RESUMO
Chiral gold(I)-cavitand complexes have been developed for the enantioselective alkoxycyclization of 1,6-enynes. This enantioselective cyclization has been applied for the first total synthesis of carbazole alkaloid (+)-mafaicheenamineâ C and its enantiomer, establishing its configuration asâ R. The cavity effect was also evaluated in the cycloisomerization of dienynes. A combination of experiments and theoretical studies demonstrates that the cavity of the gold(I) complexes forces the enynes to adopt constrained conformations, which results in the high observed regio- and stereoselectivities.
RESUMO
An efficient synthetic approach to the tricyclic 1,7a-dihydro-1,3a-ethano-indene and 1,8a-dihydro-1,3a-ethano-azulene skeletons from suitable propargyl vinyl ethers is based on a one-pot, multistep process entailing a gold(I)-catalyzed propargyl Claisen rearrangement/Nazarov cyclization, a [4+2] cycloaddition of the formed six- or seven-membered ring-fused cyclopentadiene system, and a final protection step for the easy isolation and purification of the products by chromatography.
RESUMO
The tandem gold(I)-catalyzed propargyl Claisen rearrangement/hydroarylation reaction of suitable propargyl vinyl ethers, followed by in situ reduction of the resulting carbonyl group, provides functionalized indenes in good to excellent yields. The reaction occurs at room temperature in dichloromethane in the presence of 3 mol % [IPrAuCl]/AgBF4 as the best catalytic system. Instead, cyclization of the allene intermediate either does not take place or is very slow with phosphine ligands. A variety of substituents and functional groups present on the substrate are tolerated. The effect of the aryl ring substituents and the results of a density functional theory computational study suggest that the final hydroarylation is the rate-determining step of this cascade process. Further in situ chain elongation, prior final work up of the tandem process, can be carried out by Wittig olefination of the aldehyde functionality, thus incrementing the diversity of the products obtained.
RESUMO
The tandem gold(I)-catalyzed propargyl Claisen rearrangement/Nazarov cyclization of propargyl vinyl ether derivatives, followed by in situ reduction of the resulting carbonyl group, provides functionalized cyclopentadienes fused with various N-hetero- and carbacycles, including indoles, in good to excellent yields. The reaction occurs with high regioselectivity, with the position of the double bonds in the five-membered ring depending on the type of (hetero)cycle bearing the propargylic moiety and the side chain on the latter.