Formation of O- Heterocycles

Recently, Zhang prepared a series of four trans-perhydroindohc acid isomers as organocatalysts to oversee a reaction to 3,4-dihydro-2H-pyrans 128. In this 

The tetrahydropyranol ring system was also formed in a domino Michael-ketabzation reaction. The reaction between cyclohexanone and nitro alkenols in the presence of prohne-based organocatalysts was used to successfully prepare fused cyclohexyl pyran derivatives [111]. Rodrigues and Coquerel reported an NHC-catalyzed Michael-based spirocyclization from 1,3-dicarbonyl compounds bearing 

The MARDi domino process was also successfully applied to the synthesis of azepanes, azepines, and thiepines. 

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Formation of O-Heterocycles as Major Products of the Gaseous Oxidation of -Alkanes... 

The gaseous oxidation of n-alkanes can, in suitable circumstances, yield substantial amounts of O-heterocycles of the same carbon number as the initial hydrocarbon. A comparative study has been carried out of the formation of O-heterocyclic products during the combustion of n-butane, n-pentane, and n-hexane. The way in which the yields of such compounds vary with reaction conditions has been investigated. As a result of the optimization of the amounts of O-heterocycles it has been possible to obtain maximum yields of these compounds of up to 30% from n-pentane but only about 10% from n-butane and n-hexane. An attempt is made to account for the observed differences in the amounts and nature of the O-heterocyclic products formed from the three n-alkanes. 

In the present work, therefore, a comparative study of the production of O-heterocycles during the cool-flame combustion of three consecutive n-alkanes—viz., n-butane, n-pentane, and n-hexane—was carried out under a wide range of reaction conditions in a static system. The importance of carbon chain length, mixture composition, pressure, temperature, and time of reaction was assessed. In addition, the optimum conditions for the formation of O-heterocycles and the maximum yields of these products were determined. The results are discussed in the light of currently accepted oxidation mechanisms. 

Formation of O-heterocycles with participation of carbonyl oxides 91 CRV335. 

With R and RO2 fully equilibrated in the temperature region 600-800 K, and considerable evidence that A ioc k.g [23,78], the rate of formation of O-heterocycle is given by equation (1.12). 

Hydrazidoyl halides in synthesis of N,O-heterocycles 80JHC833. Intramolecular [4 + 2]-cycloaddition with formation of O-heterocycles ... 

Oxidative cyclization of dienes and polyenes mediated by transition-metal-oxo species (formation of O-heterocycles) 07S2585. 

In the presence of a double bond at a suitable position, the CO insertion is followed by alkene insertion. In the intramolecular reaction of 552, different products, 553 and 554, are obtained by the use of diflerent catalytic spe-cies[408,409]. Pd(dba)2 in the absence of Ph,P affords 554. PdCl2(Ph3P)3 affords the spiro p-keto ester 553. The carbonylation of o-methallylbenzyl chloride (555) produced the benzoannulated enol lactone 556 by CO, alkene. and CO insertions. In addition, the cyclobutanone derivative 558 was obtained as a byproduct via the cycloaddition of the ketene intermediate 557[4I0]. Another type of intramolecular enone formation is used for the formation of the heterocyclic compounds 559[4l I]. The carbonylation of the I-iodo-1,4-diene 560 produces the cyclopentenone 561 by CO. alkene. and CO insertions[409,4l2]. 

High pressure and selectivity in reactions with formation of O- and N-heterocycles 97T2669. 

The Pschorr reaction, originally applied to the synthesis of phen-anthrene and its derivatives,has been adapted to the formation of new heterocyclic systems.In its original form, it consisted of treating a diazonium salt with copper powder in acid solution in this way, rans-o-amino-a-phenylcinnamic acid was converted into phen-anthrene-9-carboxylic acid, Eq. (20). Variants of the reaction include cyclizations such as that in Eq. (21). The reaction may be horaolytic... 

