Acridizinium ring

Fields et al. showed that a variety of unsymmetrical alkenes added regiospecifically to the acridizinium nucleus and pointed out that the great majority of cases could be rationalized by the assumption that the more negatively polarized end of the alkene was preferentially attracted toward position 6, the previously demonstrated center for nucleophilic attack on the acridizinium ring. At the same time they reported that the addition of acrylonitrile to yield a 12- rather than a 13-cyano adduct is the reverse of what would be expected from the polarization of the acrylonitrile molecule. Possible explanations for this exception are offered in Section V. 

In the first cycloaddition reaction of the acridizinium ion, that with maleic anhydride, it had been observed that addition had occurred with great stereoselectivity, although it iVas not ascertained whether the product (4) was syn or anti with respect to the benzenoid ring. It was later demonstrated by use of NMR and IR evidence (derived from the... 

When ring A was replaced by an imidazo ring, it was found ° that the new system (39) was less reactive than acridizinium (probably due to... 

The most recent extension of the 4 + 2 cycloaddition to aromatic quaternary salts has been carried out with isoquinolinium salts (42), in effect, dispensing with ring A of the acridizinium ion. Although there was an earlier claim that the addition of an ynamine to 2-methyl-isoquinolinium iodide led to a 2 1 adduct, the assigned structure... 

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). 

Although no 4-aminoquinolizinium derivatives are known, a benzo derivative, 6-amino-acridizinium ion (72 Scheme 50), has been prepared (67JOC733). In the presence of hydroxide ion the amino group loses a proton to form a benzo[6 ]quinolizin-6-imine (73). Alkaline hydrolysis opens the ring to give the amide (74). 

The quaternization of 2-pyridinecarbaldehyde with 2,3,6-trimethoxy-9-phenanthryl-methyl bromide (183) yielded a salt (184) which, when cyclized, afforded a dibenz[/i,/]acridizinium derivative (185). Reduction of the quinolizinium ring of (185) afforded ( )-cryptopleurine (57JA3287). 

Compounds with bridgehead nitrogen atoms between two six-membered rings are necessarily cationic and are therefore very unreac-tive. The positions in these molecules that are best conjugated with the positive nitrogen [e.g., the 4- and 8-positions in acridizinium (11.11)] will be the most strongly deactivated. 

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