Halo-2-pyrones in the Ullmann reaction. 1

The Ullmann reaction was used for the first time for the synthesis of poly-2-pyrones. 3-Halo-2-pyrones were obtained by halogenation of the corresponding 6-aryl-2-pyrones. The Ullmann reaction was carried out by heating a mixture of the 3-halo-2-pyrone with activated copper in a stream of an inert gas. The reaction products were the previously undescribed bis-2-pyrones, in which the -pyrone rings are interlinked by means of the 3-C atoms.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 173–176, February, 1979.     

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.     

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.     

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.     

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).     

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.     

A one-step synthesis of γ-pyrones

The potassium enolate of 4-methoxy-3-buten-2-one reacts with acid chlorides anhydrides and acylimidazoles by C-acylation and $\text{in situ}$ cyclization to afford 2-substituted γ-pyrones directly.     

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.     

Synthesis of substituted γ-pyrones

Substituted -pyrones were synthesized by the acylation of phenylacetaldehyde and its enol acetate with acetic and propionic anhydrides in the presence of 70% HClO₄. The mechanism of the reaction is examined, and the IR and PMR spectra of the synthesized -pyrones are presented. Some of their transformations were studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 152–154, February, 1972.     

Solid-state and solution photocycloadditions of 4-acyloxy-2-pyrones with maleimide

Solid-state photosensitized reactions of 4-acyloxy-2-pyrones (1b,c) with maleimide (2) afforded endo-endo double-[4+2] cycloadducts (3b,c) with high stereoselectivity. Sensitized photoreactions of 1a-d with 2 in solution gave exo-endo double-[4+2] cycloadducts (4a-d). 2-Pyrones 1a-d were photolyzed to give carboxylic acids (5a-d) via their valence isomerization in the solid state and in solution. Such kinds of photoreaction of the 4-acyloxy-2-pyrones were dramatically different from regio- and stereoselective [2+2] cycloadditions of 4-alkyloxy-2-pyrones. The photoreaction mechanisms of 1 with 2 and 1 itself were analyzed by powder X-ray diffraction analysis and MO calculations.     

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.     

Properties and cage lactone synthesis of diels-alder adducts from 2-pyrones

New cage lactones were synthesized by the photocyclization of the tricyclodienolactones 1a, 1b, 2 which were available from the Diels-Alder reactions of 2-pyrones with p-benzoquinones or norbornadiene. Retro-Diels-Alder reactions of cross-adducts between 2-pyrone-mono-adducts and p-benzoquinone were also described.     

Synthesis of pyrone-annulated 2-oxabicyclo[3.3.1]nonanes by palladium-catalyzed cyclization of 4-hydroxy-2-pyrones with allylic bisacetates

Cyclization of 4-hydroxy-2-pyrones with allylic bisacetates by a palladium catalyst is described. Pyrone-annulated 2-oxabicyclo[3.3.1]nonane derivatives were regioselectively produced from the reaction of 4-hydroxy-2-pyrones with 1,4-diacetoxy-2-cyclohexene at high temperature. The reaction would proceed via a migration of the π-allylpalladium intermediate.     

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.     

A convenient synthesis of substituted γ-pyrones

An efficient and general synthetic method for various 2-mono- and 2,6-disubstituted γ-pyrones has been developed. This utilizes the $\text{C}$-acylation (70–85%) of β-methoxy-α,β-enone lithium enolates $\text{4}$ by acid chlorides $\text{3}$ followed by the acid-catalyzed cyclization (>80%) of the resulting enols $\text{5}$ to γ-pyrones $\text{6}$.     

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.     

IMDA cycloadditions of 3-alkynyl tethered 2-pyrones for the synthesis of medium-sized macrocycles

Alkynyl tethered 2-pyrones underwent intramolecular Diels-Alder cycloadditions to provide bicyclolactone-fused medium-sized macrocycles in good overall yields. Lactone ring opening reactions furnished various ring fused medium-sized macrolactams and macroazanes with good overall yields.     

A novel non photochemical ring contraction of 4-pyrones to cyclopent-2-enones

Unlike comenic acid or most other 4-pyrones, methyl or ethyl comenate reacts with aromatic amines under mild conditions in the sense of ring contraction to 3-arylamino-2,4-dihydroxy-2-cyclopentenone-4-carboxylates. The structure of the new cyclopentenone derivatives were determined from their characteristic spectroscopic behaviour and was confirmed by X-ray crystallographic studies of compound Vb .     

Trifluoromethyl substituted 4-pyrones via self-condensation of trifluoroacetoacetates

When 3-(acetoxy)-4,4,4-trifluoro-2-butenoates 7 and 8 are heated at 100°, in the presence of catalytic amounts of zinc chloride, they undergo self-condensation to yield 2,6-bis(trifluoromethyl)-4-pyrones 1 and 2 respectively. Compounds 1 and 2 were further converted to the corresponding pyridine derivatives 3 and 4 via ammoniolysis.     

Generation of diverse 2-pyrones via palladium-catalyzed site-selective Suzuki-Miyaura couplings of 3-bromo-4-tosyloxy-2-pyrone

Based on different reactivity of the (pseudo)halide substituents in the 2-pyrone (3-Br and 4-OTs), palladium-catalyzed sequential site-selective Suzuki-Miyaura cross-coupling reactions of 3-bromo-6-methyl-4-tosyloxy-2-pyrone are described, which afford the diverse 2-pyrones in good yields.