CONFORMATIONAL FLEXIBILITY OF 6-MEMBERED 1,4-DIHYDROCYCLES

Using ab initio quantum-chemical method with 6-31G basis set it was sh own that six-membered 1,4-dihydrocycles with different bridged groups (CH2, NH, O, S, C=CH2, C=NH, C=O) possess high conformational flexibil ity. Transition from a planar equilibrium conformation to the boat wit h angle between planes of the double bonds +/- 160 degrees causes an i ncrease of energy less than 2.4 kcal mol(-1). Presence of cyclic conju gated system and an increase of aromaticity degree decrease the confor mational flexibility of dihydrocycle. All molecules have similar pi-el ectronic structure and this fact, probably, causes the similarity of t heir conformational behavior. (C) 1997 Elsevier Science B.V.