Pyrano pyran-5-ones

Pyrano[3,2-g]benzoxazine, dihydrosynthesis, 3, 714 Pyranobenzoxazines synthesis, 6, 190 Pyranobenzoxazoles mass spectra, 3, 615 Pyrano[2,3-a]carbazoles synthesis, 4, 235 Pyrano[2,3- 6]carbazoles synthesis, 4, 235 Pyrano[3,2-a]carbazoles synthesis, 4, 235 Pyranochromones synthesis, 3, 821 Pyranochromones, dihydrosynthesis, 3, 81 817 Pyranocoumarins crystal data, 3, 623 mass spectra, 3, 610 Pyranodipyranones synthesis, 3, 794 Pyrano[3,2-6]indol-4-ones synthesis, 4, 302 Pyran-2-ol, dihydrodehydration, 3, 762 Pyran-2-ol, 3-methyltetrahydro-synthesis, 3, 775 Pyran-2-ol, tetrahydro-6-substituted synthesis, 3, 775 Pyran-4-ol, tetrahydro-IR spectra, 3, 594 Raman spectra, 3, 594 Pyranols, tetrahydro-bond lengths, 3, 621 synthesis, 3, 777... 

In a similar manner, pyrano[4,3-A]quinolizines 88 can be synthesized starting with substituted pyran-2-ones 87 (Scheme 5) <1999S1884>. 

Both natural and non-natural compounds with a 2ff,5ff-pyrano[4,3-fc]pyran-5-one skeleton are of interest in medicinal chemistry. Several natural products, such as the pyripyropenes, incorporate this bicyclic ring system. The group of Beifuss has described an efficient microwave-promoted domino synthesis of the 2ff,5H-pyr-ano[4,3-fo]pyran-5-one skeleton by condensation of a,/3-unsaturated aldehydes with 4-hydroxy-6-methyl-2]-f-pyran-2-one (Scheme 6.244) [428]. It is assumed that in the presence of an amino acid catalyst a Knoevenagel condensation occurs first, which is then followed by a 6jr-electron electrocyclization to the pyran ring. While the conventional thermal protocol required a reaction time of up to 25 h (refluxing ethyl... 

Scheme 6.244 Domino Knoevenagel condensation/6jr-electron electrocyclization in the synthesis of 2H,5H-pyrano[4,3-b]pyran-5-ones.

N-Methyl-4-hydroxy-2-quinolone 183 in a three-component reaction gives spiro pyrano[3,2-c]qionoline-2-ones (02MI2), but indolones 213 with 4-hydroxy-6-methyl-2-pyridone 177 react in a complicated way (97BCJ1625). Ethoxycarbonyl-methylene indolone (Z = COOEt) forms pyrans 224, while dicyano analogs (Z = CN) yield substituted quinoline 225 (Scheme 85). 

As part of extensive studies lasting over 30 years on the structures of chromophores involved in nonenzymatic browning reactions, two intensely orange, previously unknown, compounds have been identified (2R,8aR)-l and ZS, 8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2//,7//,8a//-pyrano[2,3-3]pyran-3-one <1998CAR215>. Additional studies on the single Maillard reaction products of these compounds have also been reported <1998JFA3912>. 

Hydroxylamine converts pyran-2-ones into pyridine IV-oxides by attack at the 1,2-bond for example, the pyran-2-one (278), obtained by heating 3-acetyI-4-hydroxy-6-methyl-pyran-2-one (dehydroacetic acid) and DMF dimethyl acetal in xylene, reacts with two moles of hydroxylamine under mild conditions to give the pyrano[2,3-6]pyridine JV-oxide (279) (77JHC931). Hydrazine converts pyran-2-ones into l-amino-2-pyridones (78JHC759). 

The reactivity of 47/-benzopyran-4-ones in Diels-Alder reactions is well documented <1987T3075>, and recently high asymmetric induction has been achieved in the reaction of 3-alkoxycarbonyl-substituted chromones with chiral auxiliaries and Danishefsky s diene <1991JOC2058>. It should be noted that 3-formylchromones can react as heterodienes in the stereoselective inverse electron Diels-Alder reaction with enol ethers <1994T11755> to provide a route to pyrano[4,3-A][l]benzopyrans a heterocyclic nucleus which occurs naturally in the fungal metabolite fulvic acid <1984CC1565>. The thermal Diels-Alder reaction of 477-pyran-4-one 405 in the presence of an excess of Danishefsky s diene 404 provided cycloadduct 406 (Equation 32) <1996H(43)745>. 

The reaction of carbonsuboxide with 2 equiv of an internal alkyne affords a mixture of pyran-2-ones and pyran-4-ones (Equation 261). 3,3,7,8-Tetraalkyl-2//-cyclobuta[ pyrano[2,3-internal alkynes with an excess of carbonsuboxide (Equation 262) <2003JHC321>. 

Carbanion-induced ring transformations of pyran-2-ones include their conversion into unsymmetrical biaryls <07TL7283>, axially chiral quateraryls <07JOC7765>, 2-amino-isophthalonitriles <07T10971>, arylated indans <07T10300> and pyrano[3,4-c]pyran-l,8-diones <07TL8998>. 

Triketides are relatively rare. Triacetic acid lactone (4.2) has been detected in Penicillium patulum. It is also produced by fatty acid synthase in the absence of the reductant NADPH. Radicinin (4.3) is a major phytotoxin isolated from Ahernaria radicina (Stemphyllium radicinum) which causes a black rot of carrots. It is also formed by other Ahernaria species. Its pyrano[4,3- ]pyran structure, the identification of which had eluded purely chemical degradative studies, was established in one of the earlier applications of NMR spectroscopy to natural product structure elucidation. The biosynthesis of radicinin from acetate units was studied in 1970 by both radio-isotope methods using carbon-14 and by carbon-13 enrichment studies with NMR methods of detection. This was one of the first applications of this NMR technique to biosynthetic problems. These results established the labelling pattern for radicinin shown in 4.3. 

Naphthyridin-2(1 //)-ones 66 were prepared from pyrano[4,3-/ ]pyridine-2,7-diones 67 by successive treatment with NH3, HBr and Zn in acetic acid (1993H1). It was believed that the attack of the ammonia molecule on the C(5) atom led to pyran ring opening. Subsequent decarboxylation afforded a tautomeric mixture of substituted tetrahydropyridines 68a,b. Dinitrile 69 generated by elimination of ammonia underwent cyclization under the action of HBr to form 1,6-naphthyridine derivative 70. Dehydrobromination with Zn in acetic acid gave the final products 66. 

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