Oxygen lone pairs, nonequivalence

If the symmetry of the system is broken, as in glycosyl derivatives, it is best to adopt the nonequivalent (p-type and n-type) representation of the lone pairs on oxygen. This is illustrated with a-D-xylopyranosyl fluoride in Fig. 2. With this representation it is important to note that the p-type lone pair is tilted at a dihedral angle of ca. 30% to the axial C—F bond. 

Soon, it was pointed out by Fuchs et al. (89) that the equivalence of oxygen lone electron pairs is not consistent with X-ray structures of carbohydrates. They scrutinized 111 structures and found torsion angles O in Oendo—C—O—R fragments appreciably larger than 60°. If lone pairs were equivalent, one might expect 0 = 60° to be preferred whereas nonequivalence implies 0 > 60°. Indeed, Cosse-Barbi et al. (69,257) have recently shown that the endo anomeric effect is stronger in furanoses than in pyranoses. They... 

Figure 60. Conformations 197A and 197B as a result of no a interactions involving (a) equivalent and (b) nonequivalent lone electron pairs of the exocyclic oxygen atom as donors.

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