Photochemistry of estra‐5,7‐diene‐3β,17β‐diol 17‐acetate and its 9,10‐seco isomer, 10‐desmethyl analogues of pro‐ and previtamin D. 1. “Reversible” isomerization reactions

In analogy with the photochemistry of 7,8‐didehydrocholesterol (provitamin D), irradiation of the 10‐desmethyl steroid estra‐5,7‐diene‐3β,17β‐diol 17‐acetate leads to a photoequilibrated mixture of steroid‐5,7‐dienes and 9,10‐seco‐5(10),6,8‐trienes. In addition, a thermally unstable bicyclo[2.2.0]hexene derivative is found. From the composition of the quasi‐photostationary state, the ratios of the quantum yields of the various interconversions are estimated. Differences with the photochemistry in the 10‐methyl series are discussed in relation to conformational differences.