Benzo pyrylium salts reactions

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. 

The further behavior of benzo[c]annelated adducts 100 depends on the structure of substituents in the initial cation 30, nature of the nucleophile, thermodynamic parameters of the final products, and conditions of the experiment. The reaction may be stopped at the step of adduct 100 (b, 1 in Scheme 5) or may be continued with the formation of ring-opened intermediate 101 (b,2). However, the latter step has some specific features, in comparison with monocyclic pyrylium salts, as a consequence of the presence of the annelated benzenoid ring in benzo[c]pyrylium cations. 

A specific behavior in reactions with primary amines was described for l-aryl-3-carboxy-benzo[c]pyrylium salts 62 (89KGS454). While their monocyclic analogs undergo decarboxylation with heteroatom exchange,... 

For benzo[c]pyrylium salts having different alkyl substituents in positions a(l) and a (3), the reaction with secondary amines loses its predictable regioselectivity. Thus, reaction with dimethylamine, independent of the solvent, gives rise to /3-naphthylamine 185, from l-ethyl-3-methyl-substituted salt 183 (81KGS1608), and a-naphthylamine 186 from 1-benzyl derivative 184 (88UP1). In the former case, the nucleophilic attack occurs in position 3, whereas in the latter case, it takes place in position 1. 

Benzo[c]pyrylium salts having a methyl group in position 1 form, in the reaction with the same azomethines, 1-styryl-substituted benzo[c]py-rylium salts 94 (Section III,B). 

Benzo[c]Pyrylium Salts Syntheses, Reactions, and Physical Properties... 

Kuznetsov, E. V., Shcherbakova, I. V., Balaban, A. T., Benzo[c]pyrylium Salts Syntheses, Reactions, and Physical Properties, 50, 157. 

Bcnzo[/)]furan and derivatives, recent advances in chemistry of, Part 1, occurrence and synthesis, 18, 337 Benzo[c]furans, 26, 135 recent advances in the chemistry of, and related compounds, 73, 1 Benzofuroxans, 10, 1 29, 251 2//-l-Benzopyrans (chrom-3-enes), 18, 159 Bcnzofc] pyrylium salts syntheses, reactions, and physical properties, 50, 157 1,2-and 2,1-Benzothiazines and related compounds, 28, 73... 

Pyrylium salts, pyrone derivatives, and their benzo derivatives show various interesting reactivities and appear in many natural products. Therefore, many organic chemists have an interest in the chemistry of pyrylium salts 1 and 2 and pyrones 3 and 4 (Scheme 1) (82AHC66 83AHC187). This review is a survey of the literature from 1980 onward, concentrating especially on nucleophilic reactions and carbocyclic annulation reactions that retain the resulting pyran ring. Related benzopyrone derivatives, coumarins (5) and chromones (6), are also included. 

A large proportion of work on benzo[c]pyryliums is by Russian and Hungarian workers and is described in relatively inaccessible journals, however it is well reviewed as Benzo[c]pyrylium salts syntheses, reactions and physical properties , Kuznetsov, E. V., Shcherbakova, I. V., and Balaban, A. T., Adv. Heterocycl. Chem., 1990, 50, 157. 

The reaction proceeds normally with 5- or 6-membered 1-methylcycloalkenes 33, 36 (or for better yields with the isomeric methylenecycloalkanes 34, 37). In the case of 6-membered rings, the products (35) may be converted by dehydrogenation into benzo[6]pyrylium salts and then isoquinolines. 

Thiophen Analogues of Isoquinoline.—4,7-Dimethylthieno[2,3-c]pyridine has been synthesized by hydrolysing the acetal function of (252). This compound was obtained by the reaction of 2-(3-lithio-2-thienyl)-l,3-dioxolan with diacetyl followed by oximation and reduction. 6,7-Dimethylthieno[3,2-c]pyridine was obtained in an analogous manner, (2,5-Dimethyl-3-thienyl)acetone gave (253) on acylation. Treatment with perchloric acid gave the pyrylium salt (254), which upon reaction with NH3 gave (255). The same reaction sequence with (3-benzo-... 

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. 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

The reaction of 4-1 dimerization is similar to the primary step of recycli-zation of 2-benzopyrylium salts in acidic nucleophilic media (cf. Section III,C,4,b,i), but the reactive electrophile is the initial cation in this case, and not a proton. Probably for this reason, the 4-1 dimerization of 2-benzopyrylium perchlorates is not observed in acidic nucleophilic media, in contrast to a-1 dimerization (cf. Section III,F,2,b). At the same time, the scope of 4-1 dimerizations is less restricted in terms of structural requirements for 2-benzopyrylium salts in comparison with a a-1 dimerization. Thus, in the latter case, the presence of a methyl group in position 1 of benzo[c]pyrylium cation is compulsory, whereas for 4-1 dimerizations, the nature of the substituent in this position may be different, leading to a variety of 4-1 dimers and, as a consequence, to a wide variety of their transformations. 

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