The cytotoxic natural product phakellistatin 3 belongs to the family of cyclic peptides containing a 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) moiety. Recently, phakellistatin 13, a potential biogenetic precursor of phakellistatin 3, was isolated and shown to possess more potent and selective activity against tumor cell lines. We completed the first synthesis of phakellistatin 13 and determined the solution conformation. We then showed that photosensitized oxidation of phakellistatin 13 generated phakellistatin 3 and isophakellistain 3, and compared the solution conformation of the active isomer phakellistatin 3 with the crystal structure of the inactive isomer isophakellistatin 3.