Vibsanin E (4), a tricyclic vibsane-type diterpene, has been prepared in 50% yield from vibsanin C (2), a seven-membered ring vibsane-type diterpene by reaction with BF3.OEt2 at -78 degrees C. This chemical correlation not only established structure, including absolute configurations, but also has demonstrated a possible biosynthetic route to 4 via 2 derived from vibsanin B (1). The structure of 3-hydroxyvibsanin E (5), another example of a tricyclic seven-membered ring vibsane, isolated from the leaves of Viburnumawabuki, has been established by extensive analyses of 2D NMR data and comparison of its spectral data with those of 4.

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Abstract. M r = 306.82, monoclinic, P2/c, a = 11.542(3), b=10.859(3), c=11.632(3)A, fl= 91.09 (2) ° , V= 1457.5 (7)A 3, z=4, Dm= 1.391 (1), D x = 1.398 Mg m -a, 2(Cu Ka) = 1.5418 A, /t = 2.362mm -1, F(000)= 648, T= 293 K, final R = 0.046 for 2478 independent reflections. Both N atoms of the imidazole ring are protonated and hydrogenbonded to a CI-ion and an N atom of the cyano group of an adjacent molecule, respectively. The CI-ion is further linked with the N atom of the neighboring guanidine group by a hydrogen bond, consequently forming a 20-membered ring consisting of two cimetidine molecules and two C1-ions related by a center of symmetry, respectively.