Ring shapes of pyranoses and furanoses

The conformational preferences of the aldopentoses, which have no hydroxymethyl group at C(5), are mainly governed by minimising steric repulsion between the hydroxyl groups. Thus, D-arabinopyranose favours the C4 conformer, and a-D-lyxopyranoside and a-D-ribopyra-noside are conformational mixtures and the other aldopentoses are predominantly in the C4 form. 

Gauche Free energy Axial-axial Free energy fkJ mor1) 

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