Intramolecular palladium-catalyzed trimethylenemethane cycloadditions: Initial studies

Article abstract of DOI:10.1021/ja00019a036

The potential application of [3 + 2] cycloadditions to polycarbocycle construction is considerably enhanced by the ability to perform such reactions intramolecularly. The feasibility of such processes is explored in the context of Pd-catalyzed cycloadditions of 2-[(trimethylsilyl)methyl]allyl carboxylates, wherein trimethylenemethane (TMM) precursor fragment (donor) and the electron-deficient olefin (acceptor) is joined by a tether of simple methylene groups of 3,4, 5, and 8 members. Several versatile synthetic routes to these substrates were developed. 2-Bromo-3-(trimethylsilyl)propene proves to be a key reagent for construction of the donor portion. Acceptors bearing esters, cyano groups, and especially sulfones have been examined. The diastereoselectivity of the reaction has been explored both in terms of ring juncture and the diastereofacial selectivity with respect to an oxygen substituent at the allylic position of the acceptor. Excellent cycloadditions to give the bicyclo[3.3.0]octyl and bicyclo[4.3.0]nonyl systems are observed, whereas larger rings cannot be obtained in this series. The choice of catalyst proves critical, the most useful being either tetrakis(triphenylphosphine)palladium and DPPE or, more generally, triisopropyl phosphite and palladium acetate. The first cycloaddition of a 1,1-dialkylated TMM precursor, which fails in intermolecular cases, has been observed in this intramolecular series to give a bridgehead-substituted bicycle. A rationale for the observed diastereoselectivity is presented.