Enantioselective Cyclopropanations with Carbenoids Generated from Diazoalkanes

3 Enantioselective Cyclopropanations with Carbenoids Generated from Diazoalkanes 

Commencing in the mid-1980s, Pfaltz disclosed a collection of pioneering studies with the introduction of a new family of ligands involving semicor-rin-type structures (Equation 7) [40, 41], which captured the imagination of the community at large and evolved into the popular bisoxazoline scaffolds [42], 

Evans extensively examined the neutral bisoxazolines 49 in Cu(l)-catalyzed cyclopropanation reactions (Equation 8) [44]. The reaction between styrene and ethyl diazoacetate afforded the corresponding products 50/51 as a 73 27 mixture of trans-and ds-diastereomers with impressively high enan- 

Other important classics in catalytic asymmetric cyclopropanations are the chiral rhodium catalysts discovered and developed extensively by Doyle (Equation 9) [18, 46, 47]. The ligands employed, as well as the derived cata- 

Cyclopropanation reactions have not been limited only to applications with olefinic substrates. Doyle documented the asymmetric synthesis of cyclopropenes in the presence of Rh-MEPY catalysts (63, Equation 11) [53, 54]. Treatment of propynal diethyl acetal (80) with methyl diazoacetate in the presence of the Rh catalyst 63 thus afforded cyclopropene 81 in 42 % yield and 98% ee [53]. 

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