Benzopyrylium oxide

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. 

A comparison of the spectra of the ortho isomers, 5 with 7, reveals that fragmentation due to oxidation of the styryl double bond apparently is absent in 7. Instead, the MS of the latter exhibits ions corresponding to the formation of a benzopyrylium ion see Scheme 15. 

Nilsen and Undheim have prepared the unsubstituted betaine 166 by oxidation of 4-acetoxyisochromene (164). Thus, compound 164 with tetrachloro-l,2-benzoquinone (TBQ) or 2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone (DDQ) forms adducts 163 and 165 which in trifluoroacetic acid give 2-benzopyrylium-4-olate (166). Compound 166 cannot be isolated because dimerization readily occurs. Surprisingly, the TBQ adduct (165) gives the endo dimer (168) (19%) together with a small amount of the exo... 

Homophthalaldehyde (188) gives isoquinoline, isoquinoline 2-oxide, 3,4-benzopyrylium salts, and 2-alkyl- and 2-aryl-isoquinolinium salts (189) by reaction with NH3, NH2OH, H+ or RNH2, respectively. 

Hydrogenation to chromanes, oxidative degradation, and oxidation to pyrylium salts in acid medium have been known for a long time.3 A series of substituted benzopyrylium salts has been recently prepared by oxidation of chromenes with tritylium perchlorate.97 Oxidation of 4-phenylchromenes to coumarins with Cr03-pyridine complex or with Se02 in dioxan has been reported.186... 

The oxidation of benzopyrylium perchlorate to coumarin is brought about by manganese dioxide in chloroform or acetonitrile (68AC(R)25l). 

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

The unsubstituted 2-benzopyrylium cation 1 was first synthesized by Blount and Robinson (33JCS555) starting with indene. Its oxidation by lead tetraacetate affords homophthalic dialdehyde, which is cyclized to 2-benzopyrylium tetrachloroferrate on treatment with hydrochloric acid and ferric chloride. 

Upon acylation of some benzyl carbonyl compounds (25, R = H, Me 51, R = OH) dibenzo[a,tropylium salts 65 have been isolated in low yields (5-15 %) along with the major products, 2-benzopyrylium salts. Veratryl acetone 25 (R = Me) as well as homoveratric aldehyde 25 (R = H) (or carboxonium ions 31 which are formed from them) may undergo an oxidative a-cleavage, resulting in the benzyl cation 64. The formation of the same cation from homoveratric acid 51 is the result of decarbonylation of the acylium ion 63. Further interaction of the benzyl cation 64 with the substrate, followed by cyclization and oxidation, results in the polycyclic tropylium salts 65 (82ZOR589). 

The only known example of a reaction of 2-benzopyrylium salts with hydroxylamine is a normal O - N exchange and formation of isoquinoline N-oxide 181 (70KGS1308). 

The only known reaction of a 2-benzopyrylium salt with ozone leads to a destructive oxidation of the cation (51JOC1064). 

Due to an excess of electron density at the exo methine group in 212, these anhydrobases are readily oxidized to isocoumarines 258 (78KGS1320), and depending on the nature of the cationic substrate, compounds 212 may form a charge-transfer complex 259 with the initial salt 184 or may act as one-electron donors in reaction with the 1-benzopyrylium salt 260 (77KGS1138 77TH1). 

Although some radicals and cation radicals are postulated for chemical and electrochemical transformations of 2-benzopyrylium cations (Sections III,F,1 and IV,B)> attempts to record their electron spin resonance (ESR) spectra failed, obviously because of a low stability of these radicals. However, the structural combination of hydroxy aryl and 2-benzopyrylium fragments favors the formation of radical cations 301-303, and their ESR spectra were recorded on oxidation of the corresponding 2-benzopyrylium salts with lead tetraacetate (87RRC417). 

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. 

The platinum-catalysed intramolecular domino annulation reaction of o-alkynylben-zaldehydes has been described as a versatile approach to naphthalenes with annulated carbocycles or heterocycles of various sizes (Scheme 32).94 A plausible mechanism for the platinum(II)-catalysed annulation reaction shows that the double annulation process most probably proceeds through the benzopyrylium cation (117), which results from the nucleophilic attack of the carbonyl oxygen at the alkyne, activated by the Lewis-acidic platinum salt. A subsequent intramolecular Huisgen-type 3 + 2-cycloaddition of the second alkyne is assumed to generate intermediate (118). Rearrangement to (119) and the formal 4 + 2-cycloaddition product (118) leads to the aromatized final (116), liberating the active catalyst. In the case of FeCl3 as the Lewis acid, we assume that intermediate (118) is oxidatively transformed to (121). 

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. 

Oxidative general breakdown of flavylium salts was utilised in early structural work on the natural compounds. Baeyer-Villiger oxidation is such a process, whereby the two halves of the molecule can be separately examined (after ester hydrolysis of the product). Flavylium salts can be oxidised to flavones using thallium(III) nitrate, and benzopyrylium itself can be converted into coumarin with manganese dioxide. ... 

Subject to the restrictions set out below, phenols react with 1,3-dicarbonyl compounds to produce 1-benzopyryliums or coumarins, depending on the oxidation level of the 1,3-dicarbonyl component. 

Pyrylium salts. In contrast to the non-aromatic py-rans, their oxidized products the P. s. possess an aromatic electron sextet. P. s. belong to the oxonium salts but are, in contrast to most other oxonium salts of ethers, stable, especially when the 2,4,6-positions are substituted. The widely distributed natural products, the anthocyanidins (see anthocyanins), see also flavo-noids, are isolated as 2-phenyl-1-benzopyrylium chlorides. 

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