Copper-Catalyzed 1,5-Addition of Grignard reagents to Enantioenriched Donor-Acceptor Cyclopropanes with Inversion

A highly regioselective and stereoselective 1,5-addition of alkyl groups to enantioenriched donor–acceptor cyclopropanes 1 and bicyclic cyclopropanes such as 6-aryl-1-methoxycarbonyl-3-oxabicyclo[3.1.0]hexan-2-ones 4 using a Grignard reagent with a catalytic amount of Cu(OTf)2 (0.1 equivalent) afforded optically active diesters 2 or trans-α,β-disubstituted γ-butyrolactones 5 with an excellent level of stereoinduction. An excess of the Grignard reagent is necessary to perform the 1,5-alkylation with high yields. In the reaction of the enantioenriched bicyclic lactone 4, a highly stereoselective 1,5-addition and subsequent highly trans-selective protonation of the magnesium enolate via keto-enol isomerization furnished the trans-α,β-disubstituted-γ-lactones with three contiguous chiral centers with excellent enantioselectivity. Based on the results, we also proposed the mechanism through cluster ion pairs or a simple ion pair to rationalize the high stereoselectivity of the reaction.