Two simple calciferol analogues (1) having no five-membered (D) ring fused to the C ring, show the same smooth thermal isomerisation to the corresponding precalciferol analogues as calciferol itself. The kinetics of this reaction have been studied; the activation energy and entropy were found to be 21.6 (± 0.2) kcal/mole and −17.2 e.u., respectively. The insensitivity of the reaction towards even rather drastic changes in the conditions is striking and the reaction mechanism formerly given10, 15 for the vitamin D ⇆ previtamin D interconversion seems to remain an attractive working hypothesis.
There is one difference between the analogues and the calciferol system itself; in the former the equilibrium is far more on the “precalciferol” side. It is argued that this difference (and perhaps also the relative long wavelength ultra-violet absorption of precalciferol) may be due to a strain in the precalciferol molecule originating from the presence of the C8-C9 double bond in combination with the trans fused five-membered ring. In agreement herewith, 13α-precalciferol analogues having the five-membered ring cis-bonded and lacking this strain, are found to show a position of the equilibrium (and an ultra-violet absorption) similar to that of the analogue II and different from that of precalciferol.