NATURAL WAXES 337 



l)ctween carbons 5 and 6. However, on reduction of cholesterol, as,\Tn- 

 nietry develops at Cb, and two important classes of naturally occurring 

 steroids can be prepared, depending upon the configuration around this 

 carbon. A third point of asymmetry not occurring in cholesterol, but of 

 considerable importance in the phytosterols, is on C24 (see page 351). 



The In'droxyl group on Ca assumes a position in dihydrocholcsterol, 

 stigmasterol, crgoslerol, sitosterol, and fucostcrol similar to that in choles- 

 terol. Onlj^ the compounds which possess this configuration for the 

 hydroxyl group (the so-called B-type) are precipitable bj'^ digitonin. When 

 the positions of the hydrogen and hydroxyl groups on C3 are reversed, one 

 obtains a compound which is no longer precipitable by digitonin. Such a 

 configiu'ation occurs in epi-compounds. According to Fieser"^ no exceptions 

 to this rule of precipitability with digitonin exist, and Fernholz-"" has demon- 

 strated that the same relationship exists for cholesterol derivatives where 

 the side chain on Cn has been shortened or even replaced by a ketonic 

 group, as occurs in the sex hormones. The precipitability of the sterols 

 with digitonin is not affected by the alio or normal configuration; therefore, 

 the linkage between the A and B rings is apparently not concerned with the 

 reaction. 



The two classes of stereoisomers in which a difference in configuration 

 obtains on the C5 atom are called the normal and the alio series. Copro- 

 sterol, a natural saturated sterol formed from cholesterol, which is present 

 in feces, belongs to the normal series; dihydrocholcsterol, which is prepared 

 by reduction of cholesterol, is a member of the alio type of sterols. The 

 differences are not concerned with the arrangement of the atoms and 

 groups around Cn, since the differences are still maintained in the com- 

 pounds formed when coprosterol and dihj^drocholesterol are reduced to 

 their corresponding hydrocarbons, namely, coprostane and cholestane. 

 Windaus-"^ has suggested that the hydrocarbons vary in the nature of the 

 union between rings A and B. In the case of coprostane (also referred to 

 as pseudocholestane) the two rings correspond to a cis-decalin or to two 

 "bed" forms. In cholestane, the rings A and B are pictured as two "chair" 

 rings which occur in frans-decalin. The results of Ruzicka et al."^^^ have 

 confirmed this "\'iew by demonstrating that differences in densities and 

 molecular refractions exist l^etween the two hydrocarbons. In the case of 

 this series of stereoisomers, it is also necessary to develop the concept of the 

 structural formula by fixing the position of the hydrogen on C5. The 

 methjd group attached to Cio is used as a reference point. The compounds 

 in which the hydrogen and methyl group are in cis relationship are con- 



^ E. Fernholz, Z. physiol. Chetn., 232, 97-100 (1935). 



»' A. Windau.s, Ann., U7, 233-258 (1926). 



2«2 L. Ruzicka, M. Furter, and G. Thomarin, Helv. Chim. Acta, 16, 327-336 (1933). 



