Experimental Data on These Effects

Conforniatioiial Equilibria of Tri-D cetyl and Tri-O-benzoyl D-Xylopyranose Derivatives as a Function of the Anomeric Group (see Fig. 7) 

A tendency toward stabilization of the a form is usually quite pronounced in the acetyl and benzoyl derivatives of D-xylopyranosyl halides (see Fig. 7a) which, in solution, exist mainly or completely in the Ci conformation, with all substituents in axial positions. This was first pointed out for 

Conformalioda] EqiuHliria rfTri- X cetyl- and Tri-O-benioyl D-ribopyranose Derivatives as a Fnnctioa of the AnonMiic Gronp  

The preference for the axial position diminishes with lowered electronegativity of atoms linked tothe anomeric center thatis,F 0 N C for the first row of the Periodic Table. For the latter two elements, N and C, the anomeric equilibrium depends on the overall polarity of the substituent. Thus, derivatives of sub tuted D-arabinopyranose (see Fig. 8) contain 94% of the isomer having a nitro group in the axial position at equilibrium. N.m.r. measurements and other studies of substituted N-pentopyranosyl derivatives showed that the preference for the axial position decreases in the order NO2 N-PPhs N3 NHCOCF3 NH2  

6- anhydroheptonamides, the carbamoyl group has a strong preference for the equatorial position (see Fig. 10). For example, 3,4,5,7-tetxa-O-acetyl- 

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Experimental data on these effects are lacking, and important future work might be oriented in this direction. Some relatively fragmentary data on alloys do, however, show the potential importance of these considerations. Some of these are discussed in Ref. 91. There is some evidence that the Arrhenius preexponential term on copper is higher than that on nickel, whereas the reaction rate on nickel is the higher of the two, i.e., a compensation effect occurs. 

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