Evolution of Propargyl Ethers into Allylgold Cations in the Cyclization of Enynes

摘要

Understanding the mechanisms and stereochemistry by which enynes react with metal catalysts is central for the application of these transformations in synthesis. Recently, the similarities of metal-catalyzed additions of nucleophiles to 1,6-enynes with polyene cyclizations, which proceed with an anti stereochemistry,have been emphasized. However, substrates with strongly electron-donating groups on the alkene react nonstereospecifically via open carbocations. We have now found that propargyl alcohols, ethers, and silyl ethers 1 react with gold(I) catalysts by a new type of intramolecular 1,5-migration of OR groups (Scheme 1). This reaction leads to the tricyclic compounds 2, which are related to the sesquiterpenes globulol (3 a), epiglobulol (3b), and halichonadin F (3c). Significantly, the migration proceeds via allylgold cations 4 by a syn addition of the alkyne and the OR group to the alkene. A related 1,6-migration was also found in 1,7-enynes.