The Ratio between Endocyclic and Exocyclic Cleavage of Pyranoside Acetals Is Dependent upon the Anomer, the Temperature, the Aglycon Group, and the Solvent

Abstract

Several cis-fused decalin pyranosides with intramolecular nucleophiles of high effective molarity were studied to determine the ratio between endocyclic and exocyclic cleavage in specific-acid-catalyzed solvolysis reactions. The molecular design that allows a differentiation between endo- or exocyclic cleavage is the symmetry and asymmetry of the respective oxocarbenium ion intermediates. The synthesis of the molecular probes involves eight steps from a known compound, and proceeds via a key intermediate functionalized with three different oxidation states. A crystal structure confirmed the relative stereochemistry of the probes. A quantifiable percentage of endocyclic cleavage for β-pyranosides was found for all reaction conditions, whereas α-pyranosides show exclusively exocyclic cleavage. The percent of endocyclic cleavage for β-pyranosides is dependent upon the temperature, the aglycon group, and the solvent. At lower temperatures endocyclic cleavage increases. The ΔH⧧ and ΔS⧧ for endocyclic and exocy...