Photochemistry of Hantzsch 1,4-dihydropyridines and pyridines

The photochemistry of some Hantzsch 4-phenyl-1,4-dihydropyridines bearing a substituent on the phenyl ring (the three isomeric chloro derivatives and the 4′-nitro derivative) has been studied. All of these compounds underwent inefficient aromatization to the corresponding pyridines (quantum yield <10⁻⁴ at 366 nm, <10⁻² at 254 nm). This process is scarcely affected by molecular oxygen and is initiated by proton transfer (from C₄-H), probably to the solvent, from the excited singlet. In turn, the thus formed pyridines were photoreactive with comparable or higher efficiency. Thus, the 4-(3′-chlorophenyl) and 4-(4′-chlorophenyl) Hantzsch pyridines underwent positional rearrangement to form two isomers each. The reaction occurs via Dewar benzene--prismane path. In the case of the minor isomer a further 1,3-shift take place at the Dewar benzene level. The 4-(2′-chlorophenyl) derivative underwent C-Cl bond homolysis, which led to cyclization of the phenyl group onto one of the ester groups forming a pyrane ring.     

Recyclization of 2-vinylpyridinium quaternary salts

It is shown that in the reaction of 2-vinylpyridinium quaternary salts with methylammonium sulfite under isomerization recyclization conditions a water molecule adds initially to the double bond of the vinyl substituents, after which the resulting carbinols undergo recyclization to N-alkylaniline derivatives. The reaction is accompanied by a parallel process involving dealkylation of the pyridine ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 229–232, February, 1982.     

Recyclization of diethoxymethyl substituted benzimidazo-fused thiazolium salts

1. Download : Download high-res image (50KB)
2. Download : Download full-size image

     

Recyclization of selenophenopyrylium salts in alkaline media

Hydroxy derivatives of benzoselenophene that contain a hydroxyl group in the benzene ring are formed by the action of aqueous alcoholic sodium hydroxide on selenophenopyrylium perchlorates. The structure of the synthesized compounds was proved by IR and NMR spectroscopy.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1342–1344, October, 1973.     

Recyclization of pyrimidinium salts into 1,2,4-triazole derivatives

The interaction of the iodomethylates of pyrimidinyl-2-acetic acid derivatives with monosubstituted hydrazines, in addition to the products of a Kost-Sagitullin rearrangement, leads also to N-substituted triazoles. The structure of the triazoles was demonstrated by NOESY NMR experiments. The structure of the reaction products was determined on the basis of the response observed in the spectra between the methyl group protons of the triazole ring and the spatially close proton of the substituent in position 1 and a conclusion was drawn on the direction of the primary attack of nucleophile in the recyclization process of the pyrimidinium salts into a 1,2,4-triazole. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1175–1179, August, 2007.     

2-Benzopyrylium salts. 26. Recyclization of 2-benzopyrylium salts to α-naphthols

The behavior of 2-benzopyrylium salts with a CH₂ group in the 1 position at pH 7 was studied, and it was established that they undergo recyclization to -naphthols under these conditions. A mechanism for the conversion is proposed. One of the intermediates, viz., 3-hydroxy-1-tetralone, was isolated.See [1] for communication 25.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1184–1187, September, 1982.     

Iridium-catalyzed asymmetric transfer hydrogenation of quinolines with Hantzsch esters

The iridium-catalyzed enantioselective transfer hydrogenation of quinolines with Hantzsch esters was developed with up to 88% ee using [Ir(COD)Cl]₂/(S)-SegPhos/I₂ as a catalyst.     

Recyclization of 2-(2-acylethyl)pyridinium salts

Recyclization of 2-(2-acylethyl)pyridinium salts on treatment with nucleophiles may occur in different ways, to give heterocyclic compounds of various classes, namely indoles (the Kost-Sagitullin reaction products), 4-aminoindoles, and 3-hydroxy-1,2,3,4-tetrahydroquinolines, the formation of which constitute novel reactions in the recyclization of pyridinium salts.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1217–1225, September, 1990.     

Recyclization of 3-alkyl- and 1,3-dialkylisoquinolinium salts to naphthylamines

3-Methyl- and 3-benzylisoquinolinium salts undergo rearrangement to 2-alkylaminonaphthalenes under the influence of alcohol solutions of alkylamines. The rearrangement of 1,3-dimethyl- and l-methyl-3-benzylisoquinolinium salts leads to both 1- and 2-alkylaminonaphthalenes with predominance of the former.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1272–1277, September, 1980.     

Biomimetic asymmetric hydrogenation: in situ regenerable Hantzsch esters for asymmetric hydrogenation of benzoxazinones

A catalytic amount of Hantzsch ester that could be regenerated in situ by Ru complexes under hydrogen gas has been employed in the biomimetic asymmetric hydrogenation of benzoxazinones with up to 99% ee in the presence of chiral phosphoric acid. The use of hydrogen gas as a reductant for the regeneration of Hantzsch esters makes this hydrogenation an ideal atom economic process.     

Recyclization reactions. 26. Recyclization of heteroarenium salts by action of acid anhydride in the presence of base

When oxazolium, 1,3,4-thia- and -selenadiazolium, or pyrilium salts react with acid anhydride in the presence of base, they are recyclized to pyrrol, pyrazole, and benzene derivatives, respectively. The typical features of recyclization involving anhydride in a number of five-membered and six-membered systems are discussed.For Communication 25, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1689–1695, December, 1988.     

Synthesis of pyrylium salts and pyridines containing heterocyclic substituents

A method of synthesizing pyrylium salts with heterocyclic substituents in the ring, has been investigated. The synthesis was carried out in two stages: alkaline condensation of aromatic and heterocyclic aldehydes with methyl ketones to give 1, 5-diketones, followed by cyclization to pyrylium salts by treatment with triphenylmethyl perchlorate. Pyridines containing heterocyclic substituents were obtained by treating the pyrylium salts with ammonia.     

Recyclization of 3,5-diacetyl- and 3,5-dicyanopyridinium salts under the influence of bases

Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 848–849, June, 1990.     

Nitropyridines 11. *Recyclization of quaternary nitropyridinium salts into substituted nitroanilines

Substituted nitroanilines have been obtained by the recyclization of nitropyridinium quaternary salts under the action of bases.