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

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 Δ H ⧧ and Δ S ⧧ for endocyclic and exocyclic cleavage were determined to be 19.2 ± 1.4 kcal/mol and −12.6 ± 6.1 eu, and 22.8 ± 1.1 kcal/mol and 3.7 ± 3.8 eu in methanol, respectively. These values support the theory of stereoelectronic control in the cleavage of pyranoside acetals. Pyranosides with phenyl aglycon groups exhibit significantly lower percentages of endocyclic cleavage than pyranosides with alkyl aglycon groups. Although an exact percentage of endocyclic cleavage of pyranosides in water could not be determined, it appears to be approximately the same or greater than that which occurs in methanol. The addition of non-hydrogen-bonding/non-nucleophilic solvents increased the percent of endocyclic cleavage. The results are interpreted to support some extent of nucleophilic assistance in the endocyclic solvolysis of pyranosides, stereoelectronic control on the site of cleavage, and the possibility of endocyclic cleavage at the active site of glycosyl transfer enzymes.