GMELIN REACTION 111 



conjugation of double bonds is thus interrupted, as it is by reduc- 

 tion which replaces^ C — CH=C< by ^ C — CH2 — C<^. For the 

 absorption spectra the presence of a carbonyl group or a hydroxy- 

 methylene group between pyrrole rings instead of a methylene group 

 makes little difference. The reaction takes place first on one of the 

 methene groups (a or c) and later on both. The second step of the 

 Gmelin reaction thus leads from bilatrienes (verdins) to biladienones 

 or biladienediols, called bilipurpurins by Siedel or biliviolins type II 

 by Lemberg, which closely resemble the biladienes-(a,b) (biliviolins). 

 The third step, involving the second of these two methene groups, 

 leads to bilenediones or bilenetetrols (choletelins), which closely 

 resemble bilenes-(b) ("urobilins"). In spite of their name choletelins 

 are not the end products of the oxidation which probably now 

 involves the methene group b and finally leads to colorless breakdown 

 products (substituted maleimides). The main types of the oxida- 

 tion products up to the choletelin stage are summarized in Table III. 



In the nomenclature of the Fischer school the compounds which 

 contain carbonyl groups between pyrroles are described by using the 

 prefix "0x0" in front of the name of the corresponding compound with 

 methylene. We prefer to follow the accepted nomenclature of 

 ketones. 



Some other oxidation reactions follow a similar pattern. Oxidation 

 with bromine in the presence of methanol follows mainly reaction d 

 in Figure 10 (Siedel and Grams, 2556). The same probably holds 

 for the oxidation of biliverdin zinc complexes with iodine or bivalent 

 copper salts studied by Lemberg {1679 ,1680,17 11) . Two atoms of 

 iodine are required for the conversion of the biliverdin zinc complex 

 into the zinc complex of a bilipurpurin. 



Although the principle of the Gmelin reaction and these related 

 reactions can be considered as well established, many of the details 

 remain to be worked out, and the formulas given in Table III can 

 only be considered as provisional {cf. Sections 5.3., 5.4.). Generally 

 the reaction leads to complicated mixtures, which have to be separated 

 by chromatography. The variety of reactions depicted in Figure 10 

 is not the only complication. With the nonsymmetrically substituted 

 bile pigments the oxidation of the bilatriene stage can begin either 

 at the methene a or c, leading to a mixture of pigments. Most of 

 the oxidation products obtained in a crystalline state have so far 

 been prepared from synthetic, symmetrically substituted bile pig- 



