758 VIII. ruoviTAMiNS d and vitamins u 



in place of cholesterol as the starting material.''^ It may also be prepared 

 by epimerization of the hydroxyl group of 7-dehydrocholesterol.'^^ This is 

 accomplished by oxidation with aluminum icr/-butyroxide to 7-dehydro- 

 cholestenone which, on subsequent reduction with aluminum isopropoxide, 

 produces the epi-isomer''*^ in a yield of 1.25%. 



Epi-ergosterol has been obtained in a 1.3% yield by reduction of ergosten- 

 one with aluminum isopropoxide. ^^^ It has not been obtained in pure form. 



7-Dehydrostigmasterol can be synthesized from stigmasteroF*^ by the 

 Windaus method used for 7-dehydrocholesterol. However, it is not cer- 

 tain whether or not this sterol should be classed as a provitamin D, because 

 of the slight antirachitic activity of its irradiation product. No synthesis 

 is reported in the literature for 22,23-oxidoergosterol. Rosenberg" sug- 

 gests that it might be synthesized by oxidation of the maleic anhydride 

 addition product of ergosterol ester followed by thermal decomposition of 

 the addition product to the ester, and saponification of the latter compound 

 to the provitamin. 



C. CONVERSION OF THE PROVITAMINS D TO THE 

 VITAMINS D 



1. Introduction 



In order to effect the change of the physiologically inert provitamin D 

 to the active form, it is necessary to supply a definite quantity of energy. 

 This can be accomplished by the use of ultraviolet light present in sunlight 

 or obtained from artificial sources of radiant energy, or by cathode rays and 

 other similar types of radiation. 



Although the healing action of sunlight on rickets had been known for 

 many years, it was not realized until the work of Huldschinsky,^'''^! ^^d 

 especially that of Hess, Pappenheimer, and Weinstock,'^" that the shorter 

 ultraviolet waves of sunlight, or the still shorter waves of artificial light, ac- 

 count for this therapeutic action. The association of this curative effect 

 with a chemical reaction caused by the ultraviolet light became evident as 

 soon as it was demonstrated almost simultaneouly by the Steenbock"-^^ 

 and by the Hess'^^ groups that inert rations and oils can be activated as 

 antirachitic agents by irradiation. 



As soon as the observation was reported that antirachitic potency could 

 be conferred on foods by the process of irradiation, the nature of the chem- 

 ical reaction involved became the concern of many chemists. It was evi- 

 dent that the product which was acted upon was of a sterol-like nature, in 



'38 A Windaus and O. Kaufmann, Ann., .543, 218-224 (1939). 

 139 A. Windaus and K. Bucliholz, Ber., 71, 576-578 (1938); 7;^, 597-599 (1939). 

 "» A. F. Hess, A. M. Pappenheimer, and M. Weinstock, Proc. Soc. Exptl. Biol. Med, 

 20, 14-16 (-1922). 



