Novel trimeric indole alkaloid from Tabernaemontanadivaricata

Monoterpene indole alkaloids play an important role in natural products due to their unique structures and biological activities. In particular, bis-indole alkaloids are well known for their anti-tumor bioactivity, for examples, vinblastine and vincristine. Plants of genus Tabernaemontana are rich in monoterpene indole alkaloids, especially its dimers. This genus includes about 120 species and iswidely distributed in the tropical and subtropical areas of Asia and Australia. Many of these species are used in folk medicine for the treatment of skin diseases, hypertension, abdominal pain, and as a remedy for cancer. Recently, we isolated a new type of tetrakis-indole alkaloid as alasmontamine A from T. elegans, and new niacin-analogue related alkaloids from T. divaricata. In our search for structurally and biogenetically interesting natural products, we have isolated a novel trimeric indole alkaloid, divaricamine A. In this conference we would like to describe the isolation and structure elucidation of divaricamine A.

The whole plants of T. divaricata collected at Okinawa, Japan were extracted with MeOH, and the extract was partitioned between EtOAc and 2% tartaric acid. The aqueous layer was adjusted at pH 9 with saturated Na₂CO₃aq and extracted with CHCl₃. CHCl₃-soluble alkaloidal materials were subjected to an LH-20 column (MeOH) three times followed by an ODS column (MeOH/H₂O/TFA) three times and Cholester HPLC column to afford divaricamine A.

The structure including the absolute stereochemistry of divaricamine A was elucidated on the basis of spectroscopic data(¹H, ¹³C, ¹H-¹H COSY, TOCSY, HSQC, HMBC, ROESY, HSQC-TOCSY) and its calculated CD spectrum. This is the first example of vobasine-vobasine-iboga-type alkaloid with both C-N and C-C linkage patterns. In addition, divaricamine A showed a moderate in vitro antimalarial activity against Plasmodium falciparum 3D7 strain.

Author(s): Yusuke Hirasawa

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