Enantioselective Synthesis of Spiro Ethers and Spiro Ketals via Photoaddition of Dihydro-4-pyrones to Chiral 1,3-Dioxin-4-ones

Abstract

A versatile and highly stereoselective synthesis of spiro ethers and spiro ketals is presented. The key step in the developed synthetic sequence is based on diastereoselective intramolecular photoaddition of dihydro-4-pyrones to chiral 1,3-dioxin-4-ones. Subsequent fragmentation of the produced four-membered ring provides spiro ether structures. The spiro ethers can be transformed to their corresponding spiro ketals, with retention of configuration at the spiro center, via Baeyer−Villager oxidation. The configuration of the spiro center is defined by the facial selectivity at the photocycloaddition step. Two examples of complete stereofacial selectivity were achieved. The unique and important application of the developed sequence was demonstrated in enantioselective synthesis of a less thermodynamically stable spiro ketal 43.