Benzopyrylium salts, 4-alkoxy

There are numerous examples of benzopyrylium salts, benzopyransand benzopyranones, and frequently they have trivial names that reflect their long history (see Box 5.1). Many are natural products, and frequently these compounds contain hydroxy or alkoxy groups (sometimes in the form of a sugar residue). Polyhydroxylated natural products based upon 2-phenylbenzopyrylium (flavylium) salts and with ether linkages to sugars are called anthocyanins, whereas without their sugars they are known as anthocyanidins. 

In preparing 6-alkoxy and 6,7-dialkoxy derivatives, it is convenient to react esters 12 with an acid anhydride in the presence of a Lewis acid or, preferably, perchloric acid to produce a benzopyrylium salt (14), presumably via 13 (70KGS200) (Scheme 2). Reaction of 14 with ammonia gives then the isoquinolinols 15 in high yields, where X = alkoxy, R1 = H, Me, Pr, /-Pr, and R2 = H or alkyl group (88JMC1363). 

Various alkoxy-substituted phenylacetones 34 are used to synthesize a series of 2-benzopyrylium salts 35 by acid-catalyzed acylation [66DOK( 166)359 66ZOR1492 70KGS(2)196,70KGS(2)213 70ZOR1118]. Interestingly, in all cases the alternative pathway of acylation of 34 (R1 = H and R2, R3 are not equal to H) was not observed, and isomeric 2-benzopyrylium salts 36 were not obtained. 

Contrary to the oxonium intermediate 115 (Section III,C,2), the intermediate 211 having the alkoxy leaving group readily opens its heterocyclic ring under these conditions. Diketones 205 (R3 is not H) thus formed are then cyclized into a-naphthols 204 (or /3-naphthols of type 209). If anhydro-bases 212 were formed, they would have to take part in the analogous conversions described previously. Obviously, the same mechanism may be applied to the scheme of formation of a-naphthols 204 from 2-benzopyrylium salts under the action of dimethylamine hydrochloride in ethanol (77KGS996) (cf. Section III,C,4,a,iv). 

As described in Section III, practically all known conversions of 2-benzopyrylium salts are the result of their reactions with nucleophiles, and 2-benzopyrylium, similar to its monocyclic analog [82AHC(Suppl)], behaves chemically more like a carbenium ion, and not like an oxonium salt. This fact can be easily understood if one takes into account resonance formulas a-f, where an oxonium form f is added for the presence of an alkoxy (hydroxy) substituent in position 6. 

Cyclocondensation of o-hydroxyacetophenones, orthoformic esters and arylaldehydes in the presence of a strong acid, usually HCIO4, leads to 4-alkoxy-2-aryl-l-benzopyrylium salts 4 [31] ... 

The first pyrylium salt without hydroxy or alkoxy substituents made by the protonation or alkylation of 4-pyrones was obtained in 1911 by Baeyer and Piccard fi"om 4-pyrones and Grignard reagents. Isolation of these compounds was often difficult, and their characterization was confiised by a lack of a clear understanding of their electronic structure. However, there was much interest in benzopyrenes (coumarins, flavones, and chromones) and benzopyrylium (flavylium, chromylium) salts isolated fi om plants (anthocyanins, anthocyanidines) to which they conferred a large diversity of colors. 

---