Kufer, Werner and Scheer, Hugo
STUDIES ON PLANT BILE PIGMENTS. 14: THE DIAZO REACTION OF BILIRUBIN: STRUCTURE OF THE YELLOW PRODUCTS.
In: Tetrahedron, Vol. 39: pp. 1887-1892
The reaction of bilirubins with aromatic diazonium salts in alcoholic solvents leads to an
equimolar mixture of two types of products. One is the well-known 9-azopyrromethenone. The other is a
yellow product (A,,, z 420nm) identified as 9-alkoxymethylpyrromethenone. the alkoxy-substituent
being derived from the solvent. Thus, reaction of the symmetrically substituted bilirubins llla (lc)
and XIIIJ (lb) in methanol with diazotized sulfanilic acid yields one mole of the azopigments (4b and
4a). respectively, and one mole of the corresponding 9-methoxymethylpyrromethcnones (2b and 2a).
Bilirubin IXa (la) consequently yields a mixture of all four products. The two resulting 9-methoxymethylpyrromethenoncs
were separated by chromatography and identified as 2a and 2b. They can react further
with the dia7onium salts lo give the corresponding 9-aznpyrromethenones, but the reaction is much slower
than that of bilirubin. which explains the observed product distribution. These results are discussed in
relation to earlier work.
ALMOST a hundred years ago