Copper complexes of chiral, C2‐symmetric semicorrin ligands were found to be efficient catalysts for the cyclopropane formation from olefins with diazo compounds. In the presence of 1 mode‐% of catalyst, alkyl diazoacetates reacted smoothly with terminal olefins such as styrene, butadiene, and 1‐heptene to give the corresponding optically active cyclopropanecarboxylic‐acid derivatives (Table 1 Scheme 2). With one of the catalysts, enantioselectivities up to 97% ee were obtained (Table 2). Usually, the reactions were carried out using bis(semicorrinato)copper(II) complexes as precatalysts. In order to produce active catalysts, these complexes had to be activated first by heating in the presence of diazoacetate or by treatment with phenylhydrazine. Experiments with (semicorrinato)copper(I) complexes, prepared in situ from copper(I) tert‐butoxide (Scheme 4), suggest that the actual catalyst is a [mono(semicorrinato)]copper(I).

An efficient synthesis of chiral semicorrin ligands is described (see 6-9, Schemes 2 and 3 ) . Both enantiomers are readily obtained in enantiomerically pure form starting either from D-or L-pyroglutamic acid (1). Semicorrins of this type possess several features that make them attractive ligands for enantioselective control of metal-catalyzed reactions. Their structure is characterized by C, symmetry, a conformationally rigid ligand system, and two stereogenic centers adjacent to the coordination sphere. In a metal complex, the two substituents at the stereogenic centers shield the metal atom from two opposite directions and, therefore, are expected to have a pronounced effect on the stereochemical course of a reaction occurring in the coordination sphere. The structure of these two substituents can be easily modified in a variety of ways. A series of (semicorrinato)copper(II) complexes (see 1 W 4 , Scheme 4 ) has been prepared, and in one case (14), the three-dimensional structure has been determined by X-ray analysis (Fig. I).Esterification of L-pyroglutamic acid ((-)-1) in MeOH, catalyzed by acidic ion-exchange resin, gave the corresponding methyl ester in essentially quantitative yield without notable racemisation. Methyl pyroglutamate Scheme I CN HOOC

Optical yields of up to 97% ee characterize the cyclopropanation of olefins with diazoacetates in the presence of the semicorrin complex CuL2 (R = CMe2OH). The chiral, C2‐symmetrical semicorrin ligands LH are readily accessible in optically pure form from pyroglutamic acid as starting material.

Copper complexes of chiral semicorrin ligands (preparation described in the preceding paper) are found to be efficient catalysts for the preparation of optically active cyclopropanes such as (III) and (IV) from the alkenes (I) and the diazo compounds (II).

The syntheses of the semicorrin ligands (V) and their conversions to the chiral copper chelates (VI) are reported.