Zinc complexes 3-amino imines

P-Amino imine complexes can also be synthesized from template reactions of p-amino ketones and amines. This approach has been applied to the amino ketone (45), which can be formed by acidic hydrolysis of a zinc(II) macrocyclic complex (Scheme 9).94-95... 

An alternative access to L-amino acids was found by using chloroform as solvent in the asymmetric Strecker synthesis with galactosyl imines [24], This interesting reverse of asymmetric induction compared to the reactions shown in Scheme 5 can be explained by considering the zinc complex A as the crucial reactive species. Due to the exo anomeric effect, which is a delocalization of the 7r-electrons into the a of the ring C-O-bond, the imines adopt the conformation represented in Scheme 8. 

In the approaches toward a direct asymmetric Mannich reaction by enolate formation with the metal of the catalyst also, the well-proved systems of the analogous aldol reactions were widely applied. Here, it is referred to some of these protocols wherein a metal enolate is involved, as least as assumed and plausible intermediate [183]. Shibasaki and coworkers used a dinuclear zinc complex derived from linked BINOL ligand 371 for catalyst in direct Mannich reactions of a-hydroxy ketones 370 with Af-diphenylphosphinoyl imines 369 to give ti-configured a-hydroxy-P-amino ketones 372 in high yield, diastereoselectivity, and enantioselectivity (Scheme 5.97) [184]. The authors postulate the metal to form a chelated zinc enolate by double deprotonation of the a-hydroxy ketone. This enolate approaches with its Si-face to the Si-face of the imine, as illustrated by the transition state model 373, in agreement with the observed stereochemical outcome. It is remarkable that opposite simple diastereoselectivity arises from the Mannich reaction (anti-selective) and the previously reported syn-selective aldol reaction [185], although the zinc enolates... 

A number of chiral catalysts originally devised for aldol reactions (see Chapter 4) have also successfully been deployed in enantioselective Mannich additions. One such example is the dinuclear zinc complex 220 developed by Trost (Equation 18) [154]. The complex is generated in situ from Et2Zn and the corresponding chiral diamino triol ligand derived from prolinol. Excellent enantio- and diastereoselectivity were obtained in reactions with a-hydroxyketones such as 218. From imine 219, the syn a-hydroxy-/l-amino ketone 221 was isolated in > 99% ee and > 15 1 dr. Additional investigations revealed that appropriate choice of the N-substituent on the imine could selectively provide either the anti or the syn a-hydroxy- -amino ketones [155]. 

High anti-diastereoselectivity is observed for several aromatic imines for ortho-substituted aromatic imines the two newly formed stereocenters are created with almost absolute stereocontrol. Aliphatic imines can also be used as substrates and the reaction is readily performed on the gram scale with as little as 0.25 mol% catalyst loading. Furthermore, the Mannich adducts are readily transformed to protected a-hydroxy-/8-amino acids in high yield. The absolute stereochemistry of the Mannich adducts revealed that Et2Zn-linked complex 3 affords Mannich and aldol adducts with the same absolute configuration (2 R). However, the diastereoselectiv-ity of the amino alcohol derivatives is anti, which is opposite to the syn-l,2-diol aldol products. Hence, the electrophiles approach the re face of the zinc enolate in the Mannich reactions and the si face in the aldol reactions. The anti selectivity is... 

Addition of lithiated alkoxy ethynyl anion with chiral Al-sulfinyl imines proceeds with dr >95 5, which can be reversed in the presence of BF3. Excellent diastereoselec-tivity has been reported for zinc-mediated addition of methyl and terminal alkynes to chiral IV-f-butanesulfinyl ketimines (to form 3-amino oxindoles). Zinc-BINOL complexes have been used to achieve enantioselective addition of terminal alkynes to N-(diphenylphosphinoyl)imines (up to 96% cc) and terminal 1,3-diynes to IV-arylimines to trifluoropyruvates (up to 97% yield and 97% ee). ... 

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