2- pyranones

Good yields of a range of 6-substituted 4-hydroxypyran-2-ones are available by the cyclisation of the stable and storable bis-potassium salts of 5-hydroxy-3-oxopent-4-enoic acids which are obtained by hydrolysis of the corresponding esters the free acids are unstable. The value of this approach is illustrated by the first synthesis of the naturally-occurring 3,3-dimethylallyl conrauanalactone 20 06CC4732 . 

Pyran-2-ones are valuable synthetic reagents and manipulation of the basic molecule enhances their value. The Sonogashira coupling of terminal alkynes with 4,6-dichloropyran-2- 

The Cu(0Tf 2-catalysed HDA reaction between benzaldehydes and 1,3-dimethoxy-l-trimethylsilyloxypenta-1,3-diene occurs with excellent diastereo- and enantioselectivity in the presence of chiral Schiff bases. The absolute configurations of the 5,6-disubstituted 5,6-dihydropyran-2-ones produced were established 06JOC4141 . 

3-Diazo-3,6-dihydropyran-2-one 21 is a stable vinylcarbene precursor. Its reaction with 1,4-cyclohexadiene is highly dependent on the chiral Rh catalyst used but results in both C-H insertion and cyclopropanation. Asymmetric cyclopropanation also occurs with various alkenes and reduction of the products provides a route to cycloheptadienes 06JA16038 . 

In the presence of an imidazole-derived carbene, substituted cinnamaldehydes conjugatively add to tropone leading to the cyclohepta[6]pyranones 22. The overall process corresponds to an [8+3] annulation of tropone 06JOC8964 . 

The most common method for the preparation of pyrazoles from other heterocycles is from pyranone-type compounds. Condensation of 2,3-dihydro /7-pyran -ones 787 with various aryl hydrazines in the presence of montmorillonite KSF clay under mild conditions proceeded rapidly to afford enantiomerically pure 5-substituted pyrazoles 788 (Equation 172) 2004TL6033 . Comparable results were obtained when arylhydrazines were reacted with 2-formyl glycals under microwave irradiation 2004TL8587 . Phenylhydrazine and hydrazine were reacted with 3-acetyl-4-hydroxy-6-methyl-2/7-pyran-2-one to afford 4-acetoacetyl-3-methylpyrazolin-5-ones, which were employed in the synthesis of bipyrazoles and pyrazoloisoxazoles 1999JHC1291 . Reaction of 3,3-dialkyl-6-(trifluoromethyl)-2,3-dihy-dro -pyrones with hydrazine hydrate afforded 3-(trifluoromethyl)-5-substituted-pyrazoles 1998RCB1365 . 

3- (3-aryl-3-oxopropenyl)chromen-4-ones with hydrazine yielded pyrazolyl-2-pyrazolines 2004EJO4672 . Reaction of 3-formylchromenes with phenylhydrazine or with tosylhydrazine under solvent-less microwave irradiation afforded 

The acetonide-protected 4,5-dihydroxy-2-chloroglycidic ester 18 reacts with Mg halides to give 4-halo-3-hydroxypyran-2-ones in good yields together with small amounts of a rearrangement product. It is proposed that loss of acetone from the enohc form of this ester is followed by attack of the released hydroxy function at the ester carbonyl group. Protected 

5-dihydroxy-2-oxo-3-halopentanoates behave in a similar manner 05T2541 . 

Reaction of l,l-bis(trimethylsilyloxy)ketene acetals with 3-silyloxyalk-2-en-l-ones catalysed by MesSiOTf produces 5-ketoalkenoic acids, which are readily cyclised to the pyran-2-one by TFA 0583189 . 

Coupling of 4-tosyloxypyran-2-ones with organozinc reagents or with electron-rich alkenes are useful routes to 4-substituted derivatives 05CL796, 05OL5585 . Both 4-hydroxypyran-2-ones and the related coumarins yield coumestans on reaction with catechols in the presence of O2 and catalysed by lacasse enzymes 05SL3126 . 

Several naturally-occurring pyranone-based polypropionates have been synthesised from pyran-4-ones 05CC1687, 05OL641, 05OL819, 05OL2837 and an enantiopure cyclohexadiene-fused pyran-4-one analogous to one of these has been synthesised 050L407 . An enantioselective synthesis of (-t-)-candelalide has been described 

Maltol (3-hydroxy-2-methyl-4H-pyran-4-one) is obtained from carbohydrates as outlined in 4.2.4.4.4 and has a caramel-like odor. It has been found in a series of foods (Table 5.17), but in concentrations that were mostly in the range of the relatively high odor threshold of 9mg/kg (water). 

Maltol enhances the sweet taste of food, especially sweetness produced by sugars (cf. 8.6.3), and is able to mask the bitter flavor of hops and cola. 

Ethyl maltol [3-hydroxy-2-ethyl-4H-pyran-4-one] enhances the same aroma but is 4- to 6-times more powerful than maltol. It has not been detected as a natural constituent in food. Nevertheless, it is used for food aromatization. 

