Nonhomoannular cisoid conjugated dienes exhibit negative lowest energy pi-->pi* Cotton effects when they have P diene chirality and positive CEs when they have M diene chirality. We investigated this relationship further with a variety of such dienes by MM2 conformational energy-minimization calculations and by an X-ray crystal structure of a steroidal 19 nor 1(10),9(11) diene. CEs are stronger when each double bond of the diene is endocyclic in a different ring and weaker when only one of the double bonds is endocyclic or when neither double bond is endocyclic. They are also stronger when axial allylic and homoallylic substituents with CH/pi interactions are present that exert consignate chirality contributions.
Copyright 2002 Wiley-Liss, Inc.