Alkyl-Benzopyryliums

Alkyl groups oriented a or y to the positively charged oxygen in benzopyryliums have acidified hydrogens that allow aldol-type condensations.  

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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. 

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). 

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. 

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). 

Deprotonation reactions if the 2-benzopyrylium cation has an a-alkyl substituent or a group with a mobile hydrogen atom in any other fragment of the molecule. This conversion takes place over a wide range of conditions, and it often accompanies nucleophilic additions. Moreover, the problem of the acid-base and nucleophilic-electrophilic interactions for 2-benzopyrylium salts concerns not only the primary step of their reactions with nucleophiles, but also the ring-open forms, as shown in Section III,C,4. 

Since the reactions of 2-benzopyrylium salts with nucleophiles (in accordance with the ANRORC scheme) are seldom stopped at the ringopening step (as a rule, formation of 1,5-dicarbonyl compounds takes place), it is expedient to examine at once the reactions completed by formation of a new ring. Such a process may occur with participation of both the added nucleophile and the a-alkyl substituent in the heterocyclic... 

When 2-benzopyrylium cations have a substituent with a fairly mobile hydrogen atom (a-alkyl group or heteroatom group in any other position of the cation), deprotonation of such a substituent occurs even under mild conditions by an acid-base interaction as the primary step (Section III,A). Although deprotonation in both cases leads to compounds whose structures can be depicted by two resonance formulas, either with charge separation (betaines) or without (anhydrobases), on discussing products of C-deprotonation, the term (and the corresponding formula ) anhydro-base is more often used, whereas products of O-deprotonation are called betaines. ... 

Stabilization of an organic cation occurs whenever the electron-deficient carbon atom is conjugated with aryl groups or with atoms containing unshared pairs of electrons such as oxygen, nitrogen, or sulfur (7). Examples of cations which are useful in polymerization include triphenylmethyl (I), cycloheptatrienyl (tropylium) (II), and various alkyl (III), aryl (IV), and benzopyrylium (V, VI) derivatives. 

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). 

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 ... 

Very reactive electrophiles can attack the carbonyl oxygen of coumarin. For instance Meerwein s salt [Et30]BF4 effects O-alkylation to give the benzopyrylium ion 2 ... 

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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