4-Pyrones carboxylic acids

The intramolecular reaction oF allcenes with various O and N functional groups offers useful synthetic methods for heterocycles[13,14,166]. The reaction of unsaturated carboxylic acids affords lactones by either exo- or endo-cyclization depending on the positions of the double bond. The reaction of sodium salts of the 3-alkenoic acid 143 and 4-alkenoic acid 144 with Li2PdCl4 affords mostly five-membcrcd lactones in 30-40% yields[167]. Both 5-hexe-noic acid (145) and 4-hexenoic acid (146) are converted to five- or six-mem-bered lactones depending on the solvents and bases[168]. Conjugated 2,4-pentadienoic acid (147) is cyclized with Li2PdCl4 to give 2-pyrone (148) in water[i69]. 

The base-promoted ring contraction of 3-bromo-2-pyrones to 2-furoic acids cf. Scheme llOd) is a well exemplified reaction 01CB1992,69JCS(C)1950,73JCS(P1)1130> which has also been applied to the obtention of benzofuran-2-carboxylic acids frorn 3-bromocoumarins 08CB830,70KGS(S2)166), Similar base treatment of 3-amino-2-pyrones provides pyrrole-2-carboxylic acids (Scheme IlOe) 75JHC129). 

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

Ring contractions of pyran derivatives are occasionally valuable. The contraction of 3-halo-2-pyrones to 2-furoic acids under the influence of alkali has been studied and the conditions defined.58112113 The method is adaptable to the preparation of 3-furoic acid via furan-2,4-dicarboxylic acid58 and of 3,4,5-triphenylfuran-2-carboxylic acid.113 Another ring contraction involving halides is the conversion of 4-chloromethylpyrylium salts into furylmethyl ketones as indicated in Scheme 21.114 Pyridine oxides may be transformed with unexpected ease into furans through treatment with a thiol (Scheme 22).115... 

Diynes have already been used for building polycylic compounds in the presence of CO2 and a stoichiometric amount of Ni(0) bicyclic pyrones were obtained [117]. With the electrocarboxylation method, linear or cyclic mono-carboxylic acids were obtained as main products from non-conjugated diynes depending on the ligand associated to Ni [118, 119]. Thus ring-fomation occurred with the Ni-bipyridine complex at normal pressure of CO2 on the other hand, in the presence of PMDTA as ligand and with a 5 atmosphere pressure of CO2, linear adducts were mainly formed as illustrated in Eq. (16) ... 

The second example described here is dormant seeds from Rosa canina. Extracts of these seeds also inhibit germination of seeds of several plants (10). In Figure 5 a scheme is given for extraction and separation oF"three different inhibitor compounds. All these are present in the acid fraction. The first essential step is chromatography on Sephadex LH-20, which separates inhibitor I from inhibitor II and III. Inhibitor I was identified as abscisic acid. The other two inhibitors were separated by methylation with diazomethane, fractional distillation, and column chromatography. The second inhibitor is the a-pyrone 1 . Reaction with diazomethane transforms it into the bi-cyclic compound 19. This bicyclic compound is even more active than the parent a-pyrone 1 . Since we sought structural requirements for bioactivity here also,we tested several synthetic a-pyrones ( 0 - 22) for bioactivity. These compounds had no inhibitory activity. We alio tested the cyclopropane derivatives 23 and 24 In Table II, the bioactivity of the bicyclic compound T9 and two such derivatives is compared. The presence of several carboxylic acid groups seems to be essential (or at least helpful) for bioactivity in this case also. 

Pyridine, pyrone and pyridone carboxylic acids undergo decarboxylation when heated, and the general order of reactivity is a > y > p. In pyridine, the carboxylic acids, as expected, exist mainly in the zwitterionic forms and decarboxylation of the a and y isomers under fairly mild conditions is a consequence of the relative stability of the ions of the type (48). 

The Lewis acid-mediated condensation reaction of a,/3-unsaturated carboxylic acids with 4-hydroxy-2-pyrones has been utilized to prepare Fleischmann s a,a-bispyrone 2 and Praill s a,7-bispyrone 3 and confirm their assignments <2000TL1901>. 

