Unsaturated carbohydrates. Part VII. The preference shown by allylic ester groupings on pyranoid rings for the quasi-axial orientation

Abstract

Consideration of the proportions of anomeric 2,3-didehydro-3-deoxyaldose ester present in equilibrium mixtures, together with their conformations as revealed by n.m.r. spectroscopy, leads to the conclusion that in pyranoid compounds containing an endocyclic double bond allylic esters groupings favour the quasi-axial orientation. Quantitatively, this preference is found to be 0·8 and 1·3 kcal./mole for the acetoxy- and benzoyloxy-groups, respectively. The “allylic effect” exerts important control over conformational and configurational features in unsaturated pyranoid compounds.