Diquinane framework

More recently, the bis-cyclopropanation of dienyne 325 has also been investigated by Fensterbank, Malacria, and Marco-Contelles to construct highly strained cyclopropyl-substituted diquinane frameworks 327 in a completely diastereoselective manner (Scheme 84). 3 It is noteworthy that the formal metathesis product was also observed in these reactions, albeit as a minor product, and that a simple introduction of a methyl group to one of the two ene moieties substantially affects the reaction. 

An electron-enriched 1,3-diene moiety as in the substrate 381 can act as a nucleophile toward an activated alkyne moiety (Scheme 94). Iwasawa340 has reported an elegant synthesis of a diquinane framework 382, which is catalyzed by various metals and the rhenium(i) complex appears to be the best catalyst among the metal complexes examined. Minor product 384 presumably is formed through an insertion of a carbenoid species into the neighboring activated benzylic C-H bond. The same carbenoid species can undergo a 1,2-H shift to give the major product 383. 

Radical cyclization reactions have found widespread use in the preparation of polyqui-nanes. A highly functionalized diquinane framework was prepared by Malacria and coworkers using a radical cascade process [66], It was anticipated that the (bromomethyl)-silyl ether 161 would serve as a suitable radical trigger. Thus, a solution of PhjSnH and AIBN was added over 5 h to a benzene solution of 161 and 10 equiv of acrylonitrile at reflux. Further heating for 5 h followed by Tamao oxidation of the crude product mixture allowed isolation of diquinane 162 in 51% yield as a single stereoisomer (by H-NMR) (Scheme 10-53). 

In cyclic systems, the constraints of the frameworks bearing the reacting n-systems tend to limit the stereochemical outcomes of the reaction in a synthetically beneficial manner. The preservation of enantiomeric purity associated with the ODPM rearrangement of the (+)- and (-)-forms of compound 21 into the corresponding enantiomers of cyclopropa-fused diquinane 22 fScheme 9.4 is particularly significant because of the extensive application of this basic process in the synthesis of a range of polyquinane-containing natural products (see Section M). 

The cytotoxic sesquiterpenoid quadrone, 38, has an embedded diquinane carbon framework that allowed the Piers s group to leverage the chemistry of the Weiss-Cook reaction. The monoketal 35 is readily available from 12 (R = R" = H). Several transformations converted this compound into the vinyl cyclopropane 36. Thermal rearrangement of the scaffold employing a Cope process afforded tricyclic 37. This compound was readily elaborated into 38. 

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