Benzopyrylium salts chromones

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... 

Whereas benzopyrylium salts are resistant to electrophilic substitution, even in the benzene moiety, the benzopyrones [coumarins (81), isocoumar-ins (82), and chromones (83)] are readily halogenated in either ring, with... 

Method B involves the preparation of precursor of 2-alkyl-l-benzo-pyrylium salts, as shown in Scheme ll.50 2-Alkylbenzopyrylium salts have been prepared by condensation of salicylaldehyde with appropriate ketone in acetic acid or by alkylation or reduction of coumarin or chromone derivatives. Reaction of 2-alkylbenzopyrylium salts with salicylaldehyde gives directly a spirodibenzopyran or 2-vinynologue benzopyrylium salt 17 which then can be converted into the spirodibenzopyran by piperidine or pyridine. 

Benzopyrylium Salts, Coumarins, Chromones, Flavonoids and Related Compounds... 

Electrophilic aromatic substitution of other benzo-fused v-deficient systems generally follows predictable pathways. Thus, benzopyrylium salts are in general resistant to electrophilic substitution even in the benzo-fused ring. Chromones behave somewhat similarly, although substitution can be effected under forcing conditions. Coumarins, on the other hand, undergo nitration readily in the 6-position while bromination results in substitution at the 3-position as a consequence of addition-elimination. 

Similarly, chromanones are converted into benzopyrylium salts in this case though the oxidant is triphenylmethyl perchlorate. Hydrolysis affords the chromone (60CB1466, 60CI(L)1192>. 

When this reaction is applied to o-hydroxyacetophenone, benzopyrylium salts are formed, through initial C-alkylation and subsequent cyclization to the chromone. Under the reaction conditions, O-alkylation of the ketone and aromatization to the salt occur (72CHE935). 

Chromones behave in a similar manner to pyranones. For example, benzopyrylium salts are formed with strong acids (71CB348) and with Grignard reagents (73SC231). Similarly, 3-phenylisocoumarin affords 1,3-diphenylisobenzopyrylium (51JOC1064). 

The rearrangements of phenols which are accompanied by hydroxy group transpositions are called the Wessely-Moser reaction (equations 50 and 51). In essence, these rearrangements are recyclizations of flavonoides 114 via the ring-opened form 115 to give the novel structures 116. Compounds that can participate in these rearrangements are flavones (114, R = H, R = Aryl), flavonoles (114, R = OH, R = Aryl), isoflavones (114, R = Aryl, R = H), chromones (114, R = H, R = Alkyl), chromonoles (114, R = OH, R = Alkyl), xanthones (114, R R = benzo) as well as benzopyrylium salts (e.g. see Reference 95). 

Isoflavones result from a Suzuki coupling reaction with 3-iodochromones (Scheme 11) <05TL3707> and ring expansion of chromones by treatment with Me3SiOTf and a diazoester affords 2,3-benzoxepins via the benzopyrylium salt and cyclopropane derivative <05TL4057>. 

Silyl enol ethers, or allylsilanes will add at C-2 to 1-benzopyrylium salts generated by C-silylation of chromones in the case of silyl ethers of a,P-unsaturated ketones, cyclisation of the initial adduct is observed (cf. 8.12.2). ... 

Addition of a proton to carbonyl oxygen produces a hydroxy-benzopyrylium salt chromones undergo this protonation more easily than the coumarins, for example passage of hydrogen chloride through a mixture of chromone and coumarin in ether solution leads to the precipitation of only chromone hydrochloride (i.e. 4-hydroxy-1-benzopyryhum chloride). O-AUcylation requires the more powerful alkylating agents ... 

Grignard reagents react with chromones at the carbonyl carbon the resulting chromenols can be converted by acid into the corresponding 4-snbstitnted 1-benzopyrylium salts. ... 

By conversion into a benzopyrylium salt with a leaving group, nucleophiles can be introduced at the chromone 4-position treatment with acetic anhydride presumably forms a 4-acetoxy-benzopyrylium. ... 

Benzopyrylium Salts. - Dichloromethoxybutane has been used in the cycliz-ation of the benzyl phenyl ketone (134) to give (after addition of perchloric acid) the pyrylium salt (135). The latter reacted normally with methylamine to form the isoquinolinium salt (136). A complex series of transformations has been suggested to account for the formation of the pyrylium salt (138) by the reaction of the chromone (137) with malononitrile-acetic anhydride. ... 

Spirothiopyrans 45b including a benzopyrylium ring have been prepared in one step by condensation of 2-aminovinyl-3-formyl chromone-4-thione 47 with 1,2,3,3-tetramethylindolinium salts in ethanol (Scheme 25).90 The precursor 47 is prepared from 3-carboxymethylene-2-methyl-chromone-4-thione 48. First, oxidation of 48 with pyridinium dichromate in CH2C12, and then condensation with dimethyl formamide dimethyl acetal in benzene gave compound 47. 

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. 

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