ABSTRACT
The aromatic character of indolizine is expressed by three main mesomeric contributors, two of which incorporate a pyridinium moiety; other structures, incorporating neither a complete pyrrole nor a pyridinium, are less important. Indolizine is an electron-rich system and its reactions are mainly electrophilic substitutions, which occur about as readily as for indole, and go preferentially at C-3, but may also take place at C-l. Indolizine and its simple alkyl derivatives are sensitive to light and to aerial oxidation, which lead to destruction of the ring system. The quinolizinium ion occurs naturally only rarely, for example as a fused ylid in the alkaloid sempervirine; however, there are hundreds of indole alkaloids which have the same tetracyclic system, but with the quinolizine at an octahy-dro-level; in addition, many simpler quinolizidine alkaloids, such as lupinine, are known. In contrast, (3.3.3)cyclazine has no aromatic resonance stabilisation and is unstable and highly reactive, displaying some diradical character.
