Conformations, anomeric effect interconversion

The conformational analysis by MM2 calculations indicates a preference for the twist-boat conformation in 2,3-dihydro-5//-l,4-dioxepin (47) and for a chair in 6,7-dihydro-5//-l,4-dioxepin (46) as well as in the methoxy derivative (65). It has been suggested that the stability of the different conformations should be governed by the conjugation of the oxygen atoms with the n system, in (46) and (47), by this conjugation and the anomeric effect in (65). The barrier for the chair-twist boat interconversion has been calculated to be 2.50 kcal mol" in (47) and 3.78 kcal mol" in (65) <89JOC6034>. 

The anomeric effect also influences the conformational equilibria in pyranoses with an electronegative atom, usually oxygen or halogen, at C-1. For example, the equilibrium mixture of the P-pyranosyl chloride shown contains 98% of the conformer in which chlorine is axial. These two conformations are not interconverted by mutarotation but by chair-chair interconversion. The anomeric effect is sufficiently large so that all four substituents occupy axial positions in the more stable conformer. 

---