BILANES, BILENES, AND RELATED SUBSTANCES 



141 



had given too high oxygen values. Later analyses of Ileilmeyer and 

 Krebs {1218), Watson {2981), and Fischer and co-workers {823), 

 established that stercobilin contained only six oxygen atoms, the 

 same number as niesobilene-(b), but four hydrogen atoms more than 

 the latter. Its dehydrogenation with concentrated sulfuric acid leads 

 to mesobilatriene. 



MEM 



M M E 



A A^A A^A A,, A A 



HO N C N C fj C N ^OH 

 H H. H H H, H 



Fig. 2-t. Tetrahy(lroinesobilene-(b), according to Fischer. 



Fischer and co-workers discovered that, in contradistinction to 

 mesobilene-(b), stercobilin was optically active. They suggested the 

 formula shown in Figure 24. Siedel has recently suggested a different 



\^/\. 



H.Hh 



,H 



OH 



Fig. 25. Siedel's modification of Fischer's formula. 



position of the double bond in the pyrrole rings I and IV (Fig. 25), 

 but this formula would appear to demand a more strongly basic 

 character for tetrahydromesobilane and -bilene than they possess. 

 The four asymmetric carbon atoms in these two pyrrolic rings 

 explain the optical activity. 



The presence of the extra four hydrogen atoms does not affect the proton 

 dissociation from the tertiary nitrogen; the difference in basicity, if any, 

 between mesobilene-(b) and tetrahydromesobilene-(b) is slight. The hydro- 

 philic character of these compounds may be used preparatively. They pass 

 from the ether-acetic extract of feces or urine into the aqueous phase when 

 the extract is washed with dilute sodium acetate. In this way a mesobili- 

 violLn which is often found accompanying them (o/. Section o.2.3.) may be 

 removed {2972,297 Jf). 



6.3.2. Properties. Tetrahydromesobilane has not yet been ob- 

 tained crystalline. In conden.ses with p-dimethylaminobenzaldehyde 

 in hydrochloric acid to a red pigment which has an absorption band of 

 the same position and strength as that of the pigment obtained from 

 mesobilane, cf. above {12 18, 298 Jf). This is fortunate, since all earlier 



