Abstract
Data are presented in support of the view that the photochemical conversion of ergosterol into precalciferol, involving a cleavage of the cyclohexadiene‐ring, belongs to a generally occurring type of photochemical isomerisations. Under the influence of ultraviolet light 1,3‐cyclohexadiene itself is converted to trans‐1,3,5‐hexatriene. Irradiation of α‐phellandrene (1‐methyl‐4‐isopropyl‐1,5‐cyclohexadiene), results in a mixture of compounds, mainly consisting of (geometrical isomers of) 3,7‐dimethyl‐1,3,5‐octatriene. By ultraviolet‐ and infrared spectroscopy, hydrogenation, gas liquid partition chromatography and ozonolysis the identity of these irradiation products could be established. Indications were found that optically active α‐phellandrene might partly racemise during irradiation.
Prolonged irradiation of cyclohexadiene and α‐phellandrene resulted in a large decrease of the extinction in the region 220‐300 mμ. On irradiation of the compounds at liquid air temperature in solid solution such a decrease was not observed. From this it was tentatively concluded that in the course of the photochemical reaction at room temperature a relatively slow di‐ or polymerisation takes place.