136 



VITAMIN D GROUP 



given in Table I, taken from Bergmann.^'^ ^he bivalves may exhibit some 

 cholesterol, but chiefly their sterols are difficultly separable mixtures of 

 C28 and C29 sterols, with di-unsaturated C-24 epimers of C28 sterols pre- 

 dominating.^^' "^ The snails, however, show chiefly cholesterol, like the 

 higher animals, but with small amounts of phytosterol-like sterols. 



The sterols of plants, especially the higher plants, or phanerogams, are 

 collectively known as phytosterols. By far the commonest phytosterol is 

 sitosterol, a sterol which exists in three isomeric forms designated a, 13, and 

 7, which are so difficultly separable that they are usually considered as one. 



Fig. 



Cholestanol R = — CH(CH3)-CH2-CH2-CH2-CH(CH3)2 

 (3-Sitostanol R = — CH(CH3)-CH2-CH2-CH(C2H5) •CH(CH3)2 

 3. Examples of saturated-ring sterols. 



Ergosterol R = — CHCCHg) •CH=CH.CH(CH3)-CH(CH3)2 



7-Dehydrocholesterol R = — CHCCHs) ■CH2-CH2-CH2-CH(CH3)2 

 Fig. 4. Ergosterol and 7-dehydrochoIesterol, commonest examples of provita- 

 mins D. 



Sitosterol is usually accompanied by traces of dihydrositosterol (sitostanol) 

 and a provitamin which might be expected to be 7-dehydrositosterol but 

 which is probably ergosterol. Other sterols occurring with sitosterol in the 

 higher plants are campesterol, brassicasterol, stigmasterol, and spinasterol. 

 In the lower plants, or cryptogams, sitosterol appears in the green algae, 

 but in the brown algae it is replaced by fucosterol. In the fungi the principal 

 sterol is ergosterol, accompanied by small amounts of 5-dihydroergosterol 

 and traces of several other sterols not of interest in connection with vita- 

 min D. 



From the structural formulas it can be seen that the sterols which pre- 

 dominate in the higher forms of life differ only in their side chains. Their 



^^ W. Bergmann, /. Marine Research (Sears Foundation) 8, 137 (1949). 

 23 W. Bergmann and E. M. Low, /. Org. Chem. 12, 67 (1947). 



