Acyclic sugars: the ring-opening of 2-acetamido-2-deoxyhexopyranoses in trifluoroacetic acid

During an investigation of solvent effects on the anomeric equilibria of 2-acetomido-2-deoxyhexoses, we have observed a novel, reversible reaction of these sugars with trifluoroacetic acid that is described herein. Fig. 1 shows the H-n.m.r. spectra of 2-acetoamido-2 deoxy-D-glucopyranose (1) in trifluoroacetic acid, recorded at intervals after dissolution. A sharp doublet (J = 5.5 Hz) at delta = 7 and a singlet at delta = 2.6 begin to appear within a few min. The increase in the intensity of these resonances is accompanied by a concomitant decrease of the H-1 peak at delta = 5.4 and the methyl resonance at delta 2.4 of 1. The reaction is essentially complete after 3 h, and Fig. 1c shows the spectrum after 19 h. During this conversion, the specific rotation changes from initial alpha D of +49 deg. to a final value of –54.4 deg. Evaporation of the trifluoroacetic acid in vacuo yielded an unstable oil that showed the presence of acetamido and trifluoroacetyl groups (Vco 1680 1795 cm-1). Addition of D2O led to rapid reversal of the reaction. Fig. 2 shows the n.m.r spectra of the oil in D2O, recorded at intervals. The doublet at delta 7 and the singlet at delta 2.6 disappear, whereas the resonances of 1 reappear. An interesting feature of the reverse reaction is the growth and decay of doublet at delta = 6.5 and a methyl resonance at delta 2.15, characteristic of a short-lived intermediate. Analysis of curves of the progress of reaction suggests that consecutive first-order processes are operative