Methods and intermediates for the synthesis of selective inhibitors of dipeptidyl peptidase IV (DPP-IV) are provided. Coupling of a carboxylate salt with a boro-proline derivative provides a protected form of a DPP-IV inhibitor, which may be deblocked to yield the medicinal compound. The carboxylate salt can be a crystalline form of a sodium salt or a dicyclohexylammonium salt. The inhibitor can be used in the treatment of diabetes.