Conformations glucopyranoside ring

P2j Z = 2 D, = 1.24 R = 0.05 for 2,421 intensities. This molecule is a cardenolide / -D-glucopyranoside. The pyranoside conformation is 4C4, with Q = 58 pm, 0 = 6°. The terminal lactone ring is planar. The conformation was compared with that of digitoxigenin by means of molecular-mechanics calculations. 

This sequence serves to exemplify the formation and aspects of reactivity of toluene-p-sulphonate esters in monosaccharide systems, and further to illustrate the selective protection afforded to hydroxyl groups by the formation of cyclic acetals by reaction with carbonyl compounds. Thus reaction of methyl a-D-glucopyranoside (26) with benzaldehyde in the presence of zinc chloride gives the 4,6-acetal (27) (Expt 5.118), wherein two fused six-membered rings of the frans-decalin type are present. As a cognate preparation the reaction of benzaldehyde with methyl a-D-galactopyranoside results in a similar conversion to a 4,6-acetal, but in this case the product is the conformationally flexible system of the cis-decalin type, the most likely conformation being that shown below. 

All the glucopyranosides substituted to restrict complex formation to the 2,3 position showed marked evidence of reaction with cuprammonium. This behavior would be compatible with ring conformations Cl, B2, and 3B except that three of the substances, the 4,6-ethylidene and 4,6-benzylidene compounds, could not exist in either the B2 or 3B conformations for steric reasons because of the spatial requirements of the second ring hence, these three substances must exist in the Cl conformation. 

Apparently the first definite assignment of a chair form to a six-membered, cyclic-acetal ring was made in order to explain the occurrence of only one stable isomer of methyl 4,6-0-benzylidene-,8-D-glucopyranoside, which will have the conformation L. The stable forms of 4,6-0-benzyli-dene derivatives of methyl /3-D-mannopyranoside, methyl 8-D-allopyrano-side, and methyl S-D-altropyranoside will differ from L only in the configurations at C2 and 03. "... 

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