Viburnum awabuki

Kubo, M. Minami, H. Hayashi, E. Kodama, M. Kawazu, K. Fukuyama, Y. (1999) Neovibsamin C, a macrocyclic peroxide-containing neovibsai -lype diterpene from Viburnum awabuki. Tetrahedron Lett., 40,6261-5. 

In 2008, the group of Williams [15] reported the first total synthesis of ( )-2-0-methylneovibsanin H (33) (Scheme 14.6). The diterpene, isolated from the leaves of Viburnum awabuki [16], was prepared by employing an acid-catalyzed domino sequence. Thus, the key transformation was the conversion of advanced intermediate 28 to cyclohexene 32 upon treatment with an excess of sulfuric acid in anhydrous methanol. Acid-mediated silyl deprotection first revealed alcohol 29, which readily underwent an intramolecular oxa-Michael addition to yield tricyclic 30. It was postulated that solvolysis and nucleophilic addition of methanol to the intermediary allyl cation then furnished acid 31, which underwent Fischer esterification. The resultant highly functionalized cyclohexene 32 was isolated in 50% yield as a mixture of diastereomers at C2 (diastereomeric ratio (dr) = 85 15). The observed stereochemistry at the newly created stereocenters, i.e., at C2 and C5, was postulated to arise from the preexisting sterically congested stereocenters in the starting material (i.e., 28). Cyclohexene 32 was eventually taken on to provide the natural... 

FIGURE 2 Vibsanins A-F, which were isolated from Viburnum awabuki by prof. Kawazu. 

In 1980, Kawazu reported the isolation of vibsane-type diterpenoids from the leaves of Viburnum odoratissimum var awabuki (K. Kouh) Zabel ex Rumpler, and they were shown to consist of a unique fumulane skeleton with an additional C-5 unit. Fig. 1. 

---