140 



VITAMIN D GROUP 



cholesterol is not ergosterol. He compared irradiated cholesterol with ir- 

 radiated ergosterol on chicks and rats in a series of experiments designed 

 to be free from alternative interpretations. His findings proved beyond 

 question that the vitamin D from the provitamin of cholesterol is different 

 from the vitamin D from ergosterol, and therefore the provitamin of cho- 

 lesterol is not ergosterol. 



CsHi; 



CgHjT 



oxic'ation 



AcO' 



Cholesteryl acetate 



CsHi; 



AcO' ^ ^ -0 



7-Ketocholesteryl acetate 



benzoylation 



HO' ^ -- OH 



7-Hydroxycholesterol 



CsHi? 



reduction 



pyiolysis 



BzO' ^^ '^ ^0B2 



7-Hydroxycholesteryl dibenzoate 



CsHi; 



BzO' -^ ^^^ HO 



7-DeIiydrocholesteryl benzoate 7-Dehydrocholesterol 



Fig. 5. Synthesis of 7-dehydrocholesterol (method of Windaus et al.^^). 



2. Syntheses 



A few months before the appearance of Waddell's paper, Callow^^ sug- 

 gested, on theoretical grounds, that there might exist in nature a provi- 

 tamin D which "is a cholesterol derivative with the double bonds in the 

 critical position, a demethyl-dihydro-ergosterol." This substance, now 

 usually known as 7-dehydrocholesterol, was synthesized by Windaus et al.^^ 

 in the following manner (Fig. 5). Cholesteryl acetate was oxidized by chro- 

 mic acid to 7-ketocholesteryl acetate. This was reduced by aluminum 

 isopropylate to 7-hydroxycholesterol, the dibenzoate of which gave upon 

 strong heating in vacuo the benzoate of 7-dehydrocholesterol. The latter 

 was saponified to the free provitamin. 7-Dehydrocholesterol is highly acti- 



" R. K. Callow, Sci. J. Roy. Coll. Set. 4, 41 (1934). 



38 A. Windaus, H. Lettr^, and F. Schenck, Ann. 520, 98 (1935). 



