Hexonic acids, 3-deoxy lactone

Starting from D-galactono-1,4-lactone, 2-keto-3-deoxy-D-hexonic acids with either the D-erythro (119) or D-threo configuration (120) were obtained in only three steps.318 Epimerization at C-4 occurred during the base-catalyzed /1-elimination reaction on the galactonolactone derivative 136. 

Three structurally isomeric forms have been established for the six-carbon saccharinic acids. In the order of their discovery, these are the sac-charinic or 2-C -methylpentonic acids, the isosaccharinic or 3-deoxy-2-C -(hydroxymethyl)-pentonic acids, and the metasaccharinic or 3-deoxy-hexonic acids. Although none of these six-carbon, deoxyaldonic acids has been crystallized, six are known in the form of crystalline lactones (saccharins). All the possible metasaccharinic acids of less than six-carbon content have been obtained, in the form of crystalline derivatives, by the sugar-alkali reaction. Only one example of a branched-chain deoxyaldonic acid (the racemic, five-carbon isosaccharinic acid) of other than six-carbon content has been so obtained. The formation of saccharinic acids containing more than six carbon atoms remains to be explored. 

A lactone and a free sugar of this series have been obtained during investigations of nucleophilic displacements at C-5 of a hexonic acid derivative, in reactions (similar to those previously reported1) by which related enamines were formed. Treatment of 2,3,4,6-tetra-O-benzyl-N,N-dimethyl-5-0-(methylsulfonyl)-D-gluconamide with potassium acetate in N V-dimethylformamide gave 2,4,6-tri-0-benzyl-3-deoxy - L - threo - hex - 2 - enono -1,5 - lactone (53, R = O) which, with... 

Heating of 2-deoxy-D-ara6/ o-hexonic acid resulted in appreciable lactoniza-tion on melting. Subsequent decomposition of the lactone involved dehydration, decarboxylation, and scission to yield water, carbon dioxide, carbon monoxide, 5-(2-hydroxyethylidene)-2(5/f)-furanone, acrylaldehyde, and acetic acid. 3-Deoxy-D-rj6( -hexono-l,4-lactone - which cannot undergo j3-elimination -is thermally more stable and gives products resulting from scission and dehydration. 

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