The cumulation reactions, which start from carboxylic acids and esters usually lead to the formation of pyrone derivatives. Methyl (Z)-3-iodoacrylate and 3-hexyne gave, for example, 5,6-diethyl-2-pyrone in acceptable yield (4.38.). Inclusion of the acrylate into a six membered ring starting from ethyl 2-bromocyclohexen-l-carboxylate, led to a condensed ring system, giving a partially reduced isocoumarin derivative.50... 

The analogous open chain carboxylic acid, Z-non-2-en-4-ynoic acid, when treated with 4-iodoanisole in the presence of a palladium-triphenylphosphine catalyst and potassium carbonate gave a mixture of three products, two of which were isolated (4.41.) z) the pyrone derivative arising from the attack of the anisylpalladium complex at the 4-position, followed by ring closure //) the furane derivative (major product) arising from the... 

Bromo-2-pyrones and 3-bromocoumarins give furan- and benzofuran-2-carboxylic acids by ring fission and subsequent closure, e.g. (228) — (229) (116) or (117) — (230). Pyrone rings are opened by aqueous acid in some cases, probably by successive protonation and attack of a water molecule, e.g. dehydroacetic acid (231) gives (233) which immediately forms (232) or (234) with HC1 or H2S04, respectively. 

Azinecarboxylic acids lose C02 significantly more easily than benzoic acid. Pyridinecarboxylic acids decarboxylate on heating with increasing ease in the order (3 < < y < a. 2-Pyridazinecarboxylic acid gives pyrazine at 200°C, and 4,5-pyrimidinedicarboxylic acid forms the 5-mono-acid on vacuum distillation. Pyrone- and pyridone-carboxylic acids also decarboxylate relatively easily thus, chelidonic acid (680 Z = O) at 160°C over copper powder and chelidamic acid (680 Z=NH) at 260°C give (681 Z = 0, NH). 

Among the important processes taking place when sucrose is heated is cara-melization. Caramel is actually a very complex mixture. Some constituents are volatile, such as furan derivatives, pyrones, aldehydes, and low molecular weight carboxylic acids, and these account for 10% of the mass.340 The main constituents of the nonvolatile component have also been identified these include... 

Somewhat surprisingly, maltol (2-methyl-3-hydroxy-4-pyrone), an aromatic analogue to a-hydroxy carboxylic acids, shows little inclination toward formation of dimeric complexes. Rather, the chemistry is more in parallel with that of oxalate, and an x-ray structure of the bismaltolato complex [25] shows a cis octahedral coordination similar to that found for the oxalate complex. Under mildly acidic to moderately basic solution, the major complexes are mono- and bisligand derivatives. The corresponding vanadium chemical shifts are -509 and -496 ppm, respectively [25,26], The closely related amines, 2-methyl-3-hydroxy-4-pyridinone and its N-... 

Food, flavors consist of numerous compounds, none of which alone is characteristic of specific food. Classes of compounds which emcompass food flavors are - hydrocarbons (aliphatic, ali-cyclic, aromatic) carbonyls (aldehydes, ketones) carboxylic acids, esters, imides, anhydrides alcohols, phenols, ethers alkylamines, alkylimines aliphatic sulfur compounds (thiols, mono-, di- and tri-sulfides) nitrogen heterocyclics (pyrroles, pyrazines, pyridines) sulfur heterocylics (thiophenes, thiazoles, trithiolane, thialidine) and oxygen-heterocyclics (lactone, pyrone, furan). Discussion will be limited to striking developments in heterocyclics. 

DIMETHYL-4-OXO-2H-PYRAN-6-CARBOXYLIC ACID n-BUTYLMESITYLOXID OXALATE 2-CARBO-n-BUTOXY-6,6-DIMETHYL-5,6-DIHYDRO-l,4-PYRONE 3,4-DIHYDRO-2,2-DIMETHYL-4-OXO-2H-PYRAN-6-CARBOXYLIC ACID-n-BUTYL ESTER DIHDYROPYR-ONE 0,a-DIMETHYUa -CARBOBUTOXY-DIHYDRO-7-PYRONE 2,2-DIMETHYL-6-CARBOBUTOXY-2,3-DIHYDRO-4-PYRONE ENT 9 INDALONE... 

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