MM# Modeling of Aldopentose Pyranose Rings

Abstract

<p>MM3 (version 1992, <em>ϵ</em>=3.0) was used to study the ring conformations of d-xylopyranose, d-lyxopyranose and d-arabinopyranose. The energy surfaces exhibit low-energy regions corresponding to chair and skew forms with high-energy barriers between these regions corresponding to envelope and half-chair forms. The lowest energy conformer is ⁴ <em>C</em> ₁ for α- and β-xylopyranose and α- and β-lyxopyranose, and the lowest energy conformer is ¹ <em>C</em> ₄ for α- and β-arabinopyranose. Only α-lyxopyranose exhibits a secondary low-energy region (¹ <em>C</em> ₄) within 1 kcal/mol of its global minimum. Overall, the results are in good agreement with NMR and crystallographic results. For many of these molecules, skew conformations are found with relatively low energies (2.5 to 4 kcal/mol above lowest energy chair form). The ² <em>S</em> _<em>O</em>and ¹ <em>C</em> ₄conformers of crystalline benzoyl derivatives of xylopyranose are in secondary low-energy regions on the β-xylopyranose surface, within 3.8 kcal/mol of the global ⁴ <em>C</em> ₁ minimum.</p>