Quinolizinium cation

There are two neutral benzopyridines, quinoline (benzo[6]pyridine) and isoquinotine (benzo[c]pyridine), together with the quinolizinium cation (benzo[a]pyridinium) (Box 3.1), formed by a ring fusion that utilizes the nitrogen atom. 

The remaining [3 4-3] reaction proceeds directly to form derivatives of the quinolizinium cation (75CHE467). The perchlorate (212 Scheme 112) of 2-pyridylacetonitrile was allowed to react with malonaldehyde (as its diacetal), or with a /3-diketone (213) to give 1-cyanoquinolizinium derivatives (214). 

Hydroxyquinolizines have been reported only recently. In this connection the salts of 4-quinolizone may be considered as 4-hydroxy-quinolizinium derivatives. Boekelheide and Lodge18 hydrolyzed and decarboxylated I,3-dicarbethoxy-4-quinolizone (6) to the free quino-lizone (7) and found that the compound had the characteristics of the quinolizinium cation 8 as indicated by the identity of the ultraviolet spectra determined in neutral and acidic media. 

A molecular orbital calculation of the ultraviolet spectrum of the quinolizinium cation has been made employing self-consistent field molecular orbitals of the parent hydrocarbon.34 Of the four band positions calculated on this basis, only two have been observed. The calculated and observed values are given in Table IV. 

Heteroaromatic compounds are isoconjugate with the corresponding aromatic hydrocarbons (the parent hydrocarbons). For example, the parent of quinolizinium cation (1) is naphthalene. The difference in the electronic energies between the heteroaromatic compound and the parent hydrocarbon can be expressed by first order perturbation theory as follows ... 

Many benzo[b]quinolizinium cations have been synthesized and found to inhibit the NMDA receptor (36, 37). These compounds tend to have high affinity for the open NMDA channel and are uncompetitive inhibitors (Table 11.4). They seem to be selective for the phencyclidine site of the NMDA receptor. 

Write down the structures of the intermediates in the following synthesis of the quinolizinium cation 2-methylpyridine was reacted with LDA, then Et0(CH2)2CH=0 to give C11H17NO2, which was heated with HI ( C9Hi2NO" 1) this salt was then heated with AC2O (-H CgHjoN" I ) and this finally heated with Pd-C to afford quinolizinium iodide. 

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