Ring synthesis by intermolecular methylene-free tandem enyne metathesis
Kulkarni, Amol Anant
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Enyne metathesis provides an expedient atom-economical synthesis of 1,3-dienes. With the advent of functional group tolerant, highly active ruthenium-based catalyst systems, enyne metathesis has been applied to more and more challenging systems. One such application, namely the methylene-free enyne metathesis is described in this dissertation. This protocol allows for the synthesis of cyclic conjugated dienes. This reaction was first studied for a ring expansion strategy using cyclopentene. It was observed that upon reaction with terminal alkynes, cyclopentene underwent a two-carbon expansion and furnished the corresponding cycloheptadienes in good yield. The reaction was further studied with monocyclic and bicyclic cyclopentene-bearing ring systems. Using electronically deactivated cycloalkenes (such as 2-cyclopentenone) in the presence of phosphine-free initiators, we observed the novel formation of cyclopentadienes, as a result of 2+2+1 cycloaddition reaction. This is a novel reactivity for late transition metal carbene complexes. This reaction was also successfully employed for the synthesis of 1,3-cyclohexadienes, using 1,5-COD and all-cis polybutadiene as the surrogate for cyclobutene. Systematic optimization of the reaction conditions allowed us to effect this reaction using low catalyst loading (1.6 mol %). Internal alkynes, too, underwent the transformation to the corresponding 2,3-disubstituted-1,3-cyclohexadienes. Finally, by performing a tandem sequential methylene-free metathesis followed by an Ireland-Claisen rearrangement, s-trans cyclodienes were synthesized in a stereodefined fashion in good yield. The development of reaction protocols for a one-pot transformation of these dienes into highly functionalized cycloalkenes is currently under investigation.