AbstractOptically pure 1‐aryl‐ (phenyl and 2‐chlorophenyl) 1,3‐propanediols, 1a and 1b, condense smoothly with aldehydes to provide the corresponding cyclic acetals 3. Except in cases that the aldehyde is substituted at the α‐carbon atom, a single product is obtained. With the aid of 2D‐NMR the stereochemistry was established to be that with the aldehyde substituent as well as the aryl group equatorial in the favored chair form.The condensation products with some α,β‐unsaturated aldehydes were subjected to cyclopropanation with diiodomethane/diethylzinc. Reactions proceeded in good yields. Deprotection was achieved by ozonolysis. The cyclopropanecarboxylic acids 6 were obtained in 34‐67% yields and in enantiomeric excesses ranging from 21‐78%.A stereochemical rationalization of the results is given.