Benzopyrylium ion

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). 

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. 

The fragmentation of 2-methyl-2//-chromene follows an analogous pathway to that described for (4). a-Cleavage results in loss of a methyl radical giving the benzopyrylium ion (4a) as the base peak and subsequent fragmentations are identical to those depicted in Scheme 3. The alternative loss of a hydrogen radical from the molecular ion is not observed... 

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 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... 

Vajda and Ruff [64ACH(40)225] were the first to determine the pKa of 2-benzopyrylium ions, namely different hydroxy-substituted 1-aryl-benzo[c]pyrylium perchlorates of type 10. 

The reaction of alcohols with the alkynones 14 proceeds specifically under Ag catalysis to yield l-allenyl-l//-[2]benzopyrans a benzopyrylium ion is a probable intermediate <05JOC10096>. A Pd/Cu-catalysed allylation of the carbonyl group of 2-alkynyl-benzaldehydes is followed by heterocyclisation at an sp hybridised C atom to give 3-substituted 1-allylisochromenes. MesSiCN behaves similarly to allyltrimethylsilane providing a route to isochromen-l-carbonitrile <05T11322>. 

Typical Reactivity of Pyrylium and Benzopyrylium Ions, Pyrones and Benzopyrones... 

In earlier reports, evidence for the structure of ptelefoline (authors code number Pt/13) (7) and related iV-methyl-2-quinolone alkaloids from Ptelea trifoliata had been presented.13 A somewhat confusing situation has arisen as a result of ascribing the same name to another alkaloid isolated from the same species which, however, possesses the structure (4) and code number Pt/22.9 The structure (4) of Pt/22 was established by spectral means importantly, the mass spectrum of (4) showed a base peak at M - 15 which was attributed to the benzopyrylium ion (8). It is to be hoped that either Pt/13 or Pt/22 will be re-named in the near future. 

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