Pyrones, preparation

For the preparation of 4-substituted coumarins, a phenol may be condensed with ethyl acetoacetate under the influence of sulphuric acid. Thus resorcinol (II) readily undergoes this condensation (which is represented diagrammatically above) to give 7-hydroxy-4-methyl-coumarin (III). Note that the coumarins, like all 2 pyrones, are systematically lactones. 

Additional applications are exemplified by the well-known Meystre-Miescher degradation of the bile acid side chain and, more recently, in the preparation of a-pyrones from a,iS-unsaturated lactones. ... 

Baeyer and Piccard were the first to prepare crystalline monocyclic pyrylium salts without hydroxy or alkoxy substituents, from y-pyrones and Grignard reagents in 1911. They ascribed a correct structure to these salts, although the bonds in the ring and the valency of the oxygen heteroatom remained contested topics for the next 20 years. The discussions around the formula of pyrylium... 

Mention can be made here of the preparation of 4-pyrones by diacylation of 2-propanones (213), which had been studied by Skraup and Priglinger, and by Philippi and Seka for acetone which forms on diacetylation 2,6-dimethylp3 rone. More recently, two independent research groups, Letsinger and co-workers and... 

Preparation of y-pyrone by the reaction of methoxybutenone with formic acid esters (10°C, MeONa, CgHe, 1.5 h) has been reported (73 JPP7229512 80MI2). The intermediate 292 undergoes [3,3]-sigmatropic rearrangement to form methoxydi-hydropyrone 293 which further eliminates methanol, thus transforming to y-pyrone. 

The only example of the preparation of a benzazepine from a l//-azepine involves the loss of carbon dioxide from the initially formed [4 + 2] cycloadduct of methyl 2-pyrone-5-carboxylate (8) with ethyl l//-azepine-l-carboxylate (7).106 3-Ethyl 7-methyl 3//-3-benzazepine-3,7-dicar-boxylate (9) is formed as the minor product along with the [6 + 6] 7i-dimer 10 of the l//-azepine. 

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

The [2-I-2-I-2] cycloaddition reaction of diynes 40 and carbon dioxide 41 were successfully catalysed by a NHC-nickel (Scheme 5.12) [15]. The NHC-Ni complex was prepared in situ from [NiCCOD) ] and two equivalents of carbene. Pyrones 42 were obtained in excellent yields at atmospheric pressure of CO and mild reaction conditions. 

Piva and coworkers [236] prepared functionalized butenolides and pyrones using a combination of a ring-closing and a cross-coupling metathesis. Thus, reaction of a... 

Ring contractions of pyran derivatives are occasionally valuable. The contraction of 3-halo-2-pyrones to 2-furoic acids under the influence of alkali has been studied and the conditions defined.58112113 The method is adaptable to the preparation of 3-furoic acid via furan-2,4-dicarboxylic acid58 and of 3,4,5-triphenylfuran-2-carboxylic acid.113 Another ring contraction involving halides is the conversion of 4-chloromethylpyrylium salts into furylmethyl ketones as indicated in Scheme 21.114 Pyridine oxides may be transformed with unexpected ease into furans through treatment with a thiol (Scheme 22).115... 

The starting materials for the chemical synthesis of kojic acid are also carbohydrates, but only hexoses and their derivatives have been used for this purpose. Maurer and his coworkers studied the synthesis extensively in order to clarify the mode of formation of the 7-pyrone system from carbohydrates. They succeeded in preparing di-O-acetylkojic acid (XLIII) by purely chemical means, first from D-glucose54 and then from D-galac-tose.67 The mechanism of formation of XLIII, which does not contain any asymmetric carbon atoms, was apparently not dependent on the configura-... 

Synthesis with sacrificial electrodes is employed as a direct method in several other preparations of organometallic compounds and complexes. 3-Hydroxy-2-methyl-4-pyrone derivatives of Sn 1 (and of Zn, Cu, In and Cd as well) were prepared using the metal as an anode. The low oxidation state Sn(II) compound is obtained by direct electrolysis134. 

Trisila[l.l.l]orthocyclophane (46) was prepared by the reaction of 3,3,6,6,9,9-hexamethyl-3,6,9-trisilacyclonona-l,4,7-triyne and a-pyrone. The structure of 46 determined by X-ray crystallographic analysis (Fig. 28) indicates a twisted saddle conformation (10). 

