SYNTHESIS AND PROPERTIES OF D-ERYTHROASCORBIC ACID AND ITS VITAMIN-C ACTIVITY IN THE TOBACCO HORNWORM (MANDUCA-SEXTA)

In a four-step synthesis of D-erythroascorbic acid (1), D-xylose was isomerized to a mixture of D-xylulose/D-xylose (8:2 w/w) using xylose isomerase in the presence of borate. After removal of borate, the syrupy mixture of sugars was acetonated to give mostly 2,3-0-isopropylidene-D-xylulose (5) plus some 1,2: 3,5-di-O-isopropylidene-D-xylose. Compound 5, which was isolated in 70% crystalline yield by solvent extraction, was catalytically oxidized in aqueous solution at pH 8 to give 2,3-O-isopropylidene-beta-D-three-pentulosonic acid (6) in 85% yield. Finally, 6 was converted to 1 in methanolic hydrogen chloride in 64% yield. The overall isolated yield of 1 from D-xylose was 31%. L-Ascorbic acid (2) was slightly more readily oxidized than 1 at pH 4-8 by electrochemical oxidation at a glassy carbon electrode, as well as by O2 in water. Compound 1 supported growth of Manduca sexta larvae but not ecdysis to pupae and adults.