Amino acid anions

Amino acid anion 1743-1729 Often masked by water deformation band near... 

Attempts to isolate GTF from brewer s yeast have resulted in production of very active concentrates, but the substance is too labile to be obtained in the soHd state (136). However, it has been shown that GTF is a Cr(III) complex containing two coordinated nicotinate radicals and other amino acid anions (146). Active preparations containing similar complexes have been synthesi2ed (147). Chromium deficiency may also lead to atherosclerosis and peripheral neuropathy. 

More recently Hand et al. (ref. 9) have studied the decomposition reaction of N-chloro-a-amino acid anions in neutral aqueous solution, where the main reaction products are carbon dioxide, chloride ion and imines (which hydrolyze rapidly to amine and carbonyl products). They found that the reaction rate constant of decarboxylation was independent of pH, so they ruled out a proton assisted decarboxylation mechanism, and the one proposed consists of a concerted decarboxylation. For N-bromoamino acids decomposition in the pH interval 9-11 a similar concerted mechanism was proposed by Antelo et al. (ref. 10), where the formation of a nitrenium ion (ref. 11) can be ruled out because it is not consistent with the experimental results. Antelo et al. have also established that when the decomposition reaction takes place at pH < 9, the disproportionation reaction of the N-Br-amino acid becomes important, and the decomposition goes through the N,N-dibromoamino acid. This reaction is also important for N-chloroamino compounds but at more acidic pH values, because the disproportionation reaction... 

The X-ray structure of [cp Ir(CNR)Cl2], CNR = l,3,4,6-tetra-0-acetyl-2-deoxy-2-isocyano-Q,, 3-D-glucosc, has been determined.428 The synthesis of chiral complexes of Ir111 with Q-amino acid anions, L-L, of general formula [cp Ir(Cl)(L-L )] (252), and their NMR spectroscopic characterization, have been detailed. The X-ray structures of (252), L-L = L-proline and [cp Ir(Cl)(L-His-OH)]Cl, His = histidine, are described.429 C-allylglycinate binds in a terdentate manner in (253), which has been characterized by X-ray diffraction studies.430 C-vinylglycinate forms complex (254). 

Brook has effectively modified a procedure (introduced by Hosomi) which employs a trialkoxysilane as the stoichiometric reducing agent which, in the presence of amino acid anions reduces aryl alkyl ketones or diaryl ketones to the corresponding (A)-secondary alcohols, albeit in modest ee (generally 25 40%). ... 

Reduction of ketones using amino acid anions as catalyst and hydrosilane as oxidant... 

REDUCTION OF KETONES USING AMINO ACID ANIONS AS CATALYST AND HYDROSILANE AS OXIDANT... 

One of the fundamental operations in organic synthesis remains the stereoselective reduction of carbonyl groups1241. In a process related to that reported by Hosomi et u/.[25], using hydrosilanes as the stoichiometric oxidant and amino acid anions as the catalytic source of chirality, a variety of ketones were reduced in good to excellent yield and with good stereoselectivity1261. This process reduces the amount of chiral catalyst needed and utilizes catalysts from the chiral pool that can be used directly in their commercially available form. 

Table 11.9 Reduction of Ketones Using HSi(OEt)3 and amino acid anions.

ENANTIOMERIZATION DURING REACTIONS OF ACTIVATED N-ALKOXYCARBONYLAMINO ACIDS WITH AMINO ACID ANIONS... 

FIGURE 4.17 Enantiomerization data for reactions of activated IV-alkoxycarbonyl-L-amino acids with an amino acid anion.71,72 Percentage -d-d- peptide formed in reaction with H-d-Val-CL-Na+ in dimethylformamide-water (4 1) at 23°C. 

NL Benoiton, Y Lee, FMF Chen. Racemization during aminolysis of mixed and symmetrical anhydrides of IV-alkoxycarbonylamino acids by amino acid anions in aqueous dimethylformamide. Int J Pept Prot Res 31, 443, 1988. 

FIGURE 7.31 Aminolysis of a succinimido ester by (A) an amino-acid anion generated by base [Anderson et al., 1974] and (B) a peptide anion that is in equilibrium with the peptide.114 Pg = protecting group. Appropriate solvents are tetrahydrofuran, acetone, or dimethylforma-mide with water. 

However, similar problems can also occur with the alkylammonium extractants, especially as the pH of the solution must be sufficiently high to produce an adequate concentration of the amino acid anions, i.e., more than two units above the pK of the amino acid. Thus, coextraction of hydroxyl ions competes with the amino acid anions and in addition lowers the pH of the aqueous phase and reduces the concentration of amino acid anions. To avoid such problems, a large buffer capacity is required in the aqueous phase with the buffer chosen to minimize coextraction of anions. 

Amino Acids and Peptides Amino acids Anionic detergents H(0-MeOH Octadecyl 237... 

The [Co(dien)(GlyGlyOR)X]2+ complexes prepared by Wu and Busch178 have the configuration (39), in which the dien ligand adopts a mer configuration and the ligand X lies trans to oxygen. A series of frans-(0,X)-[CoX(dien)(AA)]+ complexes (AA = amino acid anion, X = C1, N02, CN) have been prepared. 

Tee s group has reported on the catalysis of enolization of indan-2-one (200) by a-CD, [1-CD, y-CD, hydroxyethyl-jS-CD, and hydroxypropy I -/i-C D, all of which accelerate the reaction by up to 22-fold, but dimethyl-jS-CD slows it by about half.170 These workers have also looked at the effect of alcohols on the basic cleavage of m-nitrophenyl hexanoate by /1-CD.171 Finally, they have been examining the reaction of a-amino acid anions with p-nitrophenyl acetate and hexanoate in the presence of [1-CD.172... 

Returning to the main theme in this section, another case where chelation to a metal centre controls reactions involving enolates is seen in complexes of amino acid derivatives. Amino acids are commonly found in metal complexes as the chelated anions in which the carboxylate oxygen and the amino group are co-ordinated to the metal. The co-ordinated amino acid anion could be in the keto (5.6) or enolate (5.7) form. 

Figure 5-67. The formation of the same chelated amino acid anion is observed whichever mechanism is adopted.

Equation 26. Electron-deficient flavins will also oxidize nitroalkane anions in model reactions (12). The observation (11) that nitromethane anion and FloXEt yield a stable 4a-adduct is evidence that 4a-adducts are not on the reaction path for nitroalkane oxidation. That the blocking of the N(5)-position of flavin (i.e., FloxEt) prevents oxidation of nitromethane would, however, be in accord with the requirement for an N(5)-adduct (11). The nitroalkane reaction with flavoenzyme has been used to implicate N(5)-adducts as intermediates in the oxidation mechanism of amino acid oxidases. However, it must be understood that nitroalkane anions differ significantly from the carbanions generated from a normal substrate. The nitroalkane anion on loss of its pair of electrons would provide an impossibly unstable carbonium ion, whereas in the case of the amino acid anion an internal electron release obviates carbonium ion formation. 

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