Laboratory Synthesis of Amino Acids

The laboratory synthesis of amino acids is fundamentally different from the way these compounds are synthesized in the living organisms (biosynthesis). The traditional synthesis of amino acids is known as Strecker synthesis (in the honor of Adolph Strecker). This method is based on the nucleophilic attack on the carbonyl group a reaction which we already discussed in this book. In this reaction of aldehyde with ammonium chloride and sodium cyanide the product is aminonitrile. By boiling with strong acid, this aminonitrile is transformed into the corresponding amino acid. 

Because of their importance, a number of laboratory methods for the synthesis of amino acids have been developed. In the Strecker synthesis an aldehyde is treated with NaCN and NH4CI to form an aminonitrile, which is then hydrolyzed to the amino acid ... 

A laboratory synthesis that is patterned after a biological synthesis. For example, the synthesis of amino acids by reductive amination resembles the biosynthesis of glutamic acid. (p. 1164) Proteins that provide all the essential amino acids in about the right proportions for human nutrition. Examples include those in meat, fish, milk, and eggs. Incomplete proteins are severely deficient in one or more of the essential amino acids. Most plant proteins are incomplete, (p. 1160)... 

Touring the past decade a number of research laboratories have devel-oped procedures for the rapid synthesis of amino acids labeled with short-lived, positron-emitting radionuclides. These tracers make possible the study of regional amino-acid metabolism in the living organism by external detection of the y photons produced by positron annihilation. This decay mode also permits the determination of label distribution in transverse sections through the body by means of positron emission... 

All the laboratory syntheses of amino acids described in Section 24-5 produce racemic products. In most cases, only the l enantiomers are biologically active. The d enantiomers may even be toxic. Pure l enantiomers are needed for peptide synthesis if the product is to have the activity of the natural material. Therefore, we must be able to resolve a racemic amino acid into its enantiomers. 

Alpha-bromination of a carboxylic acid is a critical step in some laboratory syndieses of amino acids. Devise a synthesis of racemic alanine from propionic acid. 

The most widely used method for the laboratory synthesis of a-amino acids is a modification of the malonic ester synthesis (Section 21.7). The key reagent is diethyl acetamidomalonate, a derivative of malonic ester that aheady has the critical nitrogen substituent in place at the a-caibon atom. The side chain is introduced by alkylating diethyl acetamidomalonate in the same way as diethyl malonate itself is alkylated. 

We ll see in Section 26.7 that this DCC-induced method of amide formation is the key step in the laboratory synthesis of small proteins, or peptides. For instance, when one amino acid with its NH2 rendered unreactive and a second... 

The synthesis of an a-amino acid from an achiral precursor by any of the methods described in the previous section yields a racemic mixture, with equal amounts of S and R enantiomers. To use an amino acid in the laboratory synthesis of a naturally occurring protein, however, the pure S enantiomer must be obtained. 

In common with all the higher AB polyamides, PA-12 can be made from either die amino acid or the lactam.12 In practice, PA-12 is made from the cheaper 12-laurolactam (12-dodecane lactam or laboratory-scale synthesis it is advisable to start with the amino acid or a combination of amino acid and lactam. 

The Jacobs-Gould intramolecular cyclization of diethyl N-(6-methyl-2-pyridyl)amino-methylenemalonate to 3-ethoxycarbonyl-7-methyl-l,8-naphthyrid-4-one is another reaction ideally suited to microwave heating, although conductively heated equipment was employed for laboratory-scale experiments [45]. The product is a key intermediate in the synthesis of nalidixic acid, the first of the quinolone antibacterials. The process usually is conducted at temperatures of 200-250 °C and in high dilution, with heat transfer oils such as the eutectic mixture of diphenyl ether and biphenyl. However, it proceeded rapidly, predictably and controllably under solvent-free conditions. 

A Rh-dipamp complex was later applied by NSC Technologies for the manufacture of several unnatural amino acids with good catalyst performances (ee 95-98%, TON 5000-20000) [30] and was also very selective but with low activity (ee 98%, TON 20) in a feasibility study for a synthesis of acromelobic acid by Abbott Laboratories [31]. 

The utilization of a-amino acids and their derived 6-araino alcohols in asymmetric synthesis has been extensive. A number of procedures have been reported for the reduction of a variety of amino acid derivatives however, the direct reduction of a-am1no acids with borane has proven to be exceptionally convenient for laboratory-scale reactions. These reductions characteristically proceed in high yield with no perceptible racemization. The resulting p-amino alcohols can, in turn, be transformed into oxazolidinones, which have proven to be versatile chiral auxiliaries. Besides the highly diastereoselective aldol addition reactions, enolates of N-acyl oxazolidinones have been used in conjunction with asymmetric alkylations, halogenations, hydroxylations, acylations, and azide transfer processes, all of which proceed with excellent levels of stereoselectivity. 

The purpose for the 1998 study was to assess the ability of and determine the means whereby member facilities identify and solve problems in peptide synthesis 10 The potential for oxidation of amino acids such as methionine is always a concern for peptide chemists and biomedical researchers. A peptide mixture containing 70% correct peptide and 30% oxidized peptide was prepared and sent to member facilities to determine if the oxidized methionine would be detected (see Table 1). In addition to the oxidized peptide, a reverse synthesized peptide was sent to the participants. In previous studies, peptides had been submitted which had been synthesized in the reverse order and if only HPLC and mass spectrometric analyses was performed, the reverse synthesis would not be identified. Therefore, two peptides were designed with the second in the reverse order with two substitutions to equal the mass of the first peptide. These two peptides were readily separated by HPLC. The second peptide was sent to the laboratories, but the laboratories were given the first sequence and asked if the correct peptide had been made. Out of 20 participating laboratories ... 

Ribonuclease A was the first enzyme to be synthesized in the laboratory. Fully active ribonuclease has been synthesized,752 as have new modified enzymes. For example a 63-residue peptide made up of five segments of the native RNase sequence retained measurable catalytic activity.753 Using total synthesis, unnatural amino acids, such as 4-fluorohistidine, have been incorporated at specific positions in RNAse.752... 

The problems involved in peptide syntheses are of much practical importance and have received considerable attention. The major difficulty in putting together a chain of say 100 amino acids in a particular order is one of overall yield. At least 100 separate synthetic steps would be required and, if the yield in each step were equal to n x 100%, the overall yield would be (n1U0 x 100%). If the yield in each step were 90%, the overall yield would be only 0.003%. Obviously, a practical laboratory synthesis of a peptide chain must be a highly efficient process. The extraordinary ability of living cells to achieve syntheses of this nature, not of just one but of a wide variety of such substances, is truly impressive. 

He studied the synthesis and utilization of sulfur-containing compounds in man, rat, dog, goat, cow, ewe, bacteria, yeasts, cockroach, and algae, constantly being aware that the results are good only as the methods which were employed to obtain them. He devoted a great deal of his time to the development of better and more refined chemical and chromatographic methods for the isolation and determination of amino acids in proteins from a variety of sources, and he had even set up a laboratory in his house for the purpose, where he worked at all hours. 

---