2- Hydroxy-2//-pyrans

Insufficient information concerning the transformation of unsaturated 1,5-dicarbonyl precursors to 2//-pyrans is available. A reinvestigation of an earlier report87 on a possible 2-hydroxy-2//-pyran sodium salt formation after the decomposition of 1-benzoylpyridinium ion with hydroxide ion would be desirable. The formation of 2-hydroxy-2,4,6-triphenyl-2//-pyran (41) from the appropriate 1,5-diketone (40) was also reported.88... 

With hydroxide anions (or with water at pH >5) pyrylium salts form pseudobases 16 (2-hydroxy-2//-pyrans) that tautomerize to acyclic 3-penten-l,5-diones 17 via a thermally allowed electrocyclic reaction. Most ring transformation reactions of pyrylium salts involve a similar ring-opening. 

The reaction of 2-methyl-4,6-diphenylpyrylium is typical the immediate 2-hydroxy-2-//-pyran, which is a cyclic enol hemiacetal, is in equilibrium with a dominant concentration of the acyclic tautomer, reached probably via a proton-catalysed process, since methoxide adducts remain cyclic. Treatment of such acyclic unsaturated diketones with acid regenerates the original pyrylium salt (section 8.3.1). 

From measurements of the IR carbonyl frequencies, it was inferred that the 4-hydroxy-pyran-2-one (230) and the 2-hydroxypyran-4-one (231) exist as such in the solid state. In ethanol solution, however, (231) gives the tautomer (232) (59JA2427). 

The self-condensation of 1,3-dicarbonyl compounds provides a useful route to 4-hydroxy-pyran-2-ones and is catalyzed by acids or bases. Alcohol is continuously removed during the reaction. Amongst a number of examples, mention can be made of the detailed procedure for the synthesis of 3-acetyl-4-hydroxy-6-methylpyran-2-one (dehydroacetic acid) (550SC(3)23l) and the formation of the cyclopentyl derivative (321) (64RTC39). Deacylation at C-3 can generally be achieved on heating with acid. 

One of two products- can be obtained from the tetraketone (348), tautomeric with the furanone (349), depending on the oxidant (80TL1575). Lead(IV) acetate gives the 4-hydroxy-pyran-2-one (350), whereas the pyran-4-one results from the use of iodosobenzene diacetate (Scheme 109). 

Hydroxy pyran-4-ones Ab initio and DFT with numerous basis sets Stabilities 2003JMT(639)87... 

Steinbaugh, B.A., Hamilton, H.W., Vara Prasad, J.V.N., Para, K.S., Tummino, P.J., Fergusson, D., Lunney, E.A., and Blankley, C.J. 1996. A topliss tree analysis of the HlV-protease inhibitory activity of 6-phenyl-4-hydroxy-pyran-2-ones. Bioorganic and Medicinal Chemistry letters, 6(10) 1099-104. 

Patent Synthesis of 3,6-Dialky 1-5,6-Dihydro-4-Hydroxy-Pyran-2-One... 

Grove, J.E Phomopsolide A and B, Tiglic Esters of Two 6-Substituted 5,6-Dihydro-5-hydroxy-pyran-2-ones. J. Chem. Soc. (London) Perkin Trans. 1, 865 (1985). 

D-fMO MO-Nonitol, 2,6-anhydro-3,S,7-trideoxy-l-C-([hydroxy-(tetrahydro-2-methoxy-5,6-dimethyl-4-methylene-2 H-pyran-2-yl) acetyl] amino)-5,5-di methyl-1, S,9-tri-0-rriethyl-, 2R-[2( j(,2[S (5-q], 5P,6P] -... 

J-Pyran-2-acetamid, Ai-[[6-(2,3-dimethoxypropyl)tetrahydro-4-hydroxy-5,5-dimethyl-2H-pyran-2-yl]rriethoxymethyl]tetrahydro-a-hydroxy-2-methoxy-5,6-dimethyl-4-rTiethylene]-... 

Chemically, wood tar is a complex mixture that contains at least 200 individual compounds, among which the foUowing have been isolated (1) 2-methoxyphenol, 2-methoxy-4-ethylphenol, 5-meth5i-2-methoxyphenol, 2,6-x5ienol, butyric acid, crotonic acid, 1-hydroxy-2-propanone, butyrolactone, 2-methyl-3-hydroxy-4JT-pyran-4-one, 2-methyl-2-propenal, methyl ethyl ketone, methyl isopropyl ketone, methyl furyl ketone, and 2-hydroxy-3-methyl-2-cyclopenten-l-one. 

