Abstract
The hemilabile chiral C2 symmetrical bidentate substituted amide ligands (1R,2R)-5a-d and (1S,2S)-6a-d were synthesized in quantitative yield from (1R,2R)-(+)-3-methylenecyclo-propane-1,2-dicarboxylic acid (1R,2R)-3 and (1S,2S)-(-)-3-methylene-cyclopropane-1,2-dicarboxylic acid (1S,2S)-3, in two steps, respectively. The chiral Feist’s acids (1R,2R)-3 and (1S,2S)-3 were obtained in good isomeric purity by resolution of trans-(±)-3-methylene-cyclopropane-1,2-dicarboxylic acid from an 8:2 mixture of tert-butanol and water, using (R)-(+)-α-methylbenzyl amine as a chiral reagent. This process is reproducible on a large scale. All these new synthesized chiral ligands were characterized by 1H-NMR, 13C-NMR, IR, and mass spectrometry, as well as elemental analysis and their specific rotations were measured. These new classes of C2 symmetric chiral bisamide ligands could be of special interest in asymmetric transformations.
Highlights
Over the past few decades, C2 symmetric chiral amides and sulfonamides have proven to be efficient ligands for several asymmetric transformations [1,2,3,4], due to their great potential of bindingMolecules 2012, 17 with the metal alkoxides, especially with Ti(IV) alkoxides, through the nitrogen atom
We report the preparation of (±)-Feist’s acid as a chiral precursor in order to introduce a cyclopropane framework into the C2 symmetric chiral ligands
A series of new chiral bisamide ligands (1R,2R)-5a–d and (1S,2S)-6a–d (Figure 1) has been synthesized from the highly enantiopure Feist’s acids (1R,2R)-3 and (1S,2S)-3, which were prepared from trans-(±)-3-methylenecyclopropane-1,2-dicarboxylic acid (2) in very good yield (~92%) using (R)-(+)-α-methylbenzyl amine as a chiral reagent
Summary
Over the past few decades, C2 symmetric chiral amides and sulfonamides have proven to be efficient ligands for several asymmetric transformations [1,2,3,4], due to their great potential of bindingMolecules 2012, 17 with the metal alkoxides, especially with Ti(IV) alkoxides, through the nitrogen atom. A series of new chiral sulfonamides with a rigid cyclohexyl backbone were introduced by Wals and co-workers for the asymmetric addition of diethyl zinc to aldehydes [8]. All these C2 symmetric chiral amides and sulfonamides are capable of forming five- and six-membered rings with metal chelates, in which the transition states only allow the approach of incoming groups from the less hindered side by blocking the highly hindered face. In case of C2 symmetric chiral amide ligands, these factors could be explained as follows: firstly, due to the presence of C2 symmetric axis in the chiral ligand, a number of possible transition states, in particular chiral transformations could be minimized [23]
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