Synthetic circularized analogues of bovine pancreatic trypsin inhibitor

Two cyclic analogues of the protein bovine pancreatic trypsin inhibitor (BPTI), c-[R]_Smc and c-Cys5[R]_Abu, have been synthesized. For the first target, a semisynthetic approach featured a cyclization that took advantage of the constraint imposed by the three disulfides of the native protein. For the second target, an entirely unstructured 58-residue thiolester, prepared by total synthesis with mild Fmoc chemistry and a side-chain anchoring strategy, was cyclized by native chemical ligation.