638 



X. VITAMINS D 



CH3 



H2 I 



C CH3 CH 



/ \l H/ \ 

 H2C C C CH 



H2 H3 I I I \\ 



[.. c 



C C CH CH CH 

 C C C HC 

 C 



L "' 



H 

 CHo 



CH 



H2C 



H/" \ / % / / \ 



C C H3C CH; 



H2 H 



Ergosterol (Provitamin Do) 



H2 



C CH 



HO— I 



T-Dehj^drocholesterol (Provitamin D.3) 



CHs 

 CH 



H/ \ 

 -C CH 



H2C 



H2 H2 I I I II 



C C CH2 CH CH2 CH 



H2C 



HO. I 



)^ 

 H/ \ 



C 



II 

 CH 



C HC. 

 H2 I ^CHs 

 CH 



C 

 H 



Vitamin U2 



H3C CHa 



CH3 



H2 I 



C CHa CH 



/ \l H/ \ 

 H2C C C CH 



H2 H2 I III 



C C CH2 CH CH; 



H2C 



c 

 H2 



H2 



1 CH 



\ / 



c 



H 



Vitamin D3 



. CH, 



/ 

 CHo 



CH 



/ \ 

 H3C CH3 



III addition to the open B ring, all active members of the vitamin D 

 group have conjugated bonds at the 10, 18-; 5, 6-; and 7,8-positions. The 

 hydroxyl group on carbon 3 is another prerequisite; vitamins D are active 

 only when this hydroxyl group is free, or when the ester is one which can be 

 readily hydrolyzed in the animal body. Windaus and Rygh^^ reported 

 that non-hydrolyzable esters of vitamin D are relatively inactive biologi- 

 cally. When the position of the hydroxyl group on carbon 3 is altered by 



