Triquinanes radical cyclization transforms

The potential of sequential radical addition as a powerful method to achieve the formation of five-membered rings was fully realized in the tandem radical cyclization strategy devised by Curran for the synthesis of triquinanes. In the case of linearly fused triquinanes, such as hirsutene 90 (Scheme 3.40), this strategy implies the retrosynthetic disconnection of the tricyclic framework by the application of two sequential radical cyclization transforms at rings A and... 

The extention of radical cyclization of (bromomethyl)dimethylsilyl allyl ethers to propargyl analogs 17 has been studied by Malacria and coworkers. The intermediate exocyclic vinyl radical 18 can be either trapped by the hydrogen atom to give, after simple chemical transformations, the trisubstituted alkene 19 (equation 25) or can be added intramolecularly to give cyclic products when suitably located double bonds are present (equations 26 and 27). An attempt to apply this methodology to the stereoselective synthesis of angular and linear triquinane has also been performed. When = t rt-butyl,... 

Fraser-Reid s group have described further examples of the use of serial radical cyclization to prepare di- and triquinane natural products. The previously-reported intermediate (4) (Vol. 23, p. 264) has been ring-opened by a Vasella-Bernet reaction, and hence converted to the triquinane (5) with the silphinene skeleton (Scheme 2). Similar initial steps were involved in the conversion of (6) into silphiperfolene (7), and to transform (8) (Vol. 23, p. 265) into (-)-a-pipitzol (9) (mannose carbons numbered). ... 

Although 0-stannyl ketyl radical anions are intermediates only recently developed for synthetic applications, they already provide ready access to carbonyl-alkene cyclizations, ring scissions, and tin(IV) enolates. Unlike standard radical reactions, these transformations provide an alcohol or ketone after workup that can be further synthetically manipulated. Finally, the intermediates can be applied to natural product skeletons such as the triquinanes. 

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