Regio- and stereoselectivity in palladium-catalyzed cycloreductions of 1,6-enynes in the presence of formic acid or triethylsilane

Palladium catalyzed 1,6-enyne cycloreductions in the presence of 1.2 equiv. of formic acid (method A) would involve cycloalkylpalladium formates which proceed via two consecutive steps: β-elimination of alkylpalladium intermediates and then reduction at the less hindered olefins regio- and stereoselectivity. Triethylsilane, however, directly reduced the alkylpalladium intermediates to give the corresponding cycloreduced products (method B).