Photochemistry of the vitamin D series. II. Wavelength dependence of photoisomerization of precalciferol

In addition to the known ring opening at shorter wavelengths it was found that ring opening in Δ^5,7-steroids occurs also at λ > 300 nm. Ring closure of the corresponding seco- steroids in this region was confirmed. The existence of three rotational isomers with respect to the 5,6-bond is assumed to be responsible for the observed wavelength dependency of the photoisomerizations of previtamin D. In this case the 5,6-s-trans isomer should absorb uv light at shorter wavelengths yielding tachysterol whereas the two 5,6-rotational isomers with skew conformation should absorb at longer wavelengths yielding lumisterol and ergosterol, respectively.