Chiroptical Properties of Cyclic Esters and Ketals Derived from (S)-1,2-Propylene Glycol and (S,S)- and (R,R)-2,3-Butylene Glycol

The optically pure five-membered ring thionocarbonates (la-3a), carbonates (lb-3b), sulfites (cis- and transió, 2c, 3c), phosphite (2d), and 2-bromo- and 2-chloromethyl-1,3-dioxolanes (cis- and irons-le, 2e, 2f) were prepared from the three title diols by standard methods and their uv and CD spectra were measured in various solvents over the range 185-400 mμ at room temperature. The CD spectra of la-3a display two well-defined Cotton effects of opposite signs and of diverse intensities centered at 222-235 and 325 mμ, related to the uv π → π *and n → π* transitions, respectively. The sign of the long-wavelength Cotton effect is associated with the chirality of the heterocycle ring. A positive ellipticity for the n →π*transition is assigned to thionocarbonates of R configuration, and vice versa, a negative one for the S series. Cyclic carbonates do not show CD maxima above 185 mg; the curves of dichroic absorption for the R and S forms are antipodal and their signs correlate with-those of the long-wavelength Cotton effect of the corresponding thionocarbonates. The condensation of (S)-l,2-propylene glycol with thionyl chloride gave rise to the expected isomers of opposite rotations, assigned the cis- and irons-lc structures, exhibiting similar dichroic bands centered at 212-225 and 195-200 mg. The complex chiroptical properties of lc-3c and 2d are discussed in terms of ring conformation and asymmetric solvation of the chromophore. The condensation of (S)-l,2-propylene glycol with bromoacetaldehyde again led to the expected cis and trans isomers. The dichroic curves of the geometric isomers and of 2e do not correspond to those of the uv spectra. A CD study of the title glycols is herein included.