H-Pyran-4-one, 2,6-diphenyl-dipole moment, 3, 626 <33JCS87, 54AC(R)430) 4H-Pyran-4-one, 2,6-diphenyl-3,5-dimethyltetrahydro-dipole moment, 3, 627 (37JCS1088) 4H-Pyran-4-one, 3-hydroxy- C NMR, 3, 589 (76JOC2777)... 

Acetic acid, dehydro- — see also Pyran-2-one, 3-acetyl-4-hydroxy-6-methyl-fungicidal properties, 3, 883 H NMR, 3, 581 reactions, 3, 690 with amines, 3, 700 reduction, 3, 843 synthesis, 2, 90 tautomerism, 3, 643... 

Kojic acid — see also Pyran-4-one, 5-hydroxy-2-hydroxymethyl-, 3, 611 acylation, 3, 697 application, 3, 880 occurrence, 3, 692 reactions, 3, 714, 715 with amines, 3, 700 with phenylhydrazine, 3, 700 synthesis, 3, 810 Kokusagine occurrence, 4, 989 Kokusaginine occurrence, 4, 989 synthesis, 4, 990 Koopmans theorem, 2, 135 Kostanecki-Robinson reaction chromone and coumarin formation in, 3, 819-821 mechanism, 3, 820 flavones, 3, 819... 

Maltol — see Pyran-4-one, 3-hydroxy-2-methyl-Maltol, ethyl — see Pyran-4-one, 3-ethoxy-2-methyl-Maltol, methyl — see Pyran-4-one, 3-methoxy-2-methyl-Mandelic acid ethyl ester, 1, 310-311 Manganaindene reactions, 1, 671 Mannich bases dehydro... 

Pyran, 2-halotetrahydro-dipole moments, 3, 628 halogen displacement reactions, 2, 64 C NMR, 3, 587 Pyran, 2-hydroxy-synthesis, 3, 741 Pyran, 3-hydroxy-synthesis, 2, 91... 

H-Pyran, 2-hydroxy-2,4,6-triphenyl-IR spectra, 3, 593 2H-Pyran, 2-methoxy-synthesis, 3, 741... 

Pyran-2-one, 3-acetyl-4-hydroxy-6-methyl-synthesis, 3, 792 Pyran-2-one, 4-alkoxy- H NMR,3, 581 Pyran-2-one, 6-alkoxy-synthesis, 3, 791... 

Pyran-2-one, 5,6-dibromo-5,6-dihydro-reactions, 3, 735 Pyran-2-one, 5,6-dihydro-allylic bromination, 3, 799 dehydrogenation, 3, 724, 799 H NMR,3, 581 synthesis, 3, 841, 843 Pyran-2-one, 4,6-dimethyl-irradiation, 3, 677 photochemistry, 3, 678 Pyran-2-one, 5,6-dimethyl-chloromethylation, 3, 680 conformation, 3, 631 Pyran-2-one, 5-f ormyl-IR Spectra, 3, 595 Pyran-2-one, 6-formyl-IR spectra, 3, 595 Pyran-2-one, 5-halo-synthesis, 3, 799 Pyran-2-one, 3-hydroxy-IR spectra, 3, 595 Pyran-2-one, 4-hydroxy-methylation, 2, 57 3, 676 pyran-4-one synthesis from, 3, 816 reactions with phosphorus oxychloride, 2, 57 synthesis, 3, 792, 794, 795, 798 tautomerism, 2, 56 3, 642 Pyran-2-one, 4-hydroxy-6-methyl-methylation, 3, 692 reactions... 

Pyran-2-one, 4-hydroxy-6-methyl-5,6-dihydro-halogenation, 3, 723 reactions... 

Pyran-2-one, 4-hydroxy-6-phenacyl-benzophenones from, 3, 686 Pyran-2-one, 4-hydroxy-6-phenyl-synthesis, 3, 795 Pyran-2-one, 4-methoxy-6-substituted... 

Pyran-4-one, 3-ethoxy-2-methyl-, 3, 612 IR spectra, 3, 595 Pyran-4-one, 2-hydroxy-tautomerism, 3, 642 Pyran-4-one, 3-hydroxy-appiication, 3, 880 synthesis, 2, 91 3, 815 Pyran-4-one, 5-hydroxy-2-hydroxymethyl- C NMR, 3, 588 mass spectra, 3, 611... 

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