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

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Regioselective epoxide ring opening of 2 I,3 I-(2 I S)-anhydro-α-cyclodextrin ( 1) through intramolecular attack of hydroxyl groups of neighboring glucose rings occurs in diequatorial fashion to yield 3 I,2 II-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 P2 1, a=14.257(1), b=22.623(2), c=16.644(1) Å, β=104.82(1)°, all 19278 reflections with I>2 σ( I) yield R( F)=0.1017).