Unexpected Hydroxylation Reactions

Methyl a-D-glucopyranoside may be converted,271 in a remarkably selective reaction, into its 2,4,6-tribenzyl ether (in 62% yield) on treatment with three molar equivalents of sodium hydride in benzyl chloride at 110°, a substitution pattern that might have been predicted in view of the low, relative reactivity of HO-3 towards alkylation in the mechanistically related, Haworth procedure.268 Similar, selective benzylations have also been achieved on partially substituted derivatives of methyl a- and /3-D-galactopyranoside272 and on methyl 6-deoxy-a-L-galactopyranoside62 in all of these, an unexpectedly high relative-reactivity of HO-4 (ax) compared to that of HO-3 (eq) was noted, indicating that steric factors are not the sole influence on reactivity in these cases. Nevertheless, the primary hydroxyl... 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

In addition to quinoline alkaloids (see Table 1), Glycosmis mauritiana contains 5-hydroxy-2,3-dimethoxy-A7-methylacridone (arborinine) and a new prenyl-acridone (28).4 The structure of the latter alkaloid was indicated by spectroscopy, by the presence of a peri-hydroxyl group, and by heating it with formic acid to give the cyclization product (29), thus showing that the prenyl group was at C-2, i.e. adjacent to the hydroxyl group, rather than at C-4. An unexpected by-product of the reaction with formic acid was the acridone (22). 

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