Amino acids, aromatic optically active

These amino acids exhibit optical activity in some cases, dextrorotatory, e.g. alanine, in other cases laevorotatory, e.g. phenylalanine. Because of the conjugated double-bond system of their aromatic rings, tyrosine, tryptophan and phenylalanine absorb light in the ultraviolet region. The absorp-... 

Pybox L8 (Fig. 4) was synthesized from pyridine-2,6-dicarboxylic acid and optically active amino alcohols via an amido chloride intermediate [ 16,23 ] or via BFj-catalyzed cychzation of intermediate amino alcohols [24]. The combination of Pybox-i-Pr (L8a) and [Rh(COD)Cl]2 (Rl) exhibited catalytic activity as an in-situ catalyst for the reduction of acetophenone (Kl) to give 76% ee (S) [16].However, the complex RhCl3(Pybox-z-Pr) R4a under assistance with AgBF4 accelerated the reduction in THF to give 94-95% ees [23]. Diphenylsilane (SI) was also the best silane in this system. Most aromatic methyl ketones were reduced in 90-99% ees, and reactions of levurinate KIO and 2-octanone Kl 1 resulted in 95% ee and in 63% ee, respectively. The Pybox-Rh catalyst R4a reduced selectively 2-phenylcyclohexanone K12 to give the S-alcohols for both trans- and cis-isomers PI and P2 in 96-99% ees [25]. The catalyst R4a can differentiate only the enan-... 

Not only the smallest optically active amino acid (alanine), but also leucine, several (substituted) aromatic amino acids, heterosubstituted amino acids (methionine, homomethionine and thienylglydne) and even an iminoacid, proline, are obtainable in both the L- and D-form. 

The synthesis of 3-aryltetrahydroisoquinolines was accomplished by electrophilic aromatic substitution of polysubstituted phenols and phenyl ethers with Lewis acid-generated tosyliminium ions of 2-tosyl-3-methoxytetrahydroisoquinoline derivatives <00SL801>. In addition isoquinoline was reported to react with N-tosylated (R)- or (S)-amino acid fluorides to afford optically active dihydroimidazoisoquinolinones. The reaction proceeds via acylation followed by attack of the tosylamino group at Cl of the intermediate 2-tosylaminoacylisoquinolinium salt <00TL5479>. 

Alkylation of Schiff bases, derived from amino acid and non-optically active aromatic aldehydes by phase-transfer catalysis in the presence of cinchona alkaloid derived quaternary ammonium salts, gave ce values of up to 50% l42. 

The aldol reaction of p-nitrobenzaldehyde with acetone is catalyzed by Zn2+ complexes of a-amino acid esters in MeOH, giving the optically active aldol adduct (Scheme 8B1.12) [28]. Although the enantiomeric excesses of the product have not been determined, the extent of asymmetric induction is dependent on the structure of a-substituents in the amino acids. The Zn2+ complexes of amino acid esters bearing an aromatic substituent such as esters of phenylalanine, tyrosine, and tryptophane (Trp) are more effective in terms of both catalytic activity and asymmetric induction. The highest asymmetric induction is observed with Trp-OEt ligand. 

An enantioselective Strecker reaction involving Brpnsted acid catalysis uses a BINOL-phosphoric acid, which affords ees up to 93% in hydrocyanations of aromatic aldimines in toluene at -40 °C.67 The asymmetric induction processes in the stereoselective synthesis of both optically active cis- and trans-l-amino-2-hydroxycyclohexane-l -carboxylic acids via a Strecker reaction have been investigated.68 A 2-pyridylsulfonyl group has been used as a novel stereocontroller in a Strecker-type process ees up to 94% are suggested to arise from the ability of a chiral Lewis acid to coordinate to one of the sulfonyl (g)... 

The Strecker reaction is defined as the addition of HCN to the condensation product of a carbonyl and amine component to give a-amino nitriles. Lipton and coworkers reported the first highly effective catalytic asymmetric Strecker reaction, using synthetic peptide 43, a modification of Inoue s catalyst (38), which was determined to be inactive for the Strecker reactions of aldimines (see Scheme 6.5) [62], Catalyst 43 provided chiral a-amino nitrile products for a number of N-benzhydryl imines (42) derived from substituted aromatic (71-97% yield 64->99% ee) and aliphatic (80-81% yield <10-17% ee) aldehydes, presumably through a similar mode of activation to that for hydrocyanations of aldehydes (Table 6.14). Electron-deficient aromatic imines were not suitable substrates for this catalyst, giving products in low optical purities (<10-32% ee). The a-amino nitrile product of benzaldehyde was converted to the corresponding a-amino acid in high yield (92%) and ee (>99%) via a one-step acid hydrolysis. 

Any kind of C-Mannich base has been actually allowed to react with a variety of amines, including aminoalcohols and amino acids. Cyclic amines, - including optically active derivatives, - and amides arc also employed as are a number of aromatic heterocycles. 

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