Synthetic and mechanistic studies on enyne metathesis: A catalyst influence

We have investigated the selectivity of the intramolecular enyne metathesis catalyzed by representative first- and second-generation ruthenium carbenes. This study witnesses the very subtle and cooperative influence of different parameters on the stereochemical course of this reaction. In the case of enynes containing an internal triple bond and a monosubstituted double bond only the application of first-generation catalysts leads selectively to the formation of the expected product. If the substrate bears an internal triple bond and a 1,1-disubstituted alkene fragment first-generation catalysts are inert in this cyclization, while in the case of more reactive second-generation catalysts the transformation proceeds with high conversion, but is not selective. Only when a substrate contains a less accessible (e.g. sterically hindered) triple bond the application of second-generation catalysts can ensure a high level of selectivity. (c) 2006 Elsevier B.V. All rights reserved.