Stereoselective syntheses of naturally occurring 5,6-dihydropyran-2-ones

The history of stereoselective syntheses of naturally occurring chiral 5,6-dihydropyran-2-ones with various substituents R3 R6 is presented. Monosaccharides are often used as chiral precursors in the synthesis of 5,6-dihydropyran-2-ones. Lichtenthaler and co-workers reported the preparation of (+)-parasorbic acid in three steps from dihydropyran in 1984, which is easily available from L-rhamnose. One of the step is the Lewis acid-mediated oxidation of 2 to lactone 3.14, which is transformed into parasorbic acid through reductive C O bond cleavage followed by double-bond isomerization. Wide structural variety of other chiral precursors have been used in the synthesis of natural 5,6-dihydropyrones, out of which many have been prepared with the aid of enzymes or whole microorganisms. The preparation of chiral precursors by biocatalytic means is the enantiopure cyclic diol, which protects in the form of acetonide and oxidative C-C bond cleavage afforded the bis-primary diol.