Acridizinium ions synthesis

The discovery that benzyne (Scheme 30) adds to acridizinium ion to produce an azoniatrip-tycene (34) which, on reduction with sodium borohydride followed by hydrolysis, affords anthracene, offers a new route to the synthesis of related hydrocarbons (71JOC3002). This, the Fields anthracene synthesis, has already found application in the preparation of 1,4,5,8,9-pentamethylanthracene (75TL4639). 

The first synthesis of the benzo[6]quinolizinium ion (Scheme 98, Table 9, example 1) was by hydrobromic acid-catalyzed cyclization of the quaternary salt formed between 2-pyridinecarbaldehyde and benzyl bromide. Aromatic cyclodehydration has continued to the present as almost the only method used for the preparation of the acridizinium ion, its derivatives and benzo analogs. Because of its instability, 2-pyridinecarboxaldehyde has been replaced by more efficient derivatives. The first of these was the oxime (example 2) which not only gave a better overall yield, but also made possible the isolation of a crystalline intermediate (181 Z = NOH). The disadvantages are that it is not suitable for high temperature cyclizations involving polyphosphoric acid, and some products (182) (e.g. example 10, Table 10) may tend to form double salts with hydroxylamine hydrobromide. 

As in the earlier review1 this account will be restricted to the quinolizinium ion (1), the benzo[a]- (2), benzo [ft]- (3), and benzo[c]quinolizinium (4) ions, and to their derivatives. The quinolizones and compounds of similar structure (5) will be included, but the quinolizines (6 and 7) only where they appear as intermediates in synthesis or reaction. A number of trivial and unsystematic names have been used (pyridocolinium ion, dehydroquinolizinium ion for compound 1 acridizinium ion, 4u-azoniaanthracene for compound 3 phenanthridizinium ion for compounds 2 or 4), and numbering has also varied. Throughout this chapter the numbering will be as shown in formulas 1 to 4. 

An alcohol which is easily aromatized to a quinolizinium ion (albeit a benzo derivative) is (145 Scheme 87), an intermediate in an acridizinium synthesis (80JOC4248). In the preparation of 4-methylacridizinium bromide (146 R = Me) this route gave a better overall yield (34%) than had been obtained by other methods. 

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