Failure of the antiperiplanar lone pair hypothesis in glycoside hydrolysis. Synthesis, conformation, and hydrolysis of α-D-xylopyranosyl-and α-D-glucopyranosyl-pyridinium salts

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Abstract
(1) Two series of crystalline α-D-xylopyranosyl-and α-D-glucopyranosyl-pyridinium bromides have been made, by reaction of the acetylated α-bromides with the pyridines in an aprotic solvent in the presence of bromide ion, followed by deacetylation with aqueous HBr at room temperature. (2) 200 MHz 1H N.m.r. spectra in D2O of representatives of each series show the xylo-compounds to adopt the 1C4 conformation, and the gluco-compounds the 1S3 conformation. (3) The hydrolyses of α-D-xylopyranosyl- and α-D-glucopyranosyl-3-bromopyridinium ions in 1.0M-NaCIO4 at 25.0 °C are described by kobs/s–1=4.7 × 10–8+1.4 × 10–18/[H+] and kobs/s–1=2 × 10–7+1.4 × 10–17/[H+], respectively. (4) The products of pH-independent hydrolysis of the foregoing ions are the sugars and 3-bromopyridine, whereas at pH 12, 10% of both reactions proceed by attack on the pyridine ring: attack on the sugar yields glucose and 1,6-anhydroglucopyranose, but no xylose. (5) The pH-independent hydrolyses display strongly positive entropies of activation: at 25.0 °C, those of five xylo-compounds give a βlg value of –1.27 ± 0.06, and those of four gluco-compounds one of –1.06 ± 0.12. (6) Arguments are presented, in the light of the low α:β rate ratios for both xylo and gluco pyridinium salts (8–23 and 80, respectively, at 25 °C), that departure of a pyridine from C(1) of an aldopyranose ring does not require a conformation in which the leaving group is antiperiplanar to a lone pair of electrons on O(5). (7) The antiperiplanar lone pair hypothesis is shown to be a special case of the principle of least nuclear motion, which is known not to apply to reactions with very product-(or reactant-) like transition states.

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