Cembrane precursor

Lewis-acid promoted cyclizations of allylic stannanes have been successfully employed to prepare macrocyclic compounds. An enantioenriched a-oxygenated allylic stannane led to a 14-membered cembrane precursor in high yield with excellent diastereoselectivity (Eq. 64) [80]. 

Stemming from the fact that both cembrane and pseudopteranes skeletons are found in Pseudopterogorgia acerosa, Fenical [5] has suggested that the pseudopterane skeleton 62 might evolve from die ring contraction of a cembrane precursor 86. 

C20H34O, Mr 290.49, cryst., mp. 104-105 °C, [a]o +168° (CHCI3). A diterpene of the verticillane type, isolated from the wood of Sciadopitys veriicillata (Taxodiaceae). The verticillanes may be biosyntheti-cally derived from formal cyclization of a cembrane precursor. 

An intramolecular cycloaddition of the tetradecatrienyl nitroethyl ether 263 was used in the synthesis of the 14-membered bicyclic precursor 265 of crassin acetate 266, a cembrane lactone possessing antibiotic and antineoplastic activity (332). Nitro compound 263 was obtained from farnesyl acetate (262) in several steps and was then treated with phenyl isocyanate and triethylamine to give the tricyclic isoxazoline 264 (Scheme 6.98). Conversion to ketone 265 was accomplished by hydrogenation of the cycloadduct with Raney Ni and boric acid followed by acetylation (332). In this case, the isoxazoline derived from a 3-butenyl nitroethyl ether moiety served to produce a 3-methylenetetrahydropyran moiety (332). 

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