180 



VITAMIN D GROUP 



Fig. 10. Time-potency curves of the activation of ergosterol in alcohol, cyclo- 

 hexane, and ether. An example of the specific solvent effect. (After Bills et al.^^^) 



However, the provitamin, even under the most favorable conditions, never 

 gives a 100 % yield of vitamin. The transformation proceeds in overlapping 

 steps, with the formation of a series of products of which vitamin D is not 

 the last. The series has received special study by Windaus ei al}^ and 

 Setz,^-^ whose conclusions, modified in the light of other work, can be 

 expressed in the following scheme. Here is illustrated the transformation 

 of ergosterol into calciferol (vitamin Do), but with other provitamins the 

 changes which take place are analogous. 



Ergosterol 



i 

 Lumisterol 



i 

 Protachysterol 



i 

 Tachysterol 



Precalciferol 



it 

 Calciferol 



Toxisterol — > Suprasterols 

 (Substance 248) I and II 



