Iodination of 6-aryl-2-pyrones

The iodination of 6-aryl-2-pyrones with iodine chloride in glacial acetic acid leads to the corresponding 3-iodo derivatives. 3-Cyano-6-phenyl-2-pyrone and 3-phenylethynyl-6-phenyl-2-pyrone were obtained by replacement of the iodo group by cyano and phenylethynyl groups.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 616–618, May, 1884.     

Synthesis of 3, 4, 6-triaryl-2-pyrones

A preparative method was worked out for the synthesis of 3,4,6-triaryl-2-pyrones by 1,3-dipolar cycloaddition of selenium arylacylmethylides to diphenylcyclopropenone.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1475–1478, November, 1977.     

A facile, general synthesis of 3,4-difluoro-6-substituted-2-pyrones

Reaction of (2E)-2,3-difluoro-3-iodoacrylic acid with a variety of terminal acetylenes under cocatalysis of PdCl2(PPh3)2 and CuI gave difluorinated 2-pyrones as the sole product in good yields.     

Synthesis and stereochemistry of 3-hydroxy-6-perfluoroalkyl-2, 3-dihydro-4-pyrones

The reaction of a number of aliphatic and alicyclic acetloxiranes with perfluoroalkanoic acid esters was studied. It is shown that substituted 3-hydroxy-6-perfluoroalkyl-2,3-dihydro-4-pyrones are formed. The reverse reaction scheme, including the formation of an intermediate -diketone and cyclization of its enol form to a dihydropyrone, was confirmed in the case of the condensation of 3-methyl-2,3-epoxycyclohexanone with ethyl trifluoroacetate. The stereochemistry of the reaction products, which exist in a half-chair confirmation with a pseudoequatorial hydroxyl group, was examined by means of PMR spectroscopy.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 897–900, July, 1978.     

Synthesis of 2-hydroxy-3,5-diaryl-6-benzyl-γ-pyrones and their esters

Chemistry of Heterocyclic Compounds -     

Regiospecific friedel-crafts acylationof 6-alkoxy-2-pyrones: Preparation of substituted aromatics and anthraquinones.

Friedel-Crafts acylation of 6-alkoxy-2-pyrones gives high yields of the 5-acyl-2-pyrones which undergo regioselective cycloaddition with various dienophiles.     

Pd-catalysed regioselective C-H functionalisation of 2-pyrones

A new synthetic methodology for the catalytic C-H functionalisation of 2-pyrones is described which proceeds regioselectively at the C3 position, mirroring the observed regioselectivity in 6π-electrocyclisation/oxidative aromatisation reactions of related compounds. Insight into the reaction mechanism is provided, with support for a neutral palladium(II) pathway. Cationic palladium(II) complexes possessing 2-pyrones are unstable and readily undergo Pd(II)→P transfer at ambient temperature resulting in phosphonium salt formation (and Pd(0)L(n) species).     

Diene synthesis with 2-pyrones and 2-pyridones. 31. Stereochemistry of the successive addition of two different dienophiles to 2-pyrones

The stereochemistry of the successive addition of two different dienophiles, viz., N-phenylmaleinimide and 4-phenyl-1,2,4-triazoline-3,5-dione, to 2-pyrones was studied. The configurations of the mixed adducts (bisimides) obtained were established by PMR and mass spectrometry. It is shown that in all cases the reactions proceed with the formation of mixtures of syn and anti isomers, the ratios of which depend on the positions of the methyl groups in the starting 2-pyrones.See [1] for communication 30.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1471–1476, November, 1982.     

Reactivity of 3-amino-, 3-halogeno-, and 3-nitro-2-pyrones and thiopyrones

Substituted pyrrole and furan-2-carboxylic acids were obtained from 3-amino- and 3-halogeno-2-pyrones. The reactions of several substituted 2-pyrones and thiopyrones with different amines and carbonyl reagents were studied.     

Preparation of 6-(nitron-c-yl)- and 6-(1,3-butadienyl)-2-pyrones and their cycloaddition reactions

6-(N-Substituted nitron-C-yl)-2-pyrones 1 and 6-(4-substituted 1,3-butadienyl)-2-pyrone 2 were prepared and their cycloaddition reactions with three kinds of diene systems were investigated. Namely, the reactions of 1 with methyl acrylate, vinyl crotonate and divinylsulfone took place at the nitrone moiety to afford 3-substituted isoxazolidines 9–14 , and that of 2 with maleimide took place at the 2-pyrone moiety to give a bis-adduct 17 via elimination of carbon dioxide.     

A rapid and diverse construction of 6-substituted-5,6-dihydro-4-hydroxy-2-pyrones through double Reformatsky reaction

A rapid and diverse synthesis of biologically important 6-substituted-5,6-dihydro-4-hydroxy-2-pyrones through a double Reformatsky reaction of aldehydes to δ-hydroxy-β-ketoesters followed by lactonization is described. Due to the high functional group tolerance and reaction site discrimination between aldehyde, nitrile, and ester groups in the substrate, the protocol can provide the dihydropyrones with bromo, nitro, carboxylic acid, and β-ketoester groups, which are suitable for the further derivatizations. Furthermore, the protocol has been successfully applied to the rapid total synthesis of naturally occurring Yangonin.     

One-pot stereoselective synthesis of tricyclic bislactones from 2-pyrones and 2-methoxyfuran

[reaction: see text] A one-pot synthesis of the title compounds via highly chemo-, regio-, and stereoselective Diels-Alder reactions of 2-pyrones with 2-methoxyfuran is described.     

Alkynyl triazenes enable divergent syntheses of 2-pyrones

The 2-pyrone motif occurs frequently in bioactive natural products and is appreciated as synthetic intermediates. However, only few methods allow for diversifying functional group modifications on this relevant heterocycle. The distinct properties of 1-alkynyl triazenes promote a smooth addition of propiolic acids across the triple bond. Addition of catalytic amounts of silver salt induces cyclization to 2-pyrones. Depending on the reaction temperature, either 6-triazenyl or 5-triazenyl 2-pyrones are selectively formed. The triazenyl unit is subsequently replaced by a variety of valuable groups in a one-pot process yielding for instance 2-fluoro pyrones. The substitution occurs with an intriguing 1,5-carbonyl transposition. Moreover, the triazenyl group serves as traceless activating group for subsequent Diels–Alder cycloadditions and as a constituting unit for rare fused aminopyrazole pyrone heterocycles.

Substituted 2-pyrones are accessed in a silver-catalyzed one-pot process between propiolic acids and 1-alkynyl triazenes. Subsequent diversifying replacement of the triazenyl unit under 1,5-carbonyl transposition yields rare 2-fluoro pyrones.     

Phosphine-catalyzed synthesis of 6-substituted 2-pyrones: manifestation of E/Z-isomerism in the zwitterionic intermediate

[reaction: see text] We report a one-step phosphine-catalyzed annulation between aldehydes and ethyl allenoate to form 6-substituted 2-pyrones. The mechanistic rationale for this reaction requires explicit discussion of the E/Z-isomerism of the zwitterionic intermediate formed by the addition of a phosphine to the allenoate. Sterically demanding trialkylphosphines facilitate the shift of equilibrium toward the E-isomeric zwitterion and lead to the formation of 6-substituted 2-pyrones. Various aromatic as well as aliphatic aldehydes undergo the transformation in moderate to excellent yield.