ORGN 416 |
| Palladium and Ruthenium catalyze cycloisomerizations of ynylidene-cyclopropanes via formal [3+2] additions to yield [3.3.0] bicyclic products. In contrast, cationic gold phosphine complexes catalyze cationic pathways to afford [4.2.0] bicycles. We propose that activation of the alkyne by gold followed by nucleophilic attack by the methylenecyclopropane results in formation of a cyclohexane with a secondary carbocation that is stabilized by the cyclopropane. Gold backbonding aids in ring-expansion to furnish the [4.2.0] bicycle and generate an allylic cation, which can then be quenched to produce the desired products. Using substrates with pendant aromatic rings allowed for trapping of the allylic carbocation to afford tetracycles. Additionally, this reaction has been rendered asymmetric, providing access to chiral all-carbon quaternary benzylic stereocenters.
|
|
New Reactions and Methodology
8:00 AM-12:00 PM, Tuesday, August 21, 2007 BCEC -- 258B, Oral
Division of Organic Chemistry |
