Two stereoisomeric 3I,2II-anhydro-α-cyclodextrins: a molecular dynamics and crystallographic study Original Research Article

Regioselective epoxide ring opening of 2I,3I-(2IS)-anhydro-α-cyclodextrin (1) through intramolecular attack of hydroxyl groups of neighboring glucose rings occurs in diequatorial fashion to yield 3I,2II-anhydro-α-cyclodextrin (3) with a rigid glucopyranose–dioxane–glucopyranose tricyclic ring system, the usual diaxial opening and the gluco/altro-configurated stereoisomer 2 cannot be detected. Molecular dynamic simulations in water were used to analyze the conformations of 1–3 and the stereochemical implications of this reaction. Due to the contracted 2,3-OH side of the torus, 3 features an inverted conicity compared to the parent α-cyclodextrin. A crystallographic study on the bis-3·3 n-PrOH nonahydrate not only displays little variations between the solid-state and solution geometries of 3, but also provides a molecular picture of a unique inclusion complex in which three n-propanol molecules are distributed in the cavity of a dimeric unit of 3 (monoclinic, space group P21, a=14.257(1), b=22.623(2), c=16.644(1) Å, β=104.82(1)°, all 19278 reflections with I>2σ(I) yield R(F)=0.1017).