Studies on vitamin D and related compounds III. Short communication on the cis-trans isomerization of calciferol and the properties of “trans”-vitamin D2†‡

Isomerization of vitamin D2 in apolar solvents at room temperature under the influence of small amounts of iodine in diffuse daylight yields a compound that could be isolated in the form of the crystalline phenylazobenzoate. Spectra of solutions of the free vitamin D isomer obtained upon saponification of the ester are presented. They support the view that the isomer should be considered as trans-vitamin D2 having structure A (fig. 3).