Vibsanes

There are currently 61 known vibsane-t e diterpenes, as exemplified by (-)-5-e/7i-Vibsa-nin E 3. The first s mthesis of 3, described J. Am. Chem. Soc. 2009, 131, 8329) by Huw M. L. Davies of Emory University and Craig M. Williams of the University of Queensland, was based on the enantioselective seven-membered ring construction developed by the Davies group and the end game established by the Williams group. A key step in the s mthesis was the intramolecular hetero Diels-Alder cyclization of 1 to 2. 

Chemical Diversity of Vibsane-Type Diterpenoids and Neurotrophic Activity and Synthesis of Neovibsanin... 

Membered Ring Vibsane- A Minimal Structural Core ... 

Southeast Asia, and about 15 species are distributed in Japan [2], There is a long history of the folk medicinal use of Viburnum species. For example, the dried bark of V. opulus L., which is known as Cramp Bark, is used to alleviate painful menstrual cramps as well as a sedative [3], Native American women took black haw (V. prunifolium L.) to treat the menopause and menstrual cramps [4], The genus Viburnum has been documented to contain a variety of compounds such as iridoids, terpenoids, and aromatic compounds [5,6]. Among the chemical contents of Viburnum species, vibsane-type diterpenoids are considered to be characteristic of the Viburnum species because they have not been fotmd in other higher plants. In this review [7], we focus on the structural diversity, biological activities, and synthesis of vibsane-type diterpenoids, in particular, synthetic studies and neurotrophic activity of neo-vibsanin A and B are emphasized. 

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. 

It was necessary to determine the absolute stereochemistry of vibsanins B (1) and C (2) before discussing the structures of the newly isolated vibsane-type diterpenoids. The NMR of 1 showed two kinds of broad signals at room temperature, but a pair of the sharp signals was observed at 0 °C. This phenomenon indicated that vibsanin B (1) is present in solution as two conformational isomers [11]. 

We have already proposed such a plausible biogenetic pathway for three subtypes of vibsane-type diterpenes as vibsanin B (1) could be transformed into vibsanin C (2) by a Cope-type reaction, based on the results of thermal reactions of 1 [13,19]. Additional isolation of these furanovibsanins compel us to elaborate their biosynthetic process after 2 is produced. Thus, our proposed biosynthetic sequences leading to the furanovibsanins from 2 are outlined in Schemes 6 and 7. Biogenetic conversion of compounds 53-56 and 60 from vibsanin C (2) can be rationalized by a cationic process like (a) in Scheme 6 followed by an acetal formation between C-4 and C-7 ketones and intramolecular addition of oxygen nucleophiles. This is based on the fact that this type of tricyclic formation can be readily realized by... 

FIGURE 12 Rearranged vibsane-type diterpenoids (neovibsanins). 

It has been shown that a 7-membered ring vibsanin C (2) can be derived from vibsanin B (1), an 11-membered ring vibsane-type diterpenoid, by... 

These unique molecular architectures and significant biological activities have strongly motivated organic chemists to devote their efforts to the syntheses of the vibsane-type diterpenoids. So far, the vibsane-type diterpenoids whose total syntheses have been achieved are only four molecules, that is ( )-2-0-methylneovibsanin H (68) [45], ( )-neovibsanin B (63) [46], ( )-vibsanin... 

FIGURE 17 Synthesized diastereomers of vibsane family and neovibsanin B (63). 

We thank our colleagues for their dedication toward these projects of vibsane-type diterpenoids and whose names are listed in the literature cited in references. These works were supported by Grant-in-Aids for Scientific Research fi om the Ministry of Education, Culture, Sports, and Technology of Japan and the Open Research and MEXT-Senryaku grants from the Promotion and Mutual Aid Corporation for Private School of Japan. 

A preliminary review of vibsane-type diterpenoids, see Y. Fukuyama, M. Kubo, T. Esumi, K. Harada, H. Hioki, Heterocycles 81 (2010) 1571-1602. 

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