Aldoses mercaptalation

H. Zinner, G. Rembarz, and H. Klocking, Isopropyliden-verbindungen der D-arabinose-mercaptale, Chem. Ber. 90 2688 (1957) P. A. J. Gorin, Acetonation of aldose diethyl dithio-acetals. Can. J. Chem. 43 21X18 (1965) D. G. Lance and J. K. N. Jones, Acetonation of D-xylose diethyl dithioacetal, Can. J. Chem. 45 1533 (1967). 

Since this work began, a gas chromatographic technique has been published that has certain distinct advantages over those previously available for monosaccharides (14). The technique involves the elimination of the asymmetric carbon by mercaptalation of the aldose mixture with ethanethiol prior to silylation. This results in much simpler quantification through the elimination of multiple chromatographic peaks for a single sugar. The problem of small furanose peaks with their low quantitative accuracy is eliminated as well. The main difficulty with this techique lies... 

L-arabinose. A paper-chromatographic study of the mercaptalation reaction with n-galactose, D-glucose, and D-mannose confirmed that almost complete conversion to the dithioacetal occurred within 5 minutes, but, after several hours, an equilibrium was set up in which two 1-thio-glycosides, probably the anomeric pyranosides, the dithioacetal, and the free aldose were all present the 1-thioglycoside was particularly favored with the D-mannose structure. 3-Amino-3-deoxy-D-mannose reacted (qualitatively) similarly to D-mannose, but more slowly, probably because of the polar influence of the amino group. 

It would seem that, under mercaptalation conditions, dithioacetal formation is a fast, kinetically controlled reaction, followed by a slower reaction which, at equilibrium, gives a distribution of products, including free aldose, thioglycosides, and dithioacetal, according to their thermodynamic stabilities in the system. 

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