Methoxy-tetrahydropyrane

For instance, in structure 12-e, the C-X and C-0 dipole moments are additive, leading to a destabilization of the molecule by increasing the energy. In structure 12-a, offset of the C-X and C-0 dipole moments minimizes electrostatic interactions, thus leading to a more stable conformation. This electrostatic model was supported by the observed increase of the percentage of the equatorial conformation of 2-methoxy tetrahydropyran (14) when moving from a non-polar to a polar solvent (Table 3).12 In this model, the polar groups are not polarizable and lead to dipole/dipole (hard/hard) interactions. 

Molecular dynamics (MD) simulations show that the conformations of sorbitol and mannitol depend on the typ e of solvent. The predicted conformations agreed well with experiment, supporting the view that MD has a good predictive value for solutions of carbohydrates. Preliminary dynamics results for methoxy-tetrahydropyran (MTHP) show that the methoxy group moves more in water than in vacuum. 

In this work we discuss further the previous results from simulations of sorbitol and mannitol and compare them with new calculations and recent experimental data. We also present some preliminary data for methoxy-tetrahydropyran (MTHP) in vacuo and in water. 

The SM2/AM1 model was used to examine anomeric and reverse anomeric effects and allowed to state that aqueous solvation tends to reduce anomeric stabilization [58]. Moreover, SM2/AM1 and SM3/PM3 models were accounted for in calculations of the aqueous solvation effects on the anomeric and conformational equilibria of D-glucopy-ranose. The solvation models put the relative ordering of the hydroxymethyl conformers in line with the experimentally determined ordering of populations. The calculations indicated that the anomeric equilibrium is controlled primarily by effects that the gauche/trans 0-C6-C5-0 hydroxymethyl conformational equilibrium is dominated by favorable solute-solvent hydrogen bonding interactions, and that the rotameric equilibria were controlled mainly by dielectric polarization of the solvent [59]. On the other hand, Monte Carlo results for the effects of solvation on the anomeric equilibrium for 2-methoxy-tetrahydropyran indicated that the AM1/SM2 method tends to underestimate the hydration effects for this compound [60]. 

A number of pyrans, including 3-hydroxy-tetrahydropyran (both axial conformer, 29 and equatorial conformer, 30), 2-methoxy-tetrahydropyran 33, 3-methyl-tetrahydropyran 32, and several 4-substituted tetrahydropyrans, along with 2-methyl-l,3-dioxolane and the rigid cyclic ethers 7-oxabicyclo[2.2.1]heptane and 1,8-cineole, were studied extensively by NMR. These empirical results, in conjunction with the literature data for a variety of acyclic and cyclic ethers, were used to examine the reliability of O-substituent chemical shift models in these systems. The empirical data correlate well with predictions made from the model and it is concluded that ethereal oxygen substituent chemical shifts are due to both steric and electrostatic terms <1998J(P2)1751>. 

Figure 4 Calculated equatorial-axial conformational energy differences in kcal/mol for 2-methoxy-tetrahydropyran. The dashed line indicates the experimental value.

Three staggered conformations are possible for the rotation about the C2-02 bond in both equatorial and axial conformers of 2-methoxy-tetrahydropyran (Fig. 2.14). These are referred to as E1-E3 (34, 36, and 38) and A1-A3 (35, 37, 39) conformers. In the El conformer (34) there are no iyn-axial steric... 

H. Booth, J. Mark Dixon, and R. Simon, Experimental studies of the anomeric effect. Part V. The influence of some solvents on die conformational equilibria in 2-methoxy- and 2-(2/,2,2 -trifluoroethoxy)-tetrahydropyran, Tetrahedron, 48 (1992) 6151-6160. 

In 3-bromo-7-methoxy-2-tetrahydropyran-2-ylpyrazolo[l,5-( ]pyrimidine 66, the pyrimidine and pyrazole rings make a dihedral angle of 3.8°. The tetrahydropyran ring is in a chair conformation and its mean plane makes an interplanar angle of 52.5° with the pyrazole ring. There is a short Br- -O intramolecular contact of 2.994 A <2004JSTC356>. 

