770 VIII. PROVITAMINS D AND VITAMINS D 



the suprasterols. The latter substances would then be considered to be 

 the terminal compounds in the sequence of irradiation products formed 

 from the provitamins. 



Suprasterol2 I, C28H43OH, melts at 104°C., has a specific rotation in 

 chloroform at 18° (D line) of —76°, and shows absorption only beloAv 250 

 mju. Suprasteroh II, also C28H4;iOII, melts at 110°C., is dextro-rotatory 

 ([o;]^ = -|-62.9° in chloroform) and has spectral absorption only below 250 

 m^.*^ Both products have an hydroxyl group and form esters. The sepa- 

 ration of the two isomers has been rendered possible by the difference in 

 solubility of their allophanic acid esters. 



There appear to be 3 double bonds in both suprasterols2 I and II, as in- 

 dicated by the results of catalytic hj^drogenation and by titration with 

 perbenzoic acid.^^*^ Since only 3 double bonds exist, there must be a closure 

 of ring B, which is open in tachysterol and vitamin D. Because no ab- 

 sorption occurs in the spectral region above 220 irifx, it must be concluded 

 that the double bonds are not conjugated. However, since the typical 

 hydrocarbon formed on dehydrogenation of sterol compounds with selen- 

 ium, namely 7-methylcyclopentenophenanthrene, is not formed from the 

 suprasterol, it appears certain that the suprasterols do not have the steroid 

 nucleus. Miiller^^^ has suggested the following structure for the supra- 

 sterols: 



Suprasterolsa and 4 have been obtained by the over-irradiation of 7-dehy- 

 drocholesterol" and 22-dihydroergosterol, ^^^ respectively. The supraster- 

 ols are only slightly toxic and are entirely ineffective as antirachitic agents.*^ 



D. VITAMINS D 



A number of different compounds have been shown to possess antirachitic 

 potency. Only two of these have been prepared in a pure state from na- 

 tural sources. Several of the others have been synthesized by irradiation 

 of provitamins, and it has been possible to separate them in pure form from 

 such relatively highly concentrated preparations. It is not known how 

 many of the latter group of compounds so prepared are present in nature. 

 This inability to obtain other members of the vitamin D group from na- 

 tural sources is due to the difficulties encountered in isolation and purifica- 

 tion. This is especially the case since some of the more uncommon mem- 



