Analysis of the optical and geometrical isomer distributions in selected propylene glycol acetals

For the first time, relative distributions of optical and geometrical isomers in selected propylene glycol acetals are determined. Resolution of the four acetal isomers possible through the reaction of racemic propylene glycol (PG) with selected aldehydes is demonstrated. The four isomers are ascribed to the presence of syn and anti geometrical isomers for each optically active PG acetal enantiomer. Thus, the (+) and well as (-) enantiomer are found to have a pair of syn and anti geometrical isomers. The ratio of the (+) and (-) isomers in the product remains at an approximate 50:50 ratio, as expected. However, somewhat unexpectedly, the syn/anti geometrical isomer ratio systematically varies with the nature of the substituent comprising the side chain of the aldehyde. Mechanisms involving electronic and minimal steric effects are advanced as possible reasons for the change in the syn/anti PG acetal ratios.