Unprecedented regio- and stereoselective conversion of 1-cyclopropyl-3- ethoxycyclopentadienes to 3-(E)-alkylidenecyclopentenes

Cyclopropylalkynes 1 were converted into cyclopropyl-substituted 3- ethoxy-5-dimethylaminocyclopentadienes 4 via initially formed (β- dimethylaminoethenyl)carbene complexes 3 and their subsequent formal [3 + 2] cycloaddition to a second alkyne of type 1 (overall yields 51-59%). New regio- and stereoselective 1,7-addition and 1,6-substitution reactions have been developed that convert cyclopentadienes 4 into highly functionalized cyclopentenes 5 and 11, respectively (yields 98-99% and 61%, respectively). An in situ domino reaction consisting of a [3 + 2] cycloaddition and subsequent 1,7-addition has also been achieved (86% overall yield).