2-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). 

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... 

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... 

Another type of [5 +1] reaction leading to the synthesis of an (in effect) quinolizin-4-one (198, Scheme 105) involves the concurrent replacement of the oxygen atom of a 2-benzopyrylium salt (197) by nitrogen (from ammonia), and cyclization (77CHE1183). 

In the presence of copper(I) salts in acidic media o-ethynyl-benzaldehyde derivatives were found to cycloisomerise to 2-benzopyrylium salts (4.26.), The reaction, although working in the absence of catalyst too, was accelerated by the addition of different metal salts. The reaction was applied in the preparation of azaphilones and related molecules.30... 

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... 

Condensed heteroaromatic cations are reactive in [2 + 4] cycloaddition reactions with inverse electron demand. For instance, 2-benzopyrylium salts (389) react with vinyl ethyl ether to afford... 

The IR spectrum of the 2-benzopyrylium species has been reported (64ACH<40)217). 

Benzo-fusion to a pyrylium ring can give rise to two isomers, namely the 1- and 2-benzopyrylium ions (6) and (219) both of which are formally aromatic 107r- electron systems. 

The most important pyran betaines are the heteroaromatic pyrylium-3-olate (258) and 2-benzopyrylium-4-olate (259) systems. Numerous analogues of both are known, and it has been shown that these have a high propensity to participate in cycloadditions (72JOC3838)>... 

The distribution of charge in the resonance forms (28)-(31) suggests that nucleophiles may attack at C-2, C-4 or C-6 (or at C-2 or C-4 in 1-benzopyrylium cations, and at C-l, C-3 or C-4a in 2-benzopyrylium ions) but they most commonly add at C-2 for example, attack by cyanide ion gives a 2//-pyran (37) which exists partly or wholly as the acyclic isomer (38). Steric and electronic effects in the reactants probably have a role in determining the course of the reaction of trisubstituted pyrylium salts with nucleophiles. A mixture of both 2H- and 4H-pyrans is sometimes produced, for example, from methoxide ion and 2,4,6-triphenylpyrylium perchlorate (39) no acyclic product was detected in this reaction... 

JOC5160). Benzylmagnesium chloride is exceptional as it consistently attacks at C-4 even when this is a ring-fusion atom as in l,3-dimethyl-5,6,7,8-tetrahydro-2-benzopyrylium perchlorate (40) <64LA(678)183>. 

Competing Reactions in the Formation of 2-Benzopyrylium Salts upon... 

Two benzoannelated analogues of the pyrylium cation are known ben-zo[b]pyrylium (chromylium or 1-benzopyrylium) and benzo[c]pyrylium (isochromylium or 2-benzopyrylium) salts. So far, the 1-benzopyrylium system is considered to be the more interesting (79MI2 84MI1) because it is the basic heterocyclic system of important plant pigments (antho-cyanins), and because there are major differences between properties of its heterocyclic ring and those of monocyclic pyrylium systems as a result of benzoannelation. Thus, for instance, 1-benzopyrylium salts do not possess the ability to have recyclization reactions with heteroatom exchange. 

Benzopyrylium salts have not been found in nature, and in spite of their ability to take part in recyclization reactions, until recently it was considered that there were no specific features in their transformations relative to monocyclic pyrylium salts (79MI2), and the scope of these transformations has been considered as rather restricted (71CB2984). The reason for such a conclusion was based on the incorrect interpretation of the effects of benzoannelation on reactions of 2-benzopyrylium cations so far no review on this subject has been published. 

The goal of the present review is to demonstrate the wide potential for using newly developed methods to synthesize 2-benzopyrylium salts and to discuss in detail the numerous and unusual transformations of this class of oxygen cations in comparison with monocyclic pyrylium salts, i.e., the study of influence of benzo[c]annelation. 

We shall use both names for this class of compounds, 2-benzopyrylium... 

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. 

This approach was applied later to synthesize substituted 2-benzopyrylium salts 4 (85LA2116) via 1,5-diketones 3, and a few dioxetanes 2 were isolated as intermediates on using singlet oxygen photochemically (74JA4339). 

The only example of an ozonide (5) was reported (85LA2116) when ozone was used instead of photooxidation 5 was also converted into 2-benzopyrylium via the corresponding o-acetyl benzyl ketone 3, but not... 

Perchlorates are the most often used anions in syntheses and investigations of all 2-benzopyrylium salts. In the following sections, only anions different from perchlorate will be depicted in structural schemes. However, taking into account the danger of explosion inherent in working with solid perchlorates, it would be cautious to use BF4, CF3SO3, or other anions instead. 

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