Discovery and Biomimetic Semisynthesis of Spirophyllines A-D from Uncaria rhynchophylla .

Spirophyllines A-D ( 1 - 4 ), four new spirooxindole alkaloids all characterized by the spiro[pyrrolidin-3,3'-oxindole] core and a rare isoxazolidine ring, were isolated from Uncaria rhynchophylla . Their structures were determined by spectroscopic methods and confirmed by X-ray crystallography. Based on the biomimetic semisynthesis strategy, compounds 1 - 8 were synthesized in three steps via the key reactions of 1,3-dipolar cycloaddition and Krapcho decarboxylation from corynoxeine. Interestingly, compound 3 showed moderate inhibitory activity against the K v 1.5 potassium channel (IC 50 = 9.1 μM).