Effect of substituents and conformations on the optical rotations of cyclic oxides and related compounds. Relationship between the anomeric effect and optical Rotation

The effect of substituents on the specific rotation of substituted cyclic oxides (X = F, Cl, CN, and HCC) and related compounds was studied via geometry optimization at the B3LYP/6-311+G** level followed by calculations of the specific rotation with B3LYP/aug-cc-pVDZ and, when practical, also with B3LYP/aug-cc-pVTZ. In some cases chiral samples were prepared so that the calculated specific rotations could be compared with experimental data. With most compounds there was only a minor effect of the basis set on the specific rotations. With the oxiranes and oxetanes, the chloro derivative gave a different behavior than the other substituents, but all substituents behaved in the same fashion with trans-2-methyl-1-X-cyclopropanes. Therefore the unusual behavior of chlorooxirane probably results from an interaction between oxygen and chlorine rather than being due to the presence of a three-membered ring. Chlorine is also an unusual substituent for the tetrahydrofurans. The effect of conformation on the calculated specific rotations was examined with the axial and equatorial 2-substituted tetrahydropyrans, where the anomeric effect is operative with the axial substituent, and also the 3-substituted tetrahydropyrans that would not be subject to the anomeric effect. The unusual effect of chlorine was seen only when it is antiperiplanar with respect to the oxygen.