The effect of the position of the methyl group on the helical twisting powers of aldol condensation products of methyl cyclohexanones

The four aldol condensation products of methylcyclohexanone isomers and 4-(4-hexyloxybenzoyloxy)benzaldehyde have been prepared. These chiral products were either obtained with high enantiomeric excess or the enantiomers were separated by means of chiral HPLC. In all cases only the E-isomer was obtained. UV irradiation was used to isomerize the E-isomers to the Z-isomers. Comparison of the helical twisting powers (HTP) of the E-isomers revealed that the derivative in which the methyl group is directly next to the double bond exhibits the highest value. Although the other three E-isomers have lower HTP values, they show large HTP changes and reversal of the helical twist sense upon irradiation. Of these compounds, the one derived from the commercially available (R)-3-methylcyclohexanone is an interesting candidate for use as an alternative to compounds derived from menthone for realizing photochemically induced twist changes in twisted nematic and cholesteric materials.