Crystal and molecular structures of three 2-substituted 4-methyloxan-4-ol derivatives studied by X-ray crystallography and by PM6 calculations

Abstract

The conformational features of three 2-substituted 4-methyloxan-4-ol (tetrahydropyran) derivatives were investigated by single crystal X-ray crystallographic studies and the geometries were optimized using semi-empirical PM6 methods. The tetrahydropyran ring in each of (I)–(III) adopts a chair conformation with their hydroxyl groups occupying equatorial, axial and axial positions, respectively. The strongest hydrogen bonding interactions apparent in each of the respective crystal structures, as judged from distance and angle considerations, involve the ring-bound O2-hydroxyl group and tetrahydropyran ring oxygen atom. These hydrogen bonds lead to a supramolecular chain with a linear topology in each case but generated by glide symmetry in (I) and (II), and 2₁-screw symmetry in (III). In the latter, a two dimensional array constructed by O—H…O bonding is found leading to undulating layers. Despite the absence of additional O—H…O hydrogen bonding sites, two dimensional arrays are also generated in each of (I) and (II), but this time mediated by C—H…O contacts that lead to flat layers. The theoretical structures for each of (I) and (III) differ from their respective experimental structures in terms of the relative orientation of the aromatic substituent. By contrast, for (II), there is close agreement between the experimental and theoretical structures. (I) C₁₂H₁₆O₂, monoclinic, Cc, a = 18.7887(8), b = 6.0634(2), c = 10.4178(4) Å, β = 116.945(7)°, V = 1057.99(10) ų, Z = 4, R₁ = 0.029; (II) C₁₃H₁₈O₃, orthorhombic Pc2₁n, a = 9.3664(17), b = 10.0472(18), c = 12.240(2) Å, V = 1151.9(3) ų, Z = 4, R₁ = 0.037; (III) C₁₃H₁₈O₄, orthorhombic P2₁2₁2₁, a = 9.814(3), b = 9.966(2), c = 11.845(2) Å; V = 1158.5(5) ų, Z = 4, R₁ = 0.079.