Benzopyrylium salts, amination

The reaction of several amines with 7-methoxy-3-phenylbenzopyrylium perchlorate has been shown to yield the corresponding 2-substituted 3-phenylchromene (78JCS(P1)88), although the reaction of secondary amines with simple benzopyrylium salts gives mixtures of chrom-3-enes and chrom-2-enes (72JOC1069). 

The ability of 5-oxoniachrysenes 67 to form stable adducts with ammonia, methyl amine, and hydrazine is not usual for 2-benzopyrylium (cf. Section III,C,2), but is often encountered for 1-benzopyrylium (chro-mylium) salts (51 Mil). At the same time, no rupture of the C6—Cring bond was observed for 2-benzo- or 1-benzopyrylium salts under the conditions used. A remote similarity to C6—Crjng bond rupture may be seen in reactions of 2-benzopyrylium salts with sodium azide (Section III,C,2) or with hydrogen peroxide (Section III,C,4,b,ii). 

Since the decarboxylation of 161 to 162 proceeds in a poor yield, it was suggested that formation of 162 in benzene occurs directly from 2-benzopyrylium salts 62 through primary nucleophilic attack by amine in position 3. In this case, an enamine fragment of pyruvic acid appears in the ring-opened intermediate 163, which undergoes easy decarboxylation (82TL459). The vinylic carbanion 164, formed by the loss of carbon dioxide, captures a proton by intra- or intermolecular process, then hetero-cyclization takes place. 

The application of acetic acid catalysis in reaction of 2-benzopyrylium salts with primary amines4, in contrast to the reaction with ammonia, does not lead to a simple result. Thus, if in 30 R3 is not Aik, excluding the alternative formation of a-naphthylamines of type 153, the use of acetic acid catalysis leads to isoquinolinium salts 152 in high yields (89KPS75), whereas without acetic acid, diketones 166 were the only products of interaction between 2-benzopyrylium salts 30 and primary amines. 

The formation of a-naphthylamines 140 and 153 in reactions of 2-benzopyrylium salts with ammonia and primary amines is not always an undesirable process. The synthetic application of a-naphthylamines is connected with the chemistry of benzo[c]phenanthridine alkaloids 132 (78MI1 81H474). 

Similar to the reaction of 2-benzopyrylium salts with ammonia (Section III,C,4,a,i), an excess of competing nucleophiles in reactions with secondary amines is undesirable. Thus, the interactions of l-ethyl-3-methyl-substituted salt 183 with 40% aqueous solution of dimethylamine leads to a-naphthol 189 as the major product (79TH1). 

Interestingly, a-carboxy-substituted monocyclic pyrylium salts 195, probably because of a lower tendency towards nucleophilic addition in comparison with 2-benzopyrylium salts (Section III,C,1), react via two pathways. One is similar to reactions of 2-benzopyrylium salts (a), and the other resembles the behavior of isoquinolinium salts (b) (Section III, C,4,a,ii). In this case, monocyclic five-membered acyloins 198 were obtained in 15-40% yield. Obviously, their formation occurs by initial addition of amine (pathway a). The difficultly obtainable a-unsubstituted py-... 

The formation of acylnaphthalenes 225 occurs under mild conditions (catalysis by sodium rm-butoxide or with the use of the phase-transfer method and TEBA as catalyst), and it affords high yields (60-90%). In this connection, it is surprising to recall the opinion about the incapability of 2-benzopyrylium salts to take part in recyclization reactions with compounds possessing active methylene groups, with secondary amines, or with sulfur or phosphorus nucleophiles (71CB2984). 

On thermolysis, one cannot exclude the provisional disturbance of aromaticity by a reverse electronic process followed by an electrocyclic ring-closure to acylnaphthalene 225 as described for the reaction of 1-styryl-substituted 2-benzopyrylium salts 199 with secondary amines (Section III,C,4,a,iv). 

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. 

It is important to realise that 1-benzopyrylium salts cannot be converted into quinolines or quinolinium salts by reaction with ammonia or primary amines (cf. pyryliums to pyridines, 11.1.2.2), whereas 2-benzopyrylinm salts are converted, efficiently, into isoqninolines or isoquinolinium salts, respectively."... 

Benzopyrylium Salts.- Phenols have been converted100 into benzopyrylium salts by treatment with 3-chlorocinnamaldehyde-perchloric acid or the quaternary amine perchlorate, RCCl=CHCH=NMe2 CIO . When 3-phenylbenzopyrylium perchlorates are treated with... 

If the 2-benzopyrylium cation has no substituent in position 1, the interaction of the salt 159 with amines having a primary a-carbon atom or with aniline leads to the formation of isoquinolinium salts 160 in high yield (81KGS313 89KPS75). However, the use of amines with secondary a-car-... 

As is the case for the reaction of secondary amines with alkylpyrylium salts to give dialkylanilines, benzopyrylium and pyrylium salts react with amines to give similar products (e.g., 113—>114 Equation 8) <1982AHC(32)128, 1970KGS1308, 1981KGS1351, 1987KGS889, 1998CHE159>. 

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