Structurally Diverse Indole Alkaloids with Vasorelaxant Activity from Melodinus hemsleyanus

Six new structurally diverse indole alkaloids, melohemsines J–M ( 1 – 4 ), 11-hydroxy-Δ 14 -vincamine ( 5 ), and 11-hydroxy-16- epi -Δ 14 -vincamine ( 6 ), and 15 known alkaloids were isolated from the leaves and twigs of Melodinus hemsleyanus Diels. These new compounds and their absolute configurations were determined through spectroscopic data analyses, X-ray diffraction, and computational methods. Melohemsine J ( 1 ) is the first example of a melodinus-type alkaloid possessing a 6/6/5/5/6/5 hexacyclic skeleton and containing a tetrahydrofuro[2,3- b ]pyridine-2(3 H )-one unit. Melohemsine K ( 2 ) is an unusual aspidosperma-type alkaloid possessing a 6/5/6/5/5 pentacyclic architecture with a contracted E ring (loss of CH 2 ). Compounds 5 – 10 and 16 exhibited vasorelaxant activities with EC 50 values of 0.8–3.8 μM. In addition, compound 4 displayed moderate cytotoxicity toward the tumor cell lines HepG2 and A-549 with EC 50 values of 18.7 and 28.7 μM, respectively.