Pyrans, hydrogenation

The formation mechanism of hydrogenated flirans by eoupling between terminal alkenes and aldehydes in trifluoroacetie acid medium deseribed in the literature involves unsaturated alkoxyearbenium ions as the intermediate [4-7]. This scheme is proposed by analogy with the meehanism for formation of hydrogenated pyrans under Prins reaction conditions. 

Nature of addition of formaldehyde to alkenes in Prins reaction is studied by MP2(fc)/6-31G(d). The structure of the transition states and intermediates are found. It is shown that the hydrogenated pyrans as 1,3-dioxanes can be formed by addition of formaldehyde oligomers to alkenes. However, the activation energy of this reaction is higher than that of 1,3-diox-anes formation. 

HYDROGENATED PYRANS FORMATION FROM FORMALDEHYDE DIMERS... 

According to literature [5], hydrogenated pyrans are thermodynamically more favorable reaction products as compared with 1.3-dioxans. At the same time it is assumed that their formation occurs with the participation of formaldehyde monomer. However, the results of our calculations show that hydrogenated pyrans can also be produced from FO and alkenes (Fig. 10.6). 

FIGURE 10.6 The scheme of hydrogenated pyrans formation from alkenes and FD. Proton transfer at the stage b d is suggested to a corresponding mechanism [25]. 

TABLE 10.3 Energy Parameters of the Hydrogenated Pyrans Formation Reactions, kJ/ mol... 

The calculations show that the formation of hydrogenated pyrans can be carried out by FD and alkenes reaction accompanied by 1,3-dioxanes formation as well. However, the activation energies of these reactions are higher lhan the ones of 1,3-dioxanes formation... 

Formaldehyde Oligomers Hydrogenated Pyrans Prins Reaction Transition State... 

The role of formaldehyde dimer in 0-containing heterocycles formation by the Prins reaction have been investigated. It was shown that the 1,3-di-oxanes, hydrogenated pyrans and oxetanes can be can be obtained from formaldehyde dimers and alkenes in the gas phase. The activation energy of these reactions is different. It is lower for 1,3-dioxanes formation, and higher for oxetanes formation. Thus formation of 1,3-dioxanes happens in the conditions of kinetic control. Opposite, the hydrogenated pyrans formation happens in the conditions of thermodynamic control... 

FIGURE 11.3 The scheme of hydrogenated pyrans formation from alkenes and FD. 

Unpredictably due to the results of our calculations, the formation of ox-etanes can also be accomplished with formaldehyde olygomers participation and 1,3-dioxanes and hydrogenated pyrans formation (Fig. 11.6). 

Thus, according to the calculated data, hydrogenated pyrans and oxetanes are possible to be formed along with the formation of alkyl-substituted 1,3-dioxanes in the reaction of FD with alkenes. In this case the formation of 1,3-dioxanes is characterized by lower activation energy... 

The calculations show that the formation of hydrogenated pyrans and oxetans can be carried out by FD and alkenes reaction accompanied by... 

AB Initio Calculation Alkylated 1,3-Dioxanes Formaldehyde Oligomers Hydrogenated Pyrans The Prins Reaction Transition State... 

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). 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

Naphthalene-1,8-dicarboxylic add, its nitro-substituted derivatives, and naphthalene-1,4,5,8 tetracarboxylic acid, on treatment with sulfur tetrafluoride at 0 C, undergo dehydration to form quantitatively the corresponding anhydrides [218, 221] Unsubstituted and mononitrated monoanhydndes react further at 200-250 °C to give derivatives of 1,1,3,3-tetrafluoro-l// naphtho[I,8 c,d]pyran Dinitronaphthalene-l,8-di-carboxyhc acid anhydndes and naphthalene 1,4,5,8 tetracarboxylic acid dianhydnde give the respective tetra- and octafluoroethers only in the presence of an excess of anhydrous hydrogen fluoride [221] (equabons 113 and 114)... 

Unlike cyclohexene, its oxa analog, 3,4-dihydro-2//-pyran, undergoes facile reduction to tetrahydropyran in yields ranging from 70 to 92% when treated with a slight excess of triethylsilane and an excess of either trifluoroacetic acid or a combination of hydrogen chloride and aluminum chloride (Eq. 69).146 This difference in behavior can be understood in terms of the accessibility of the resonance-stabilized oxonium ion intermediate formed upon protonation. 

A dichloromethane solution of 4 - m e th y I - 5,6 - d i hy dro - 2 // -pyran gives 4-me-thyltetrahydropyran in 35% yield when treated with a mixture of five equivalents of triethylsilane and 2.5 equivalents of aluminum chloride in the presence of excess hydrogen chloride at room temperature for one hour (Eq. 84).136 This behavior is essentially the same as that exhibited by the disubstituted 4-methylenetetrahydropyran isomer under similar conditions.136... 

Falbe and Korte (90) studied the cobalt hydroformylation of dihydro-pyran and its derivatives. The reactions were conducted at temperatures above 180°C, with pressures of 300 atm of 1/1 H2/CO. Under these conditions, the initially formed aldehydes were hydrogenated to the alcohols in situ. As noted with acyclic vinyl ethers, formyl attachment was predominantly a. 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

Compounds are generally classified according to their fully unsaturated parent compound (but see below). Thus substituted, partially saturated, and fully saturated derivatives of, for example, pyrrole are all indexed under pyrrole. Benzo and similar derivatives are included under the most unsaturated parent system (e.g., quinoline, thienofuran, etc.). For any given heterocyclic parent only one indicated hydrogen isomer appears in the text, typically the most stable or the lowest numbered form thus all instances of pyran, whether of the 2H- or 4H-form, are indexed under 2H-pyran. The charges and additional valences for any heterocyclic parent structure are not indicated. 

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