercaptan, by comparison with alcohol, is advantageous to facilitate the ca talytic cycle. The reaction mechanism and stereochemistry are proposed and supported by some experimental evidence. Such samarium ion/mercaptan cocata lyzed reactions show the feature of remote control, which is applicable to the asymmetric synthesis of optically active delta -lactones. This study al so demonstrates the synthesis of two insect pheromones, (2S,5R)-2-methylhex anolide and (R)-hexadecanolide, as examples of a new protocol for asymmetri c reduction of long-chain aliphatic ketones.