Quinolizinium ions ring opening

Buta-1,3-dienes, l-(2-pyridyl)-, from quinolizinium ion ring-opening, 55,... 

With the benzo[Z>]quinolizinium (acridizinium) ion, ring opening has not been observed instead an unstable substance believed to be a piperidinylbenzoquinolizine was obtained (Scheme 14). 

However, in the quinolizinium ion (160) the fused ring increases the stability and makes it more difficult for nucleophilic attack, because now the aromaticity of both rings is lost in the intermediate addition product (161). Conversely, once the addition product is formed, ring opening is particularly easy (162). 

Quinolizinium ions (259) react with piperidine to give the ring-opened products (260). Subsequent closure of the ring can also occur, e.g. 2-(2,4-dinitrophenyl)isoquinolinium ion with PhNH2 at 190°C forms (258). 

Quinolizinium ions 265 react with piperidine to give the ring-opened products 266. 

Practically all the reactions of quinolizinium ions are similar to those of pyridinium salts, thus they are resistant to electrophilic attack, but readily undergo nucleophilic addition, the initial adducts undergoing spontaneous electrocyclic ring opening to afford, finally, 2-substituted pyridines however the susceptibility of the cations to nucleophiles is not extreme - Uke simpler pyridinium salts they are stable to boiling water. 

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