Tetrahydropyran conformations

The magnitude of the anomeric effect is thus defined as the difference between the conformational free energy (—AG x)o of the equilibrium of 2-substituted tetrahydropyran conformers 9 and 10 (Fig. 2.4) and the conformational free energy (—AG x) of the equilibrium of the two analogously substituted cyclohexanes 11 and 72 (Fig. 2.4) [16]. 

Typical carbon-oxygen bond distances m ethers are similar to those of alcohols (—142 pm) and are shorter than carbon-carbon bond distances m alkanes (—153 pm) An ether oxygen affects the conformation of a molecule m much the same way that a CH2 unit does The most stable conformation of diethyl ether is the all staggered anti conformation Tetrahydropyran is most stable m the chair conformation—a fact that has an important bearing on the structures of many carbohydrates... 

Table 3.8. Comparison of Conformational Free-Energy Values for Substituents on Tetrahydropyran, 1,3-Dioxane, and 1,3-Dithiane Rings with Those for Cyclohexane...

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

In the cyclization of the corresponding cis-epoxides, with the aim of obtaining the corresponding cis-2,3-disubstituted tetrahydropyrans, a similar trend was observed. For these systems, however, the 6-endo pathway was less favored, which was ascribed to difficulties in attaining a TS conformation that would allow for maximum stabilization of the developing p-orbital with the adjacent 7t-system. Alternatively, palladium-catalyzed cyclization of the tetrabutylammonium alkoxide derived from 33b results in the corresponding ris-2,3-disubstituted tetrahydro-pyran in excellent yield and selectivity (90%, dr >99 1), while the ris-epoxide gives stereoisomer 37b (86%, dr 98 2) [112]. 

The cyclic dimer, tetramer, and hexamer can be crystallized in acetonitrile, and also in chloroform (the former two oligomers). X-ray analysis of the crystals of the cyclic dimer47 disclosed that it consisted of a pair of different enantiomers of 53 and that all of the four substituents attached to the two tetrahydropyran rings occupied the axial position as illustrated in Fig. 54s). Such a conformation is in... 

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. 

Rychnovsky et al. considered the formation of achiral conformers from chiral molecules and trapping the prochiral radical with a hydrogen atom donor based on memory of chirality (Scheme 12) [41], The photo-decarboxylation of optically active tetrahydropyran 40 leads to an intermediate 43, which now does not contain a stereocenter. If the intermediate 43 can be trapped by some hydrogen atom source before ring inversion takes place, then an optically active product 41 will be formed. This is an example of conformational memory effect in a radical reaction. It was reported that the radical inversion barrier is low (< 0.5 kcal/mol) while the energy for chair flip 43 44 is higher (5 to... 

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. 

This article deals with the conformational analysis of substituted oxanes (tetrahydropyranes) and derivatives in which ring methylenes are replaced by further oxygen atoms (di-, tri-, tetroxanes, pentoxanes, and O ) or by carbonyl group(s) (oxanones, Meidrum s acid derivatives) and, if conforma-tionally of interest, systems incorporating these rings in polycyclic structures... 

The conformation of two 4-phenylsulfonyl-tetrahydropyran-2-one derivatives both in solution and in the solid state was studied (95JP0747) the tetrahydropyran-2-one ring proved to adopt a skew-boat conformation. 

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. 

Anomeric effects are cumulative, and can cause a potentially flexible ring to adjust to a more rigid conformation in order to maximize the overlap of suitable lone pair and a orbitals. It has been particularly instructive in explaining anomalous preferences for substituent orientations in tetrahydropyrans and related compounds. In the case of 2-methoxytetrahydropyran, for example, the axial conformer is three times more populated than the equatorial form (Scheme 1.2). 

Recall that in the latter, certain types of substituents adjacent to oxygen in the ring actually prefer axial arrangements. This observation has been codified in what is commonly referred to as the anomeric effect and is responsible in part for the conformations of carbohydrates. Is it possible that conformational preferences seen in substituted tetrahydropyrans will carry over into preferences in transition-state geometries for Claisen rearrangements ... 

There is a strong correlation between substituent effects on conformational preferences in tetrahydropyrans and Claisen transition states. 

X-Ray analysis of hexahydrofuro[2,3- ]pyran derivative 39 shows that the tetrahydropyran ring assumes a chair conformation with the C(7a)-0(1) bond found in the axial position <1998T8753>. This conformation agrees with that obtained by analysis of the NMR spectrum of 39. 

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>. 

The cyclization of tetrahydropyran 12 to 5-methyl-8-oxabicyclo[3.2.1]octan-6-one (13) is a particularly striking example of C-H activation by an adjacent heteroatom59. Although the ring size would be the same, and conformational differences in the transition states leading to both products should be minimal, none of the alternative bicyclic ketone 14 was observed. 

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