BILADIENES-(a,b) AND RELATED SUBSTANCES 



129 



such prototropic isomerism has been found. Moreover, not only viohns 

 and erythrins differ spectroscopically, but also their hydrochlorides and 

 complex salts. This kind of isomerism has been found neither in the bilene-(b) 



MEM 



P M M E 



H H. H 



+ 



H^ N C N OH 

 H H 



MEMP PMME 



TT^A ApA ApA A„A Anu 



HO N ^ N C ]vf C N ^OH 

 H H. H H ^ H ^ 



Fig. 16. Synthesis of mesobilierythrin (Siedel). 



nor in the bilatriene series (cf. 861, p. 701). In the present case, matters are 

 complicated by the presence of a resonating tripyrrolic system (rings II, 

 m, and IV), substituted nonsymmetrically on one side only by a pyrryl- 



MEMP PMME 



A A^A A^ A A„A -A^u 



HO N C X C X C N OH 



H 



H H H 



Fig. 17. Mesobiliviolin, differing from mesobilierythrin only in position 

 of double bonds. 



methyl group. In the bilene-(b) series only rings II and III, in the bilatriene 

 series all four rings, form the resonating system. While it is impossible to 

 state that in such a case isomerism could not occur, it must be considered 



M 

 H 



E Al A 1 Iv\m e^ 1^ A IV \M 



TTu - AA 00 AA 



MHHe MHHe 



Fig. 18. Siedel's violin and erythrin formulas as resonance forms. 



