Diterpenes cycloadduct

The stmctural complexity and biological activity of the cyathane family of diterpenes has stimulated considerable interest from synthetic chemists, as reflected in the number and diversity of approaches reported thus far [42]. Our own strategy for cyathane synthesis is based on a rhodium-catalyzed [5+2] cycloaddition. The precursor for this reaction was fashioned ultimately from commercially available and inexpensive (S)-(-)-limonene. Treatment of the ketone 139 with 5 mol% [RhCl(CO)2]2 in 1,2-dichloro-ethane gave cycloadduct 140 (Scheme 13.14) in 90% yield and in analytically pure form after simple filtration through a plug of neutral alumina [43]. 

Another means of accelerating cycloaddition reactions that has been investigated is to conduct the reactions at high hydrostatic pressure. Smith reported that the Diels-Alder reaction between furan 26 and enone 27 in various solvents and employing Lewis acid catalysts did not lead to the desired cycloadduct 28 (Scheme 17.6) [24]. The product could, however, be obtained in the absence of solvent at 5 kbar. Adduct 28 was isolated in 80 % yield as a single diastereomer and was subsequently transformed into the diterpene (-i-)-jatropholone A (29). 

Scheme 17.16) [56]. It was proposed that the observed preference for the major product 86 arose from the energetically favored cnclo transition structure of quinone imide 89. It was suggested that the alternative endo transition structure 90, leading to minor diastereomer 87, was disfavored because of an unfavorable A interaction. Cycloadduct 86 was subsequently elaborated into the proposed structure 88 of the antitubercular diterpene pseudop-teroxazole. Corey s elegant total synthesis firmly established that the reported structure of the natural product had been misassigned.

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