Schiff bases derived from o-phenylenediamine and from 2-hydroxyaniline show a two-electron wave on cyclic voltammetry in acetonitrile and further waves at more positive potentials due to oxidation of the product from the first wave. Preparative scale reaction at the potential of the first wave leads to formation of a heterocyclic ring product [45]. 

The formation of oxygen heterocycles through carbon-oxygen bond formation was also reported. Substituted 2-(o-halophenyl)-ethanols were converted to dihydrobenzofuranes using palladium and Buchwald s bulky biaryl-type ligands (3.43.). The reaction was also efficient in the formation of six and seven membered oxygen heterocycles.53... 

Possible reasons for the observed sequence of O-heterocycle formation can best be discussed in terms of the generally accepted mechanism for the formation of such compounds during combustion processes (7). 

A. Fish The suggestion that the yield of O-heterocycles is a maximum when the fuel has a C5 chain is supported by the fact that the yields from 2-methylpentane (2) (32%) and n-pentane (28%) are similar. If, as generally agreed, O-heterocycles are formed by the isomerization and decomposition of alkylperoxy radicals, their formation competes with the 0-scission of hydroperoxylalkyl radicals. 

The photoaddition of alcohols and phenols to alkenes has been observed.404 The equivalent intramolecular process results in cyclization and the formation of oxygen heterocycles. Irradiation of 2-allyl-4-(-butylphenol (388) affords 2,3-dihydro-2-methyl-5-<-butyl-benzofuran (389), whereas o-3-methylbut-2-enylphenol (390) gives 2,2-dimethylchroman (391). In both cases, therefore, the addition can be said to occur in a Markovnikov direction. 

Investigation of the reactions between ortho-diamines and dibenzoylethylene 182 causes many problems for researchers. Owing to the polyelectrophilicity of this ketone molecule as well as the possibility of a redox process in the reaction mixtures, there is a potential in such reactions for the formation of various heterocyclic systems. This leads to difficulties in establishing the structure of the reaction products and gives rise to a number of errors. In particular, during the years 1970-1975, five mutually inconsistent reports devoted to the reaction of dibezoylethylene with o-PDA (Scheme 4.53) were published [130, 131, 132, 133, 134]. In these papers five possible structures (183, 184, and 186-188) were... 

The increased susceptibility to pseudobase formation for O-heterocyclic relative to the corresponding N-heterocyclic cations that was noted above for the aromatic series is also seen in the l//-isobenzofurylium (25 X = O) and N-phenyl 1 //-isoindolium (25 X = NPh) series in Table II. For these cases ApKR. (NPh - O) 11.5, which may be converted to ApXR. (NMe - O) % 15.5 using the difference noted previously for N-methyl- and N-phenyl-3,4-dihydroisoquinolinium derivatives. This difference is in reasonable agreement with ApXR + (NMe - O) % 18.5 observed for aromatic cations (Table IV). The low stability of the cation relative to the pseudobase for O-heterocycles is also present in the 2H-furylium cations (26). Although... 

Tertiary Mannich bases having a halogen atom in position 4 with respect to the amino group (Fig. 127) can give cyclic products by intramolecular N-alkylation, with formation of the heterocyclic quaternary salts 330, as shown by the N-Mannich bases of chloroacctohydrazidc and by some brominated O-Mannich bases. ... 

The importance of alkylperoxy radicals as intermediates had long been realized (see Sect. 2) and their subsequent reaction to yield the alkyl-hydroperoxide or decomposition products such as aldehydes and alcohols had been reasonably successful in describing the mechanism of the autocatalytic oxidation of alkanes. However, even though 0-heterocycles (which cannot be derived from intermediate aldehydes) had been found in the products of the oxidation of n-pentane as early as 1935 [66], the true extent of alkylperoxy radical isomerization reactions has been recognized only recently. Bailey and Norrish [67] first formulated the production of O-heterocycles in terms of alkylperoxy radical isomerization and subsequent cyclization in order to explain the formation of 2,5-dimethyl-tetrahydrofuran during the cool-flame oxidation of n-hexane. Their mechanism was a one-step process which involved direct elimination of OH. However, it is now generally formulated as shown in reactions (147) and(I67)... 

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