Heterocyclic compounds, aromatic indolizines

The ultraviolet spectrum of the parent heterocycle in water shows three main peaks, at 221.5, 282, and 302 nm. The similarity of this spectrum to that of indolizine is apparently due to the fortuitous cancellation of the effects of the introduction of successive nitrogen atoms into indolizine. UV data are available for several quaternary derivatives of this heterocycle and their V-oxides." 2,3-Dihydroimidazo[l,2-a]pyr izine (5) has only two peaks, at 258 and 394 nm, in its ultraviolet spectrum in water. Both this compound and the unsubstituted aromatic heterocycle undergo large hypsochromic shifts on protonation. This contrasts with the bathochromic shift observed on protonation of aminopyrazines and is consistent with the hypsochromic effect obtained on protonation of the analogous imino compounds. These observations confirm the conclusion that protonation occurs at the 1-position in this heterocycle to give the cation 6. 

The aromaticity of indolizine was discussed with the aid of configuration analysis of matrices, the isoelectronic (9)annulenyl anion serving as aromatic reference compound <81CCA427>. A new aromaticity index has been applied to indolizine and other bicyclic heterocycles <87T4725>. 

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