THE ISOMERIC COMPOSITION OF D-XYLO-HEXOS-5-ULOSE (5-KETO-GLUCOSE) IN AQUEOUS-SOLUTION

-O-Isopropylidene-alpha-D-xylo-hexofuranos-5-ulose (2) was deprotected in aqueous acid solution to give a mixture of at least six isomeric f orms and one anhydro form of the parent ketoaldohexose, D-xylo-hexos-5 -ulose (3), commonly referred to as 5-keto-glucose. Structural assignm ent of each form was made based on high field H-1 and C-13 NMR studies of the mixture in aqueous (D2O) solution. The dominant isomeric form of 3 was observed to have the pyranose structure 1R,5R-D-xylo-hexo-pyr anos-5-ulose (3a, 67 %) with the next most abundant form being an anhy dro structure, 1S,5S-1,6-anhydro-D-Xylo-hexopyranos-5-ulose (3c, 18 %) . Included among the other isomers were the alpha and beta-1,4-furanos e (3d, 3e) and 1-aldehydrol beta-5,2-furanose (3f) structures. The iso mer present in least amount (3g, < 1 %) is assigned as the alpha-anome r of 3f. Experimentally determined J(C-1,H-1) values were useful in su pport of assigned isomer structures.