To clarify this behavior further, calculations were made on the angular dependence of the JCM values for six model systems (Fig. 3) in a and /3 configurations.5-6 The first four models, namely 2-methoxy-5-(tetrahydropyran-2-yloxy)tetrahydro-pyran (1), 2-methoxy-4-(tetrahydropyran-2-yloxy)tetrahydropyran (2), 2-meth-oxy-3-(tetrahydropyran-2-yloxy)tetrahydropyran (3), and 2-methoxy-2-(tetrahy-dropyran-2-yloxy)tetrahydropyran (4), represent a (a) and )8 disaccharides (b) linked by (1 —>4)-, (1— 3)-, (1— 2)-, and (1 — 1 )-glycosidic linkages, respectively. The last two model compounds, designated as 5 and 6, are 3-hydroxy derivatives of 1 and are thus related to (1 —>4)-linked disaccharides, stereochemi-cally related to the gluco and manno configurations at C-2. 

A reaction between pentacarbonyl[methoxy(2,6-dimethoxyphenyl) methylene]chromium(0) and pent-4-yn-l-ol provides direct access to the functionalized tetrahydropyran-2-one 1005 the best yield is obtained via thermal heating of the two starting materials in THF (Equation 395) <2000TL9323>. 

Isobutyl-3-methoxy-5-[3-(tetrahydropyran-2-yloxy)propyl]pyrazine (132) gave 2-(3-hydroxypropyl)-5-isobutyl-6-methoxypyrazine (133) (TsOH, MeOH, 20°C, ultrasonication, 2 h > 95%) 295 analogues likewise.298... 

Dibenzyloxy-3-isobutyl-6-(tetrahydropyran-2-yloxymethyl)pyrazine 4-oxide (171) gave 2-benzyloxy-3-isobutyl-5-methoxy-6-(tetrahydropyran-2-yloxym-ethyl)pyrazine 4-oxide (172) (NaH, Bu4NBr, MeOH, Mc2NCHO, 20°C, N2, 40 min 91% the selective transalkoxylation of the 5-benzyloxy group may be due to activation by the adjacent /V-oxidc entity).848... 

Diels-Alder reactions have been very successfully subjected to asymmetric catalysis by binaphthyl complexes. Accordingly, the synthesis of tetrahydropyranes 41 and 42 can be realized by reaction of glyoxylic esters 39 with methoxy-dienes 38 (Scheme 8) [20]. Some of these reactions take place with excellent endo-conxro and... 

Essigsaure Chlormercuri- -(2-methyl-propylester) XIII/2h, 180 Tetrahydropyran 3-Chlormercuro-2-methoxy- E14a/1, 392 (Enol-ether/R — OH/HgX2)... 

Tetrahydropyran 2-Azido-4,5-bis-[methoxy-methoxy]-3-iod-6-methyl- E21e, 5201 (En + IN3)... 

The reaction of 6-ethylthio-3,4-dihydro-2-methoxy-27f-pyran with aldehydes is promoted by SnCU, and has been used in the diastereoselective synthesis of highly functionalized tetrahydropyrans (Eq. 53) [88a] whereas the reaction of 2,2-diethoxy-6-(ethylthio)-3,4-dihydro-2//-pyran with aldehydes in the presence of SnCU gives aldol adducts of glutarates (Eq. 54) [88b]. 

A methoxy group at position-2 of tetrahydropyran is much more stable in the axial orientation than would be expected based on its A value, a manifestation of the anomer-ic effect [46,47,48,49,50,51,52,53], originally observed with carbohydrate derivatives by... 

Amberlyst-15 was also used as a catalyst for the reaction of alcohols and phenols with tetrahydropyran (Eqn. 22.33). Refluxing a mixture of an alcohol and dimethoxymethane in the presence of a Nafion-H catalyst gave the methoxy methyl ethers in very good yields (Eqn. 22.34). ° Nafion-H was also used to catalyze the conversion of diols to cyclic ethers. ... 

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