1- Benzopyrylium 1 -Benzopyryliums

Azonia substitution at a naphthalene bridgehead position gives the quinolizinium ion (16). Oxonia substitution, elsewhere, forms the 1- and 2-benzopyrylium ions (17) and (18). The two most well-known monoaza systems with three aromatie fused rings are aeridine (19), derived structurally from anthraeene, and phenanthridine (20), an azaphenanthrene. The better-known diaza systems inelude phenazine (21) and 1,10-phenanthroline (22), while systems with three linearly fused pyridine rings are ealled anthyridines, e.g. the 1,9,10-isomer (23). 

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... 

When 2-phenyl-l-benzopyrylium salts are treated with sodium azide, 2-azido-l-benzopyrans... 

The reaction of sodium azide with 2-benzopyrylium perchlorates 4 gives unstable l-azido-2-benzopyrans 5, which lose nitrogen on heating to give mixtures of 3,1-benzoxazepines 6 and anils 7.25... 

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... 

Auch Tropylium-3, Benzopyrylium-4, Flavylium-3 und Acridinium-Salze3 unterliegen dieser Dimerisierung. 

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. 

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. 

In the spirothiopyran 45b, the spiro form has two absorptions in the visible region (Vax 490 and 474nm) due to a polyene chromophore from IV-vinyl group to oxygen of the benzopyrylium component.90 The colored form of 45b produced by visible light irradiation shows the Vax at 570 nm. This colored form 45b" was confirmed by characteristic 1H-NMR spectra, as well as that of spiropyran. 

Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

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. 

Bipyrans can be prepared by quantitative cathodic reductions of the corresponding pyrylium and benzopyrylium salts. According to the nature of the substituents, dimerizations obtained from pyrylium salts take place at the C-2 or C-4 positions (Scheme 156) [274]. 

Flavonoids are natural phenolic systems containing pyrylium and pyrone rings, and provide the most prominent examples. We have met some of these systems under antioxidants (see Box 9.2). Coumarins contain a 2-pyrone system. Note that all of these compounds are fused to a benzene ring and are strictly benzopyran or benzopyrylium systems. 

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

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. 

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