Access to Optically Active 7-Membered Rings by a 2-Step Synthetic Sequence: Cu-Catalyzed Stereoselective Cyclopropanation of Branched 1,3-Dienes/Rh-Catalyzed Stereoconvergent [5 + 2] Cycloaddition
Michele Garbo, Céline Besnard, Laure Guénée, Clément Mazet
Abstract
We report a 2-step cyclopropanation/cycloaddition catalytic sequence that provides access to optically active 7-membered rings with a carboxyl-bearing tertiary stereocenter. In the first step, the Cu-catalyzed cyclopropanation of branched dienes generates vinylcyclopropanes in high yields, regioselectivity and enantioselectivity, albeit in modest cis/trans diastereoselectivity. The stereoconvergent nature of the subsequent Rh-catalyzed [5 + 2] cycloaddition with alkynes overrides this apparent limitation and affords preferentially one 7-membered ring out of the 8 or 16 possible stereoisomers that can be theoretically generated. The final regioselectivity and enantioselectivity are high in the majority of cases. The method is versatile and tolerates a broad range of functional groups.