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A shift of atoms from position that mark their equilibrium coordinates increases the Gibbs free enthalpy of a structure. Such changes implied by intraor intermolecular forces are evident from atomic parameter such as distances, bond or dehedral angles. For the six-membered cyclic ether tetrahydropyran, the chair conformation marks an undistorted molecular arrangement. Substituting a methyl group for a hydrogen atom, particularly in one of the axial positions, gives rise to changes in endoand exocyclic dihedral angles, if referenced versus the undistorted chair geometry [1]. Since tetrahydropyrans, in particular those exhibiting multiple subsitution at positions 2 and 6, constitute important natural products, we were interested to quantify strain induced by methyl substituents in tetrahydropyran. In the first part of the study, thermochemical and geometrical parameter were calculated for all isomers of dimethyltetrahydropyran using density functional theory [2]. The computed DG(298) values were correlated with bond and dihedral angles of the associated structures, in order to fit strain enhalpies to parameter that allow a classification of structures into strained, medium-strained, and unstrained isomers [2]. This information served as guideline for estimating the degree of strain of eight substituted tetrahydropyrans (X-ray analysis), in order find evidences for different efficiencies the compounds are formed in from alkenol bromocyclizations [3].

Which parameter to select for estimating the degree of strain in substituted tetrahydropyrans?