INTRODUCTION 637 



tively and quantitatively as regards their action. Four of the vitamins 

 were prepared in pure form; two of these were derived from natural sources. 

 It is believed that the other vitamins D actually do exist, since a number of 

 different provitamins D can be activated. The vi^tamins D themselves 

 have not been demonstrated in pure form. The inability to prepare pure 

 minor members of the vitamin D group is probably to be ascribed to their 

 presence in extremely small amounts, and especially to the fact that they 

 frequently occur in mixtures with other D vitamins which are present in a 

 much larger proportion. Ten different provitamins D are known. Thus 

 it must be surmised that there are ten different D vitamins corresponding in 

 structure to each of the ten precursors. In addition to these compounds, 

 the possibility exists that purely synthetic compounds may be produced 

 which have the structural features required for an active vitamin D. 

 Thus, Raoul and collaborators'-^ reported that bicholestadiene (2,4,2 ',4'- 

 bicholestadiene), on irradiation, becomes highly active as an antirachitic 

 agent. 



Vitamin D2 (activated ergosterol) and vitamin D3 (activated 7-dehydro- 

 cholesterol) are the two principal D vitamins on the basis of the extent of 

 their distribution, and of the fact that the pure compounds are known to 

 mediate the physiologic changes usually ascribed to the D vitamins. Vita- 

 mins D2 and D3 possess similar biologic activities in the case of the rat. 

 Vitamin D2 has only a feeble antirachitic potency in the case of chicks, 

 but vitamin Ds possesses a normal biopotency in this species. Vitamin D4 

 is the terminology assigned to the activated 22-dihydroergosterol,'^° while 

 vitamin D5 is believed to be activated 7-dehydrositosterol. The structure 

 of the common provitamins D and of the vitamins D is given below. Fur- 

 ther information as to the structure of related compounds is given in The 

 Lipids, Vol. I, pp. 749-755 and 782-791. 



a. General Structural Requirements for the Vitamins D. All of the D 

 vitamins have a similar structure insofar as the cyclic portion of the mole- 

 cule is concerned. The differences in structure of the several vitamins D 

 are confined to the side chain. In the activation of the provitamms D, 

 the main structural alteration involves the destruction of the steroid ring 

 B by its rupture between carbons 9 and 10. The methyl side chain on 

 carbon 10 becomes a methylene group with a double bond attaching it to 

 carbon 10; carbon 9 becomes — CH2 — by the addition of one hydrogen 

 atom. 



•2^ Y. Riioul, N. Le Boulch, J. Chopin, P. IMeunier, and A. Guerillot-Vinet, Compt. 

 rend., 234, 2493-2495 (1952). 



130 A. Windaus and G. Trautmann, Z. physiol. Chein., 2^7, 185-188 (1937). 