A combination of an intramolecular [5+21-cycloaddition of the pyrone 110 followed by an intermolecular [4 + 2]-cycloaddition with dimethyl-butadiene leads to the fused 6,7,5-tricyclocarboxylic system 112 via 111 as described by Rodriguez et al (scheme 22).1621 The prepared skeleton is found in nature in several terpenes. The starting material for this domino reaction is obtained from maltol 109... 

Dideoxyhex-2-enono-1,5-lactone derivatives (penten-5-olides) have been prepared (255-258) and employed as starting compounds in synthesis. Thus, Michael addition of benzylhydroxylamine to racemic 6-0-acetyl-2,3,4-trideoxy-D,L-g/ycerohex-2-enono-1,5-lactone (267) took place ster-eoselectively to give the unstable benzyloxyamino-2-pyrone 268, which was readily converted into the /Mactam derivative 269, a precursor of thienamy-cin (259). / -Lactams were also obtained (260) by 1,3-dipolar cycloaddition of nitrone 270 to the unsaturated 1,5-lactone 267, followed by hydrogenoly-sis and subsequent cyclization to the /Mactam 271, having a polyol side-chain at the C-3 position. 

It has been shown22 that the reaction of the diene 4 with aldehydes RCHO in the presence of a catalyst prepared from (R)-BHMOL (33) and Ti(OPr-i)3, which affords the dihydro-y-pyrones 35 in good yields and high ee, proceeds by a two-step sequence via the open-chain adducts 34, which cyclize to the products on treatment with trifluoroacetic acid (equation 20). 

Although the preparative chemistry of (vinylketene)cobalt(I) complexes is relatively limited in the literature, the methods used include all the major procedures that have been more widely exploited in the analogous chromium and iron systems. There are many similarities between the intermediates involved in the synthesis of vinylketene complexes of iron, chromium, and cobalt, but as the metal is varied the complexes containing analogous ligands often exhibit significant differences in stability and reactivity (see Sections II and VI). Comparison of such species has often been an important aim of the research in this area. The (vinylketene)cobalt(I) complexes have also been shown to be synthetically useful precursors to a variety of naphthols, 2-furanones, ce-pyrones, phenols,6,22,95 >8, y-unsaturated esters,51 and furans.51,96a... 

Gleitz J, Beile A, Wilkens P, Ameri A, Peters T. (1997). Antithrombotic action of the kava pyrone (+)-kavain prepared from Piper methysticum on human platelets. Planta Med. 63(1) 27-30. 

The intermediate J4 was prepared by an "intramolecular Narasaka type Diels-Alder reaction" of dienophile 10 and 3-hydroxy-2-pyrone (J L) in the presence of phenylboronic acid [7] ... 

A series of Tc complexes with the monobasic ligands 2-methyl-3-oxy-4-pyronate (maltolate, Hma (78)) or l,2-dimethyl-3-oxy-pyridinonate, (Hdpp, (79)) were prepared in nonaqueous solution by chloride substitution in [TcOCU]. Stepwise substitution is observed. The substitution of one... 

One question remained to be resolved regarding the structure of cyclobutadiene. Chapman reported in 1973 that in an attempt to separately prepare the two dideuterio cyclobutadienes ([1,2- H2]-1 and [1,4- H2]-1) from the two dideuterio 2-pyrones ([3,6- H2]-2 and [5,6- H2]-2), both yielded the same IR spectrum. Of course if cyclobutadiene were square only a single IR spectrum would have been expected, while if it were rectangular one would have expected different IR spectra for the two dideuterio cyclobutadienes. The IR spectra of the two dideuterio cyclobutadienes were computed, and indeed they were found to be quite different from each other. When a 1 1 mixture of the two calculated spectra was compared with Chapman s spectrum (see Figure 8) with inclusion of additional bands later observed by Michl, it was immediately obvious that Chapman... 

A similar synthesis of mechanistic interest rather than preparative value involves the thermal reaction of dimethyl 2,5-bisdiazo-3,4-diketoadipate (89, Scheme 23) with benzofuran (91)." The presumed intermediate is the pyrone cation 90 produced from the adipate 89 by the Wolff rearrangement, cyclization, and loss of nitrogen. Electrophilic substitution then affords the benzofuran 92, which can be isolated. Ring opening and cyclization of the resultant ketene 93 then affords the dibenzofuran 94 in poor (0.4%) yield. 

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