STRUf'TTTRE OF PROVITAMINS D 



755 



7-Dcliydrocholesterol 



(S) 22-Dihydrocrgosterol {Provitamin D4) 



The structure of 22-dihydroergosterol is readily established by its syn- 

 thesis from ergosterol on the absorption of one molecule of hydrogen. It 

 is evident that the reduction has occurred in the side chain, inasmuch as it 

 is no longer possible to obtain methylisopropylacetaldehyde on ozonolysis. 

 The double bonds in ring B are obviously in the same position as in ergos- 

 terol. 



22-Dihydroergosterol 



' (4) Other Provitamins D 



Provitamin D5 is 7-dehydrositosterol. Its structure can readily be deter- 

 mined by analogy to the other provitamins. It differs from provitamins 

 D2, D3, and D4 only in the side chain. The formulas of epi-ergosterol and 

 epi-7-dehydrocholesterol are readily established, since they are entirely 

 similar to their corresponding trans derivatives except for the cis position of 

 the hydroxy 1 group in position 3. The compositions of "mussel" and of 

 "periwinkle" provitamins D are not known. 



7-Dehydrositosterol 



