3.4- Dihydro-2-pyranones

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. 

Acyl ketenes also react with a variety of dienophiles such as enol ethers to give the corresponding 2-alkoxy-2,3-dihydro-4-pyranones [147]. 

Aryl-2,3-dihydro-4-pyranones. The hetero-Diels-Alder reaction of Danishefsky s diene and aromatic aldehydes is catalyzed by the metallocenium complex. The adducts are readily hydrolyzed. 

Lithiation of A,N-disubstituted enaminones 347 with lithium tetrameth-ylpiperidine (LTMP) in THF generates the a-anions, which are trapped with aldehydes or ketones to give the alcohols 348. These cyclize with loss of dimethylamine upon acidification to give dihydro-4-pyranones 349 in 37-67% overall yields. Scheme 98 (78TL315). 

Planar chiral ortho substituted benzaldehyde chromium complexes are useful compounds for a variety of asymmetric reactions. For example, planar chiral tricarbonylchromium complexes of o-substituted benzaldehydes were reacted with Danishefsky s diene in the presence of Lewis acid at room temperature to afford the chromium-complexed 2,3-dihydro-4-pyranones 25 with high dias-tereoselectivity (Eq. 15) [14]. The high diastereoselectivity of the formation of cycloaddition products 25 is also contributed to an exo-side approach of the diene to anti-oriented carbonyl oxygen of the planar chiral ortho substituted benzaldehyde chromium complexes. When the reaction takes place at lower temperature, aldol-type condensation product 26 was obtained along with the formation of pyranones. The open intermediate 26 was easily transformed to the corresponding cycloaddition product after stirring at room temperature [14]. 

By employing the same strategy. Smith and co-workers described an asymmetric synthesis of tri- and tetra-substituted trifluoromethyl dihydro-pyranones via an NHC-catalyzed redox process with a,p-unsaturated triflu-oromethylketones and a-aroyloxyaldehydes. This process accommodates variation at both the a- and p-positions within the trifluoromethylenone acceptor, as well as incorporation of the pharmaceutically relevant trifluoromethyl unit, producing synthetically useful products in good yields, dias-tereoselectivity, and enantioselectivity (up to 97% yield, >95 5 dr, 99% ee) (Scheme 7.85). 

First discovered by Otto Paul Hermann Diels and Kurt Alder in 1928, the Diels-Alder reaction involves the cycloaddition of a conjugated diene with substituted alkene (dienophile) to yield a substituted cyclohexene product. These products can be further used in the synthesis of natural products. A lesser-known reaction is the hetero-Diels-Alder (HDA) reaction. In 1974, Danishefsl et al. developed a diene that can easily produce dihydro-pyranones. ° Since then, much research has been performed with Danishefksy s diene in order to develop new Lewis-acid catalysts for Diels-Alder reactions. 

Similar work on the union of hydroxylynones and ethyl acrylate was reported by Silva et al. [78] (Scheme 6.55). Highly functionalized tri- or tetrasubstituted dihydro-pyranones could be afforded efficiently in the presence of palladium catalyst. 

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... 

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... 

Thermolysis of D-fructose in acid solution provides 11 and 2-(2-hydrox-yacetyl)furan (44) as major products. Earlier work had established the presence of 44 in the product mixtures obtained after acid-catalyzed dehydrations of D-glucose and sucrose. Eleven other products were identified in the D-fructose reaction-mixture, including formic acid, acetic acid, 2-furaldehyde, levulinic acid, 2-acetyl-3-hydroxyfuran (isomaltol), and 4-hydroxy-2-(hydroxymethyl)-5-methyl-3(2//)-furanone (59). Acetic acid and formic acid can be formed by an acid-catalyzed decomposition of 2-acetyl-3-hydroxyfuran, whereas levulinic acid is a degradation prod-uct of 11. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4//-pyran-4-one has also been isolated after acid treatment of D-fructose.The pyranone is a dehydration product of the pyranose form of l-deoxy-D-eo f o-2,3-hexodiulose. In aqueous acid seems to be the major reaction product of the pyranone. 

X-Ray diffraction shows that the p- nitrophenyl group is equatorial and the morpholinocar-bonyl moiety axial in the isochroman-l-one cis-3,4-dihydro-4-morpholinocarbonyl-3-p-nitrophenyl-l//- 2-benzopyran-l-one (175) <78JCS(P1)1351). The pyranone ring has a distorted twist conformation. 

Cyclopentenones from carbohydrates.9 This transformation involves glycosides (2) obtained by reaction of 6-hydroxy-2,3-dihydro-6//-pyranones (1) with an... 

Thus, the potential role of DMHF as a flavor impact compound and as an intermediate to form additional flavor compounds in heat-abused citrus products should be carefully considered. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4-H-pyran-4-one was also isolated from dehydrated orange juice (37) and is known as a novel nonenzymic browning reaction product. This pyranone has a high threshold value, over 200 ppm (38), and appears to possess negligible odor character. 

Catechin-(7,8-b,c)-4(3-(3,4-dihydroxyphenyl)-dihydro-2(3H)-pyranone (catechin phenylpropanoid lactone)... 

Heating also produces a series of other substances as a result of Maillard reactions. These include cyclotene, maltol and its derivatives (hydroxymaltol, dihydro-maltol), 2,3-dihydro-5-hydroxy-2-methyl4(/7) pyranone (DHM) and its 5-hydroxy derivative (DDMP) and furaneol. The sensory impact of these components can play an important role in developing the toasted , burnt , or caramel aromas of some barrel-aged wines (Cutzach et al. 1997,1999). 

The aromatic zone ZO 5 has a toasty character with fruity caramel overtones, which we identified as 2,3-dihydro-3,5-dihydroxy-2-methyl-4(H)-pyranone (7), or DDMP. 

An additional aromatic zone OZ 6 is also present in some wood extracts and in models of Maillard reactions with proline and glucose. The molecule responsible for the interesting odor of jam and burnt sugar has been identified to 3,5-dihydroxy-2-methyl-2,3-dihydro-4(H)-pyranone or hydroxymaltol [8]. 

A Nazarov cyclisation of l-(3,4-dihydro-2//-pyran-6-yl)-3-phenylpropenones affords cyclopenta[Z)]pyranones (Scheme 9) <03OL4931> and the conjugated ethoxytriene 7 derived from tetrahydropyran-2-one also yields a fused cyclopentenone <03JOC9728>. 

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>. 

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