Pyridine with enolate

It is more reactive than perchloryl fluoride and therefore not without danger. It forms, for instance, a highly explosive product with pyridine. Like perchloryl fluoride it reacts with enol ethers, esters and enamines, but at lower temperature (—78°) to yield the fluorinated ketones as well as addition... 

Alkyl-1,4-dihydropyridines on reaction with peracids undergo either extensive decomposition or biomimetic oxidation to A-alkylpyridinum salts (98JOC10001). However, A-methoxycarbonyl derivatives of 1,4- and 1,2-dihydro-pyridines (74) and (8a) react with m-CPBA to give the methyl tmns-2- 2>-chlorobenzoyloxy)-3-hydroxy-1,2,3,4-tetrahydropyridine-l-carboxylate (75) and methyl rran.s-2-(3-chlorobenzoyloxy)-3-hydroxy-l,2,3,6-tetrahydropyridine-l-carboxylate (76) in 65% and 66% yield, respectively (nonbiomimetic oxidation). The reaction is related to the interaction of peracids with enol ethers and involves the initial formation of an aminoepoxide, which is opened in situ by m-chlorobenzoic acid regio- and stereoselectively (57JA3234, 93JA7593). 

The oxidation of butanone-2, catalyzed by complexes of pyridine with cupric salts, appeared to be similar in its main features [191]. Butanone-2 catalytically oxidizes to acetic acid and acetaldehyde. The reaction proceeds through the enolization of ketone. Pyridine catalyzes the enolization of ketone. Enole is oxidized by complexes of Cu(II) with pyridine. The complexes Cu(II).Py with n = 2,3 are the most reactive. Similar results were provided by the study of butanone-2 catalytic oxidation with o-phenanthroline complexes, where Fe(III) and Mn(II) were used as catalysts [192-194],... 

The normal Hantzsch synthesis leads to a symmetrical product. The diesters formed may be hydrolysed and decarboxylated using base to give pyridines with less substitution. Note that we are using the ester groups as activating species to facilitate enolate anion chemistry (see Section 10.9)... 

The photostimulated reactions of vie aminohalo pyridines with acetone or pinacolone enolate ions lead to azaindoles in high yields (75-95%) [67]. When the amino group is protected as pivaloylamino derivative, (as in 17, Sch. 20), the analogs of substitution compounds 18 obtained by the photoinduced reaction of 2-amino-3-iodo-, 3-amino-4-iodo- and 4-amino-3-iodopyridines with acetone or pinacolone enolate ions, afford 5-, 6- and 7-azaindoles in almost quantitative yields by cyclization upon deprotection of the amino group and dehydration under acidic conditions (for example, see Sch. 20) [67]. 

Pyridine is an important cofactor in the reaction system that leads to cleaner reactions and better yields of products than in its absence.15 It can act either as a u-donor for Pb(rv) or as a base catalyzing the keto-enol tautomerism. The u-donor effect was evidenced spectroscopically by the formation of adducts of pyridine with lead tetraacetate.45,4511 Moreover, pyridine catalyzed the ligand redistribution of twcfc-methoxyphenyllead triacetate to bis( r/ -mcthoxy-phenyl)lead diacetate. Other u-donor catalysts can be used and their nature is highly important for the success of the reaction. NaOMe and HOBT showed a modest effect, but a thousand-fold increase in rate over the uncatalyzed reaction was observed when 1,10-phenanthroline was employed and near quantitative yields of arylation products were obtained (Equation (16)).44... 

Spiroacylal 2 was designed under the rationale that the constraint of the carbonyl groups into a conformation in which overlap of their 7r-orbitals with the bent bonds of the cyclopropane is assured should dramatically increase the vulnerability of the cyclopropane toward nucleophilic attack.8 Experimental support for this notion is abundant.8 Spiroacylal 2 is considerably more reactive than 1,1-dicarbethoxycyclopropane in such reactions. For instance, reaction of 2 with piperidine occurs at room temperature. The corresponding reaction in the case of the diester is conducted at 110°C.5 Reactions with enolates also occur under mild conditions.8 Compound 2 reacts with the weak nucleophile pyridine at room temperature to give a betaine.8 An illustrative mechanism for the reaction of the acylal 2 with aniline to afford 2-oxo-l-phenyl-3-pyrrolidinecarboxylic acid (3) is... 

