Studies on the stability of vitamin D2 powder preparations. IV. Isomerization of vitamin D2 and related compounds catalyzed by surface acid on excipients, and active site for catalytic reactions

Abstract

This paper deals quantitatively with the relationship between the surface acidity of excipients and isomerization of vitamin D2 (I) and its isomers, involving isocalciferol (II), 5, 6-cis-isocalciferol (III), isotachysterol (IV), precalciferol (V), 5, 6-trans-calciferol (VI), and tachysterol (VII). Isomerization rates and products were examined in hexane solution of each isomer in which CaHPO4 (pKa 0.8 to 1.5) was suspended. The results showed that the isomerization products are I→II and IV, III→II and IV, VI→II and IV, VII→II and IV, V→I, II, and IV (I is thermal transformation product of V), II→IV. Similar results were obtained when natural aluminum silicate or talc was used. The isomerization rates of vitamin D2 and its isomers were intimately related with "Bronsted acidity, " rather than "Lewis acidity, " on the solid surfaces. Natural aluminum silicate was base-exchanged with sodium chloride solution, and was re-activated in air at 300°. This sample exhibited little diminution in Lewis acidity although its activity in D2 isomerization decreased markedly with decreasing Bronsted acidity. On the other hand, catalys tsample which had been saturated with perylene, that gives colored complexes with typical Lewis acids, was found to be still effective for this isomerization. In a powder preparation, 5, 6-cis-isocalciferol (III) was obtained together with II and IV as the isomerization products of vitamin D2 (I). This differs from the results obtained suspension samples.