Abstract
The anomeric effect (relating to X-C-Y interaction) in 2-alkoxytetrahydropyrans and 2-alkylthiotetrahydropyrans was compared some years ago (1). The results are shown in Scheme 1; 0/0 and S/0 Anomeric Effect (1) Scheme 1 since the steric component of the CH3O versus CH3S axial interaction is not known in these systems, the results unfortunately do not permit evaluation of the relative anomeric effects of S-C-O versus O-C-O. One way out of this difficulty would be through evaluation of the steric part of the interaction by force-field calculation, but unfortunately, such calculations have so far had limited success for oxygen-containing saturated heterocycles. Comparison of the O-C-O with the S-C-S anomeric interaction has been effected (2,3,4) as shown in Scheme 2. In this case one can tell, unequivocally, that the O-C-O anomeric effect is the larger, for the overall ΔG° in the two systems is nearly the same and from the known conformational energies
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