Benzopyrylium salts synthesis

Benzopyrylium perchlorate, 3-ethyI-reduction, 3, 662 Benzopyrylium salts C NMR, 3, 590 chromene synthesis from, 3, 756 chromone synthesis from, 3, 829 electrophilic substitution, 2, 49 mass spectra, 3, 618 reactions... 

Another type of [5 +1] reaction leading to the synthesis of an (in effect) quinolizin-4-one (198, Scheme 105) involves the concurrent replacement of the oxygen atom of a 2-benzopyrylium salt (197) by nitrogen (from ammonia), and cyclization (77CHE1183). 

The related synthesis of benzopyrylium salts involves the condensation of salicylaldehydes with methyl ketones in a basic medium, which affords the unsaturated ketone (642). Cyclization occurs on treatment with acid. The same benzopyrylium salt is formed rapidly and in high yield if the condensation is effected under acidic conditions (Scheme 252) (73BSF3421). It has been proposed that the latter synthesis proceeds via an aldol condensation accompanied by the formation of a hemiacetal, followed by dehydration. 

The properties of 1,3-oxazinium salts are similar to those of pyrylium salts.4194,219,255-259 1,3-Oxazinium salts are intermediates for the synthesis of derivatives of pyrazole, isoxazole, pyridine, pyrimidine, quinoline, isoquinoline, chromene, benzopyrylium salts, butadiene, etc.4... 

Application of Acid-Catalyzed Acylation to the Synthesis of 2-Benzopyrylium Salts... 

The need for different catalysts in the synthesis of 2-benzopyrylium salts (39) from compound 38 is determined by the introduced substituent R3, which is introduced at the first position of the cation. Thus, formylation of 38 (R1 = Me,Ar R2 = H) with dichloromethyl butyl ether in the presence of AICI3 [according to Rieche (60CB88)], followed by treatment with perchloric acid leads to 1-unsubstituted salts 39 (R1 = Me, Ar R2 = R3 = H) (70KGS278 70KGS1013). 

This polycyclic 2-benzopyrylium salt was also obtained in good yield from the 2-benzopyrylium salt 56, through intramolecular acylation of the intermediate isochromene derivative 60. The same approach was used later by Elliott and co-workers (84CJC2435) in the synthesis of some benzofc]-phenanthridine analogs. 

The acid-catalyzed acylation was successfully applied to the synthesis of two types of polycyclic 2-benzopyrylium salts 67 and 69. The initial compounds in these cases were orf/i0-(3,4-dimethoxyphenyl)-substituted a-naphthols (66) or /3-naphthols (68), and the final salts were 5-oxoniachrysenes 67 (76KGS745 77KGS1176) and 6-oxoniabenz[a]anth-racenes 69 (89ZOR164), respectively. 

Benzopyrylium salts are more suitable than homophthalic anhydrides for the synthesis of dihydroisoquinolinium salts of type 237, by the method described previously, because of the easier availability and greater variety of the former compounds. 

The Cu-promoted enantioselective oxidative dearomatisation of aUcynylbenzaldehydes followed by a cycloisomerisation leads to azaphilones, fused 4//-pyrans (Scheme 1) <05JA9342>, while an alternative synthesis involves oxidation of a 1/7-benzopyrylium salt derived from a substituted benzaldehyde (Scheme 2) <05JOC4585>. Treatment of azaphilones with primary amines results in cleavage of the pyran ring and the formation of vinylogous y-pyridones. 

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