Cembrenoids

Total Synthesis of Several Natural Epoxy Cembrenoids... 

The gross structure of the new cembrenoid (107) was established by conversion to a mixture of cembrane diastereoisomers. Dehydration of the tertiary hydroxyl group gave a separable mixture of triene isomers, one of which (A - O) was converted to the mixture of hydrocarbons as shown in Scheme 26. The structure and relative stereochemistry, including the configuration of the double bonds, but excluding that of the tertiary hydroxyl at C4, were determined from analysis of sjrectral parameters. These conclusions were supported by an X-ray diffraction study of the hydroxy ether (107) which also revealed the stereochemistry at C4 (99). The absolute stereochemistry was... 

As mentioned previously, the acyclic diterpenoids and cembranoids from Eremophila species are unique in containing internal c -double bonds. The derivation of the two classes of compounds from a common precursor seems reasonable. Thus, the acyclic hydroxy diacid (95) and the all-c -cembratriene (108) must branch from the same intermediate. Nevertheless, at least one acyclic diterpene (96) contains a rroni-double bond and cyclization of its precursor would lead to cembrenes displaying a 3,4-tra/u double bond. No such examples have yet been found but a clue to their formation might be obtained from a comparison of the two cembrenoids, 107 and 109, which differ in configuration at C3 and C4. Their possible origins from acyclic precursors which differ in the geometry of the 2,3-double bond are shown in Scheme 47. 

Investigation of the products of Pseudopterogorgia bipinnata by Wright [56] and coworkers yielded a new class of cembrenoids, namely, bipinnatins 88 a-c and 89. 

Fig. 7.9 Examples for acyclic diterpenoid glycosides (above left), cembrenoids (below left), and labdenoids (above and below right) from the Solanaceae...

The McMurry reaction has also proved useful in the synthesis of a number of cembrenoid diterpenes, including crassin acetate methyl ether 119 [157], sar-... 

Aold, M., Tooyama, Y, Uyehara, T., and Kato, T. (1983) Synthesis of ( )-asperdiol, a marine anticancer cembrenoid. Tetrahedron Lett., 24, 2267-2270. 

Marshall has reported an innovative application of the Wittig rearrangement in the synthesis of the 14-membered cembrenoid carbocycles (Scheme 16.21) [104], The stereoselectivity of this process is coupled with the conformational preferences of the macrocyclic ether. The rearrangement reaction of the 17-membered macrocycle 187 provided a 90% yield of the 14-membered alcohols, with 188 as the major diastereomer (dr=81 7 6 7). This represents a remarkable example of macrocyclic stereocontrol (see also Chapter 1) to effect a stereoselective ring-contraction by use of a Wittig rearrangement. 

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