II. CHEMISTRY 



145 



mins only in having a short side chain terminating in a carboxyl group. It 

 is nevertheless not activatable.®^ 



The sterol, A^'^-norcholestadiene-3-ol, differs from 7-dehydrocholesterol 

 in that its side chain is unbranched and has one fewer CH2 group. Although 

 it is described as "a new provitamin D," no study of its antiricketic acti- 

 vatability has appeared. 



The natural compound, A^'^-^-cholestatriene-S-ol, or 22-dehydro-7-de- 

 hydrocholesterol, differs from 7-dehydrocholesterol only in having a double 



Original 



Fig. 7. Spectral changes during the irradiation, under identical conditions, of 

 ergosterol (upper curves) and 3,17-dihydroxj'androstadiene (lower curves). (After 

 Dimroth and Paland.^'') 



bond at C-22. It has not been isolated in sufficient purity for direct studies 

 to be made on its activatability, but as a component of the provitamin D 

 mixture occurring in Myiilus edulis it probably contributes to the vitamin 

 D potency of the irradiated mixture.^'' One would expect it to be activatable, 

 because the double bond at C-22 is known (in ergosterol) not to prevent the 

 formation of an effective vitamin D. 



7-Dehydrocholesterol and 7-dehydroepicholesterol have identical side 

 chains, the shortest and simplest found in any provitamin D of unques- 

 tioned activatability. A double bond at C-22, as in ergosterol, is favorable 

 to activation, but not essential for it, because when it is saturated, as in 

 22-dihydroergosterol, activation can still occur, although the vitamin D 



«* G. A. D. Haslewood, Biochem. J. 33, 454 (1939). 



