Aglycon equatorial

Figure 7. A plot of the calculated FPT-INDO results for the one-bond coupling constant Jpr, in 2-methoxytetrahydropyran as a function of torsion angle around the aglycon C-0 bond with the methoxy group in axial (curve labeled with squares) and equatorial (stars) positions.

In the application of this conformational rationale simultaneously to the structures of o-L- and jS-D-cardenolides, the a-L- and the jS-D-rhamno-sides (19) and (44) serve to illustrate the hypothesis that a structural correspondence can be obtained, which permits a satisfactory explanation for the fact that a-L and 0-d isomers have potencies which are, approximately, the same. On the basis of the structures shown, the aglycon component is equatorially disposed, both pyranose rings have the same chair form, and the order of substituents on the ring is the same. The only difference between (19) and (44) is a reversal of configuration of substituents on the pyranoid ring, which is tantamount merely to a vertical displacement of... 

An aglyconic derivative of KDO, having 0-7 and 0-8 temporarily blocked, was required for the synthesis of the model disaccharide Koenigs-Knorr reaction of 3 with a suitable halide derivative of KDO would be expected to occur exclusively at the (equatorial) 0-4 while 0H-5, being axially-disposed and experiencing additional steric shielding by H-7, would be practically unreactive. X-ray crystal structure analysis of KDO derivatives (14, 15) has shown that H-7 is orient-... 

In Ref. 1 (pp. 1114-1115) the effect of the axial or equatorial orientation of the aglycon in the rate of oxidation of a number of /f-D-glycopyranosides is described. When the aglycon adopts an equatorial orientation [a / anomer in the 4Ci(d) conformation] the oxidation takes place at a higher rate (two to ten times) than for the a anomer. In the case of methyl L-arabinopyranosides, the a-L anomer, which, in the (l) conformation has an equatorially attached group at C-l, is oxidized more rapidly. 

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