144 VITAMIN D GROUP 



By replacing the hydroxy 1 group with a thiol group, Strating and Backer ^^ 

 and Bernstein and Sax^^ have recently prepared 7-dehydrothiocholesterol 

 (7-dehydrocholesteryl mercaptan or cholestadiene-5,7-thiol-3). This com- 

 pound exhibits an absorption spectrum similar to that of the provitamins 

 D and undergoes similar spectral changes upon irradiation, but the thiol 

 isostere of vitamin D3 which apparently resulted was not antiricketic for 

 chicks, even at generous doses. 



Strating and Backer^* have also prepared the compound, "7-dehydro-3- 

 homocholesterol," in which the hydroxyl group is replaced by the HOCH2 

 group, i.e., 3-hydroxymethyl-cholestadiene-5,7. This exhibits the typical 

 provitamin D absorption spectrum, but no studies on its activatabiiity have 

 been reported. 



The C-3 position is a center of asymmetry. Sterols which differ from nor- 

 mal only in the steric arrangement of substitutions on C-3 are defined as 

 episterols.^^ They are not precipitable by digitonin. The epi configuration 

 is exhibited by epiergosterol and 7-dehydroepicholesterol. Both are highly 

 activatable, 7-dehydroepicholesterol acquiring about one-tenth as much 

 activity as is acquired by 7-dehydrocholesterol under similar exposure to 

 ultraviolet light .^^^ *» 



b. Orientations at C-9 and C-10 



Isopyrocalciferol is the C-9 epimer of ergosterol, and except for this differ- 

 ence the two are identical in structure. The fact that isopyrocalciferol does 

 not become antiricketic upon irradiation is evidence that the normal con- 

 figuration at C-9 is essential for activation. Lumisterol differs from ergos- 

 terol only in the epimerization of its substituent methyl group at C-10. 

 The fact that it is fully activatable is evidence that the configuration at 

 C-10 is not a determinant of activatabiiity. Pyrocalciferol differs from ergos- 

 terol in the spatial arrangements at both C-9 and C-10, and it is not activa- 

 table. 



c. The Side Chain 



The sex hormone, 3 , 17-dihydroxyandrostadiene, differs structurally from 

 ergosterol and 7-dehydrocholesterol only in the absence of a side chain, 

 the substitution at C-17 being an hydroxyl group. This compound, when 

 irradiated, undergoes the spectral changes indicative of vitamin D forma- 

 tion (Fig. 7), but the product shows no antiricketic activity .^^ 



The cholanic acid analog of ergosterol and 7-dehydrocholesterol, known 

 as 3-hydroxy-A^ '-choladienic acid, differs from the activatable provita- 



«2 J. Strating and H. J. Backer, Rec. irav. chim. 69, 909 (1950). 

 63 S. Bernstein and K. J. Sax, /. Org. Chem. 16, 685 (1951). 

 " J. Strating and H. J. Backer, Rec. trav. chim. 70, 389 (1951). 



