Is the initial solid state acyl shift of 1,2-diglycerides intermolecular?

Abstract

Although the acyl shift of 1,2-diglycerides in the solid state is commonly assumed to involve an intramolecular orthoester intermediate, an appraisal of crystal structures for starting material and end product indicates that the initial reaction in fact may be intermolecular. This conclusion is based on the fact that the 1,2-diglyceride packs in a folded chain conformation while the 1,3-diglyceride packs with extended chains and, additionally, that the reaction geometry for nucleophilic attack is most favorable for the unsubstituted hydroxyl group across the polar interface in the 1,2-diglyceride crystal structure. That no mixed acid products are found from a mixture of 1,2-diglycerides is explained by the immiscibility of near homologues in the β′L crystal structure.