Influence of the CH/π hydrogen bond on the enhancement of circular dichroism (CD) amplitude of 1,3-cyclohexadiene compounds

Abstract

Introduction of methyl group(s) at bridgehead carbons at positions 10 and 5 of steroidal 1,3-dienes has been known to influence the Cotton effect of the π–π* transition at ca. 260 nm. Thus the positive sign of the circular dichroism (CD) curve of 5α-estra-1,3-dien-17β-ol becomes negative in 5α-androsta-1,3-dien-17β-ol, even though the helical sense of the diene chromophore remains practically unchanged. In order to investigate the effect of methyl substitution on the CD of 1,3-diene steroids, we carried out time dependent density functional theory (TD-DFT) calculations at the M06-2X/6-311++G(d,p)//MP2/6-31G(d,p) level. It has been shown that replacement of the bridgehead hydrogen by a methyl group greatly influenced the sign and amplitude of CD spectra of the π–π* transition. We suggest that the change in the CD curves by methyl substitution originates from CH/π hydrogen bonds between the diene π-system and a CH₃ group homoallylic to it.