In the course of recent investigations '-4 in this series, we developed a oneflask synthesis of triacetalated, acyclic aldohexoses by use of 2,2-dialkoxypropanes or l,l-dialkoxycyclohexanes in the presence of p-toluenesulfonic acid. As shown in a preceding paper', one of the triacetals obtained from D-glucose, viz., 3,4:5,6-di-0-isopropylidene-aldehydo-D-glucose dibenzyl acetal, was readily convertible, via the trifluoromethanesu1fonate5 intermediate, into 2-azido-2-deoxy-3,4:5,6-di-Oisopropylidene-aldehydo-D-mannose, and then this gave 2-amino-2-deoxy-D-mannose derivatives.
When treated at 65" with 2,2_dimethoxypropane in 1,bdioxane solution in the presence of p-toluenesulfonic acid, D-glucose gave6 a mixture of 3,4:5,6-and 2,3 : 5,6-di-0-isopropylidene-aldehydo-D-glucose dimethyl acetal (1 and 2) in good yield. The chromatographically inseparable mixture of 1 and 2 was treated, without further purification, with trifluoromethanesulfonic anhydride. In the 'H-n.m.r. spectrum of the product (3 + 4) in chloroform-d, at lowest field, the protons on the carbon atom (C-2 or C-4) bearing the sulfonyloxy group appeared as a doublet of doublets, at S 4.92 and 5.16, and their intensity ratio was -0.44 (for 3) to 0.56 (for 4). Ester 4 showed, in t.l.c., a slightly lower mobility than 3, but chromatographic separation was extremely difficult because of the lability of the two compounds.
When the mixture (3 + 4) was treated with sodium azide in N,N-dimethylformamide',* for 1 h at room e p t m erature, only 4 underwent preferential sN2 displacement, to afford the corresponding 4-azido derivatives 6. Compound 6 and the unchanged 3,4:5,6-di-O-isopropylidene-2-O-(trifluoromethylsulfonyl)-al-d&do-D-glUCOSC dimethyl acetal (3) were then purified by chromatography on a column of silica gel. Complete replacement of the sulfonyloxy group in 3 with azide anion as just described, in contrast to 4, required 24 h, even at 50". Such a large *The Behavior of Some Aldoses with Acetal-Exchange Reagents, Part XIV. For Part XIII, see ref. 1.