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Voacanga africana (Apocynaceae) is used as an anti-diarrheal medicine in West Africa. In the present study, we investigated the effect of an extract of V. africana and its constituents on smooth muscle contraction induced by capsaicin in mouse rectum, where transient receptor potential vanilloid type 1 (TRPV1)-immunoreactive fibers are abundant. Methanol and alkaloid extracts of the root bark of V. africana were found to inhibit capsaicin-induced contraction in a dose-dependent manner (30-300 μg/ml). Major constituents isolated from the alkaloid extract were then studied for their effects on the capsaicin-induced contraction. The main active constituents were found to be Iboga-type alkaloids, including voacangine (1), 3-oxovoacangine (2), voacristine (3), and (7α)-voacangine hydroxyindolenine (4). The voacangine concentration dependently (3-100 μM) inhibited the capsaicin-induced contraction. The capsaicin-induced contraction was almost completely inhibited by the TRPV1 antagonist, N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide (BCTC). On the other hand, the Iboga-type alkaloids did not inhibit the contractions induced by 3 μM acetylcholine and 300 μM nicotine. These results suggest that Iboga-type alkaloids isolated from V. africana inhibit capsaicin-induced contraction in the mouse rectum, possibly via the inhibition of a TRPV1-mediated pathway. This inhibition may be involved in the anti-diarrheal effect of V. africana.

A new hexacyclic iboga-type indole alkaloid, voacangalactone (1), was isolated from Voacanga africana , and its structure including the absolute configuration was established by asymmetric total synthesis involving such key steps as the asymmetric Diels-Alder reaction using an aminodiene and the construction of an isoquinuclidine ring and an indole skeleton.

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Discovery of Indole Alkaloids with Cannabinoid CB1 Receptor Antagonistic Activity. -The new compound (I) is isolated from Voacanga africana together with two known indole alkaloids. These are the first examples of natural alkaloids having the title activity. -(KITAJIMA, M.; IWAI, M.; KIKURA-HANAJIRI, R.; GODA, Y.; IIDA, M.; YABUSHITA, H.; TAKAYAMA*, H.; Bioorg. Med. Chem. Lett. 21 (2011Lett. 21 ( ) 7, 1962Lett. 21 ( -1964