Pyran reactions

A review of the use of electrocyclic reactions in synthesis has been presented" and of asymmetric electrocyclic reactions that result in diastereo- or enantio-selectivity selection as a consequence of the influence of a chiral component, be it substrate or catalyst, on the electrocyclic bond-forming process." Computational studies of 0 electrocyclic ring-opening reactions of 2-pyrone and 6-fluoro-2-pyrone are pseudoper-icyclic 6-fluoro-2-pyranol are a borderline case and 2-pyranol, pyran, and 6-fluoro pyran reactions are pericyclic in character." ... 

Malatesta, V., Neri, C., Wis, M.L., Montanari, L., and Millini, R., Thermal and photodegradation of photochromic spiroindolinenaphthooxazines and -pyrans reaction with nucleophiles. Trapping of the merocyanine zwitterionic form, /. Am. Chem. Soc., 119, 3451, 1997. 

The chemoselective desilylation of one of the two different silyi enoi ethers in 10 to give the monosilyl enol ether II is realized by the Pd-catalyzed reaction of Bu3SnF. The chemoselectivity is controlled by steric congestion and the relative amount of the reagent[7,8]. An interesting transformation of the 6-alkoxy-2,3-dihydro-6//-pyran-3-one 12 into the cyclopentenone derivative 13 proceeds smoothly with catalysis by Pd(OAc)2 (10 mol%)[9]. 

Vinyl ethers and a,P unsaturated carbonyl compounds cyclize in a hetero-Diels-Alder reaction when heated together in an autoclave with small amounts of hydroquinone added to inhibit polymerisation. Acrolein gives 3,4-dihydro-2-methoxy-2JT-pyran (234,235), which can easily be hydrolysed to glutaraldehyde (236) or hydrogenated to 1,5-pentanediol (237). With 2-meth5lene-l,3-dicarbonyl compounds the reaction is nearly quantitative (238). 

Production of Acrolein Dimer. Acting as both the diene and dienoplule, acrolein undergoes a Diels-Alder reaction with itself to produce acrolein dimer, 3,4-dihydro-2-formyl-2id-pyran, CgHg02 [100-73-2], At room temperature the rate of dimerization is very slow. However, at elevated temperatures and pressures the dimer may be produced in single-pass yields of 33% with selectivities greater than 95%. 

Pyrans and related compounds react with ammonia to give pyridines. A commercially useful example is the reaction of dehydroacetic acid (derived from diketene) with ammonia to give 2,6-dimethyl-4-pyridinone [7516-31 -6] via 2,6-dimethyl-4-pyridinone-3-carboxyhc acid [52403-25-5]. Chlorination of the pyridone gives clopidol [2971-90-6] (56), a coccidiostat (72,73). 

Hydroqulnollnes. Pyrans formed by reactions of a,P-unsaturated aldehydes with 1-ethoxycyclohexene and treated with hydroxjiamine are converted ia good yield to 5,6,7,8-tetrahydroquiQolines (117). These compounds can be dehydrogenated to the corresponding quiaolines. The parent reduced product has been prepared by heating O-aHylcyclohexanone oxime (118). 

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... 

H-pyran synthesis from, 3, 759 bis(trimethylsiloxy) in pyrrole synthesis, 4, 333 chromene synthesis from, 3, 750 cycloaddition reactions with isocyanates, azetidin-2-ones from, 7, 261 dihydropyran synthesis from, 3, 771 fuiyl... 

Furfural — see Furan-2-oarbaldehyde, 532 Furfuryl acetate, o -(butoxycarbonyl)-anodic oxidation, 1, 424 Furfuryi acrylate polymerization, 1, 279 Furfuryl alcohol configuration, 4, 544 2-Furfuryl alcohol polyoondensation, 1, 278 reactions, 4, 70-71 Furfuryl alcohol, dihydro-pyran-4-one synthesis from, 3, 815 Furfuryl alcohol, tetrahydro-polymers, 1, 276 rearrangement, 3, 773 Furfuryl chloride reactions... 

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... 

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