A more sensitive test for phenols consists of dissolving or suspending 15 mg of the unknown in 0.5 mL of chloroform and adding 1 drop of a solution made by dissolving 0.1 g of iron(III) chloride in 10 mL of chloroform. Caution CHCI3 is a carcinogen.) Addition of a drop of pyridine, with stirring, will produce a color if phenols or enols are present. 

Sosnovskikh, V. Y., Irgashev, R. A., Khalymbadzha, I. A. and Slepukhin, P. A. (2007) Stereoselective hetero-Diels-Alder reaction of 3-(trifluoroacetyl)chromones with cyclic enol ethers synthesis of 3-aroyl-2-(trifluoromethyl)pyridines with to-hydroxyalkyl groups. Tetrahedron Lett., 48, 6297-6300. 

The photostimulated reactions of the isomeric dihalo-pyridines with pinacolone potassium enolate (Me CC0CH2 K+) in liquid ammonia lead to the facile replacement of both halogens parallel reactions with the potassium salts of phenylacetonitrile and -ethylphenylacetonitrile afford mixtures of mono- and di-substituted products. These displacements occur by S fjl-type mechanisms.5 ... 

Like enamines, enol ethers are sufficiently electron rich to undergo flZfl-Diels-Alder chemistry with high efficiency under mild conditions. For example, 2,3-dihydrofuran 90 reacts with triazines 89 to give pyridines with pendant protected alcohols 91 (09T975) (Scheme 31-A) or pyridines bearing annulated lactones 92 (04T8893) (Scheme 31-B) as the ultimate products, depending on the substitution pattern of the 1,2,4-triazine 89. 

An alternative reagent, trimethylsilyldiazomethane in hexane, is commercially available and appears useful for preparation of bile acid methyl esters [62], Like carboxylic acid groups, hydroxyl groups also require protection prior to GC-MS analysis. A common and mild method to prepare trimethylsilyl (TMS) ethers is to react the sample with a mixture of dry pyridine, hexamethyldisilazane (HMDS), and trimethylchlorosilane (TMCS), 3 2 1 or 9 3 1 (by volume) with, or without heating [32], However, if 0X0 groups are present in the bile acid, this reaction can yield enol-TMS ethers and multiple products. This artifact can be avoided by converting the 0X0 group into an oxime, usually a methyloxime (MO). To do this, the sample is dissolved in 50 pL pyridine with 5 mg methoxyammonium chloride and heated for 30 min at 60°C [32]. 

Stable and has to be prepared not more than a few days before use, either by irradiating aqueous pyridine with ultraviolet light according to equation (52) or by treatment of JV-(4-pyridyl)pyridinium chloride hydrochloride with alkali according to (53). Products 53 and 54 of these reactions are enolate salts derived from glutaconic aldehyde (55), and are able to condense with primary amines in the presence of acid (equation 54)... 

As one would expect, oximes and nitroso-compounds are frequently encountered starting materials throughout the chemistry of this group of heterocycles. Further examples of their reactions have appeared. Reaction with enol ethers produces dihydro-oxazines (260), which may be converted to pyridine N-oxides upon treatment with HCl. Dihydro-oxazines are also obtained upon treatment of 7,6-unsaturated dicarbonyl compounds with nitrous acid. Thermolysis of these products leads to nitrones (261). 

The basic properties of pyridines make themselves evident in nucleophilic as well as in electrophilic processes an example is the formation of acyl-pyridinium ions which can react with enolate anions (p. 205). Again the orientation of attack revealed in the isolated product has no bearing on the relative reactivities of C(2) and C(4). 

The y-lactbne is an internal ester that also forms with relative ease, provided the sialic acid can assume the open-chain conformation (Figure 8). Reaction of NANA with dicyclohexyl carbodiimide in cold pyridine, for example, produced the lactone in crystallizable form (Derevitskaya et aL, 1966). Small quantities of the 2,6,7,8,9-penta-O-acyl derivatives of NANA-y-lactone were produced by acylation of NANA in cold pyridine with acetic anhydride or benzoylchloride (Khorlin and Privalova, 1967). The Cg-carbonyl of the lactone can exist in both keto (XXVII) and enol (XVII) forms, the latter being susceptible to ozonolysis (Figure 6). 

The furo- and pyranobenzopyranones 114 and 115 are prepared by the reaction of 0-enolate of i(-keto lactone 113[132], The isoxazolc 117 is obtained by the oxidation of the oxime 116 of a, /3- or, d, 7-unsaturated ketones with PdCh and Na2C03 in dichloromethane[l 33], but the pyridine 118 is formed with PdCl2(Ph3P)2 and sodium phenoxide[134]. 

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