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Pyran-4-one, 2,2,5-trimethyl-2,3-dihydro-photodimerization, 3, 720 4H-Pyran-4-one, 2,3-dihydro-2,3,5-trimethyl-6-( 1 -methyl-2-oxobutyl)-synthesis, 3, 844 Pyranones alkylation, 2, 56 aromaticity, 3, 632, 633 C NMR, 3, 587, 635 H NMR, 3, 580 cardiac glycosides, 3, 883 chromone synthesis from, 3, 830 colour couplers... 

Pyranones, dihydro-confdrmation, 3, 632 mass spectra, 3, 617 reactions, 2, 62 with halogens, 3, 799 synthesis, 3, 797... 

In transforming bis-ketone 45 to keto-epoxide 46, the elevated stereoselectivity was believed to be a consequence of tbe molecular shape — tbe sulfur ylide attacked preferentially from tbe convex face of the strongly puckered molecule of 45. Moreover, the pronounced chemoselectivity was attributed to tbe increased electropbilicity of the furanone versus the pyranone carbonyl, as a result of an inductive effect generated by tbe pair of spiroacetal oxygen substituents at tbe furanone a-position. ... 

Compounds in carbohydrate-based synthesis of 2,3-dihydro-4//-pyranones as starting compounds for the preparation of C-saccharides, glycosylstan-nanes, and analogs of tromboxane A2. 98EJ02267. 

For this preparation Matheson, Coleman and Bell practical grade dibenzyl ketone was recrystallized once from anhydrous ether at —70°. It melted at 33-34°. Practical grade dibenzyl ketone may be used directly however, the yield of the pyranone is somewhat lower. 

This procedure is a modification of that of Letsinger and Jamison.2 The pyranone has also been prepared by treatment of... 

Treatment of pyranone 15 with 2-methylisourea generates an imine which reacts inter-molecularly to provide the 1,5-diazocine system.38... 

Six-Membered Ring Systems With O and/or S Atoms Pyranones... 

Pyran-4-ones are formed when acyl ketenes derived from dioxofurans and dioxinones react with vinyl ethers. Intermediate products are 1,3,5-triketones and reduced pyranones <96CPB956,96H(43)2457,96TL6499>. 

Complete diastereoselection is observed in the HDA reaction of Danishefsky s diene with o-substituted benzaldehyde chromium tricarbonyl complexes. Decomplexation is facile and good yields of 2-aryl-2,3-dihydropyran-4-ones result <96SL258>. Cis-2,3-disubstituted pyranones are accessible from the Lewis-acid catalysed HDA reaction between (triisopropylsilyloxy) dienes and aldehydes and dehydrogenation of the resulting dihydropyrans <96JOC7600>. 

Cyclisation of o-hydroxyphenyl ethynyl ketones under basic conditions is known to produce benzopyran-4-ones and benzofuranones by 6-endo-dig and 5-exo-dig processes, respectively. However, both cyclisations are reversible in aprotic media thereby generating anions, of which that derived from the pyranone is rapidly and irreversibly protonated and hence selective formation of the chromone results <96T9427>. 

Two syntheses of hongconin (18), a naturally occurring isochroman-4-one which exhibits antianginal activity, have been described. One utilises the annulation of phthalide unit to optically pure dihydropyran-3-ones <96JOC455>, whilst a similar Michael addition to the bicyclic pyranone levo ucosenone and subsequent enolate methylation are essentials of the second route <96JOC459>. 

Helmig D, J Lopez-Cancio, J Arey, WP Harger, R Atkinson (1992b) Quantification of ambient nitrodibenzo-pyranones further evidence for atmospheric mutagen formation. Environ Sci Technol 26 2207-2213. 

Feringa and coworkers have used the optical active furanone or pyranone as an acceptor for the diastereoselective Michael reactions (Eq. 4.133).193... 

Scheme 150).225 227 The pyran products predominate when the ratio of triphenylphosphine to palladium catalyst exceeds two whereas the linear oligomers are the major products when this ratio is close to unity. The suggested227 mechanism (Scheme 151) includes a step of insertion of C=0 into a C—Pd palladium-catalyzed reactions leading to the formation of pyranones (see Scheme 152)228 and piperidones (see Scheme 139 in Section V,A,2).211 It is useful to note that the 2,5-divinyltetrahydropyran derivative can be transformed catalytically by ruthenium trichloride into synthetically useful 3,4-dihydro-2//-pyran derivatives (Scheme 153).229... 

Stoichiometric reactions leading to the formation of pyranones and pyrans are illustrated in Scheme 154,41,230 but few synthetic applications can be envisaged for processes of this type. 

Conversion of furfuryl alcohol derivatives 48 to pyranones 49 (Achmatowicz oxidative ring expansion) is employed in the synthesis of spiroketal moiety of a natural product and cyclopentenones <00TL6879>. 

The Pd-catalysed carbonylation of alkynyl epoxides 60 and alkynyldioxolanones 61 leads to the allenes 62 which can then be converted to the same pyranones through a tandem conjugate addition-cyclisation (Scheme 40) <00JCS(P1)3188>. Carbonylation of allenyl alcohols is catalysed by Ru3(CO)i2 and yields 5,6-dihydropyran-2-ones . 

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