Vibsanin 11-Membered ring

Since the conformations of the 11-membered ring vibsanins A and P-T, which bear a 6,7-epoxide ring, are fixed, analysis of their NMR can be performed normally. On the other hand, vibsanin B (1) and vibsanols A (23) and B (24) [18], which contain a cross-conjugated diene, show complex NMR signals due to the presence of several kinds of conformational isomers. Therefore, careful structure elucidation should be performed by a combination of VT NMR experiments and MM2 calculations [13]. 

FIGURE 9—CONT D (B) Another 7-membered ring vibsanins. 

Furanovibsanins, a diverse range of 7-membered ring vibsanins, which are presumed to be produced by two ketones at the C-4 and C-7 positions, have also been found. The examples shown in Fig. 11 are furanovibsanin A (53) and its 3-0-methyl congener 54, which are presumably derived from 3-hydroxyvibsanin E. The additional examples are furanovibsanin B (55) and its 7-epimer 56, and furanovibsanins C (57)-G (61). These diterpenoids were isolated from V. odoratissimum var awabuki, collected in Tokushima [27]. 

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... 

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