5-KETO-MANNOSE (D-LYXO-HEXOS-5-ULOSE) IN AQUEOUS-SOLUTION - ISOMERIC COMPOSITION DOMINATED BY ALPHA BETA D-FRUCTOFURANOSE RELATED STRUCTURES/

Selective C-6 hydroxyl triphenylmethylation of methyl 2,3-O-isopropyli dene-alpha-D-mannofuranose (1), followed by C-5 hydroxyl oxidation and sequential removal of protecting groups in aqueous acid, yielded D-ly xo-hexos-5-ulose (5-keto-mannose, 5) as a mixture of isomeric forms. T he isomeric mixture of 5 in D2O solution was carefully examined using H-1 and C-13 NMR techniques and structural assignments were made for s even isomers. The most prevalent form of 5 observed was the ketofurano se isomer 2S,5R-D-lyxo-hexo-5,2-furanos-5-ulose 1-hydrate (5a, 52 %), with its 2S,5S-ketofuranose anomer (5b) being the next most abundant ( 14 %). Also identified in the mixture were the alpha and beta-hexofura nos-5-uloses 5c (6 %) and 5d (< 2 %), the pyranose structure 1R,5R-lyx o-hexopyranos-5-ulose 5e (10 %), and the anhydro isomer 1R,5R-1,6-anhy dro-D-lyxo-hexopyranos-5-ulose (5f, 5 %), present in a C-1(4) conforma tion. Limited spectral information suggests that the remaining isomer 5g (8 %) is a hydrated acyclic aldehyde form of 5.