Pyrylium ring benzopyrylium salts

Two benzoannelated analogues of the pyrylium cation are known ben-zo[b]pyrylium (chromylium or 1-benzopyrylium) and benzo[c]pyrylium (isochromylium or 2-benzopyrylium) salts. So far, the 1-benzopyrylium system is considered to be the more interesting (79MI2 84MI1) because it is the basic heterocyclic system of important plant pigments (antho-cyanins), and because there are major differences between properties of its heterocyclic ring and those of monocyclic pyrylium systems as a result of benzoannelation. Thus, for instance, 1-benzopyrylium salts do not possess the ability to have recyclization reactions with heteroatom exchange. 

The conversion under acidic conditions of 2-benzopyrylium salts into naphthols is not observed for isoquinolinium or monocyclic pyrylium salts, which become converted into phenols only in alkaline medium. Moreover, the ring opening of the heterocycle in 211, determined by its protonation, may be considered as the specific type of 1,4-addition of R2OH to the heterodiene fragment of the 2-benzopyrylium cation. This is a new version of the corresponding step in the ANRORC scheme. 

Evidently, deprotonation of adduct 112 precedes the ring opening. Unlike monocyclic pyrylium salts, only one pathway may be followed for the adducts of 2-benzopyrylium salts with the nucleophile possessing two carbon atoms, both of which might participate in the construction of the new skeleton. That the added component does not react as a 1,1-nucleophile may be explained by the difficult formation of intermediate 226, since it should be a fixed orf/zo-quinonoid form. 

Since compounds 10 and 30 are rather complicated, the authors could not attempt to determine the normal frequencies of the molecule, but had to restrict the analysis of spectra to the determination of characteristic frequencies. Thus, six frequencies have been found that can be used to identify 2-benzopyrylium cations. Band I appears between 1650-1610 cm-, and the pyrylium ring is responsible for this vibration [8a band according to Wilson s notation (34MI1)]. The position of this band and its intensity are dependent on the nature and position of substituents in the cation, and these changes are similar to data of monocyclic pyrylium salts [82AHC(Suppl)]. 

Similar to aroxyl-benzo[b]pyrylium systems (78ZOR1643), there is no detectable delocalization of the unpaired electron on the benzo[c]pyrylium ring in 301-303 (87RRC417). The stability of the radical cation is determined in this case by the position of the aroxyl substituent in the cation, and the most stable is 301. The formation of diradical species 303 together with the monoradical cation, was indicated on oxidation of the corresponding 2-benzopyrylium salt. 

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