BILADIENES-(a,c). RUBINS 



119 



I.S.). It accounts well for the properties of the rubins, their similarity 

 to a-hydroxypyrromethenes, their very weakly basic character (1598), 

 and their dehydrogenation to bilatrienes. The weakly basic char- 

 acter is due to the lack of nonhydroxylated pyrrolene nuclei, the 

 a-hydroxylated pyrrolenes tautomerizing to the lactam form: 



OC 



^^ 



H 



H 



HO 



/X /\A Ar A ApA -A ^„ 



H H ^^ H H 



Instead of two vinyl side chains Fischer at first assumed one con- 

 densed furane ring in addition to one vinyl group, a structure not in 

 agreement with the fact that only two molecules of hydrogen were 

 taken up in the catalytic hydrogenation of bilirubin to mesobilirubin; 

 later he assumed a dihydrofurane ring condensed to a pyrrole ring: 



M 



A A . 



CHi 

 CH, 



which in his book (861, p. 625, 637) he claimed to have finally estab- 

 lished. The evidence was based on the so-called "nitrite body" an 

 ill-defined product of the oxidation of bilirubin by nitrous acid, and 

 on the observation that one of the two molecules of hydrogen added 

 in the catalytic hydrogenation of bilirubin to mesobilirubin was taken 

 up faster than the second. The evidence was discussed by Lemberg 

 {1681) and rejected. Fischer has now confirmed the formula with 

 two vinyl side chains by synthesis and by the fact that two moles of 

 diazoacetic ester can be added to the vinyl side chains of bilirubin as 

 to those of protoporphyrin {86I(.). The "nitrite body" was shown to 

 be methylvinylmaleimide, reducible to methylethylmaleimide, while 

 previously it had been claimed that the reduction yielded a cyclic 

 isomeride of this compound. 



