ANOMERIC AND GAUCHE EFFECTS - SOME BASIC STEREOELECTRONICS

The anomeric effect and its extensions - the generalised anomeric effect and the gauche effect - are neatly interpreted in terms of competing bonding interactions - n-sigma* and sigma-sigma* - between filled and vacant orbitals. (The reverse anomeric effect is an exception, but the evidence for its existence is not unequivocal.) In a given system this represents just one of several factors controlling conformation, and perhaps reactivity. Its importance is specifically indicated by the changing pattern of bond lengths at the centres concerned. New data - both experimental and calculational - on the structure and conformation of very simple systems - aryl tetrahydropyranyl acetals and 2-substituted ethanol derivatives - are consistent with the dominant role of these stereoelectronic effects.