Xylopyranoses mutarotation

Reduction of 10 with lithium aluminum hydride (LAH) in ether furnished19 an intermediate, presumably the phosphine derivative 11, which was treated with acid to effect ring enlargement, giving the 5-phosphino-D-xylopyranose derivative 14. This compound was immediately converted by air oxidation19 into the stable crystalline compounds, 5-deoxy-3-0-methyl-5-C-(phosphinyl)-D-xylopyranose (15) and the 5-C-(hydroxyphosphinyl) derivative 16 in overall yields of 15 and 3.5%, respectively, from 10. Compound 16 was obtained in 90% yield from 15 by oxidation with bromine.19 No mutarotation was observed19 for compounds 15 and 16 in water during 48 h. 

The cover shows the 13C NMR spectrum of a- and P-D-xylopyranose at mutarotational equilibrium (35% a, 65% p. in deuterium oxide. 100 MHz, H broadband decoupling with the CC INADEQUATE contour plot. An interpretation of the plot according to principles described in Section 2.2.7 gives the CC bonds of the two isomers and confirms the assignment of the signals in Table 2.12. 

At pH 4.4, 5-thio-D-xylopyranose (215) shows a slow mutarotation, namely, [a]p +202 - +178° (half-time, 10 hours). At pH 6.6, however, the half-time is only about 10 minutes. The direction of the rotation shows that 215 crystallizes in the a-D form. The nuclear magnetic resonance spectrum of 215 in deuterium oxide shows the presence of the H-1 proton of both anomers. The diaxial coupling of 8.2 Hz for the H-1 proton at the higher field (t 5.25) corresponds to the )8-D anomer in the CJ(d) conformation of the molecule having a sulfur-containing ring. 

Investigations into the anomeric specificity of the enzyme suggested that the a-xylopyranose was utilised exclusively, a fact demonstrated by comparison of initial reaction rates with pure and mixed anomers assuming that the rate of spontaneous mutarotation of each sugar is slow [30]. 

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