Amino acid-derived catalysts configuration

Typical hydrogenations are carried out at 25°C with 1 to 2.5 atm of hydrogen and the catalyst and with an olefin to rhodium ratio of about 50. This way, a-N-acylaminoacrylic acids in ethanol are converted to the amino acid derivatives. The optical yields are comparable to those obtained with the previously mentioned homogeneous catalyst. The same absolute (R) configuration of the products is observed. The main advantage is that the insoluble catalyst can be reused many times. It may be recovered from the reaction mixture by filtration, under an inert atmosphere, with no loss of catalytic activity or optical purity in the hydrogenated product. 

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... 

A number of aromatic and heteroaromatic aldehydes 56 are hydrocyanated in the presence of this catalyst to give the corresponding cyanohydrins 57. A good catalytic activity is found only if the complexes are derived from dipeptides and if the amino acid fragments attached to the linear complex have identical absolute configuration (see eq. (15)) [89],... 

A cold soln. of 2-oxobutyric acid in ethanol treated with L-(—)-a-methylhenzyl-amine in the same solvent, 10%-Pd-on-charcoal added, hydrogenated 10 hrs. at 30 /50 p.s.i. until 1 mole of has been absorbed, the catalyst removed by filtration, the filtrate coned., aq. 30%-alcohol and palladium hydroxide-on-charcoal added, then hydrogenated at 25 /50 p.s.i. until Hg-uptake ceases L-butyrine. Y 75.9-84.9% excess of enantiomorph 81.4%.— Debenzylation with other catalysts was not successful. Asym. induction occurs during reduction of the azomethine and debenzylation can be performed with little or no loss of configurational integrity. The configuration of the amino acids is the same as that of the a-methylbenzylamine from which it is derived. The magnitude of the induced asymmetry depends on the substrate and the catalyst. F. e. s. R. G. Hiskey and R. G. Northrop, Am. Soc. 83 4798 (1961) asym. synthesis of amino acids s. a. J. G. Sheehan and R. E. Chandler, Am. Soc. 83, 4795 (1961). 

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