No abstract

Kinetic experiments on the photoisomerisation of ergosterol were carried out using different sources of light. New methods of analysis for pre‐ergocalciferol and tachysterol2 had to be developed. The quantum yield of the photoisomerisation of ergosterol with ultraviolet light of 2537 Å was found to be 0.31 at 20°. This value was obtained when ether, ethanol and petroleum ether were used as solvents. Tachysterol2 is not formed from ergosterol during irradiation, but originates exclusively from pre‐ergocalciferol. Excited ergosterol falls back to its ground state (69%) or transforms to pre‐ergocalciferol (26%) and to unidentified products (5%).

Results of kinetic investigations and measurements of quantum yields are reported, which cannot be explained by the most simple scheme imaginable for the photochemical interconversions of provitamin‐D and its isomers. At least two excited transitory species have to be assumed. More experimental data will be needed before a decision can be made in favour of one or other of the hypothetical, more complicated, pathways.

A prophylactic test (chick assay) was carried out with pure previtamin Ds. The apparent antirachitic activity was found to be 35% of that of pure vitamin Ds. This result is discussed in connection with the kinetics of the interconversion of previtamin Ds+ vitamin Ds. The observed activity may be interpreted as originating exclusively from vitamin Ds formed in the test animal. IntroductionThe previtamins D were discovered in 1949 by VeZZuz et al. 1 who showed that in a solution of (pre)vitamin D an equilibrium previtamin

S. J. Halkes et al. The synthesis of ring D substituted YP,lOcc-steroidsThe speed of evolution of wave packets initially confined to the initial cell into the final cell will essentially determine the wave-mechanical reaction rate. A conventional approximate treatment was carried out to obtain the absolute rate of proton migration in malonaldehyde mentioned in example iiz] of the preceding section.The rate constant was calculated to be -9.77 x 10" s-l over a wide range of ordinary temperatures. In this convention the evolution of wave packets was discussed within a one-dimensional motion along the IRC for which motions of the other freedoms are considered as providing the effective potential26.Such a convention is tractable based on the concept of the IRC which is clearly formulated in a closed form, and the whole process is made automatically calculable with the aid of the IRC-equations. Chemistry now assumes a certain logical character. Further experiments will be facilitated by such a logic as theoretical languages have induced in quantum chemistry, e.g. that of chemical reaction paths, which will be newly elucidated by the discovery of innovating experimental work. Professor Havinga has made a start in this direction. IntroductionInvestigations on sterols. 401*. The synthesis of ring D substituted 9P,lOa-steroids Abstract. The retro configuration (9p, IOU) was introduced into pregnane derivatives containing substituents at C(16) and/or C(17) by irradiation of a suitable 5,7-diene. In this way the title compounds became much more accessible than by the introduction of ring D substituents into 20-ketoretro-steroids. ' 39th Communication: J. Hartog, J. I. G. M. Wittelaar, L . Morsink and A . M . deWachter, J. Med. Chem. 15, 1292 (1972). S. J. Halkes, J. Hartog, L. Morsink and A . M . deWarhter, 79 in ethanol gave the desired 17u-hydroxy-9P, 10%-pregna-* Dedicated to Professor E. Hauinga as a token of our appreciation for his widespread achievements. ' P. H. Wirrsema, A . L. Loeh and J . Th. G. Ozierheek, J . Colloid Interface Sci. 22, 78 (1966).