The Congolese liana Ancistrocladus likoko (Ancistrocladaceae) produces naphthylisoquinoline alkaloids that are, chemotaxonomically remarkable, all based on the same coupling type, with the biaryl axis located between C-5 and C-8'. About 20 alkaloids, belonging to the subclass of 5,8'-linked naphthylisoquinolines, have so far been discovered in this plant species. Here, we report on the isolation and structure elucidation of six further such 5,8'-coupled monomeric alkaloids, named ancistrolikokines I (9), C3 (10), F2 (11), J (12), J2 (13), and J3 (14). They were identified in the twigs of A. likoko, along with the two new atropo-diastereomeric dimers michellamines A8 (15a) and B8 (15b) and the naphthalene-devoid dihydroisoquinoline ent-ealaine D (19). The latter had previously only been known from total synthesis and has now been identified for the first time as an authentic natural product. Three of the new alkaloids, 12-14, are the only fully dehydrogenated naphthylisoquinolines with a 5,8'-biaryl linkage, apart from one single known other example previously likewise found in A. likoko. The stereostructures of the new alkaloids were established by spectroscopic (in particular HRESIMS, 1D and 2D NMR), chemical (oxidative degradation), and chiroptical (electronic circular dichroism) methods. The new ancistrolikokines exhibited moderate to strong cytotoxic activities against drug-sensitive acute lymphoblastic CCRF-CEM leukemia cells and against cells of their multidrug-resistant subline, CEM/ADR5000. A first structure-activity relationship (SAR) study on a small library of 5,8'-coupled naphthylisoquinolines from the twigs of A. likoko suggests that the oxygenation patterns in the isoquinoline portion at C-6 and C-8 play a crucial role for the antileukemic activities within this group of alkaloids.
Keywords: Ancistrocladaceae; Ancistrocladus likoko; Ancistrolikokines; Antileukemic agents; Naphthylisoquinoline alkaloids.
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