Decomposition and racemization of amino acids

Kinetics of the Nonbiological Decomposition and Racemization of Amino Acids in Natural Waters... 

Bada, J.L., 1971. Kinetics of the non-biological decomposition and racemization of amino acids in natural waters. Adv. Chem. Ser., 106 309—331. 

The effect of temperature on the rate of racemization of amino acids in fossils was investigated and the implications of the findings on fossil dating were analyzed313. The high rate of conversion of L-aspartic acid into its D-isomer, observed in uncontaminated bone samples taken from catacombs in Rome (IV century BC) was attributed to collagen decomposition due to the humidity of the catacombs314. 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

Amino Acid Dating Techniques depend on the "rates of hydrolysis reactions of proteins and racemization, epimerization, and decomposition reactions of amino acids [they have] been applied to the age-dating of fossil bone, teeth, and shell. Activation energies range from near 20 kcal per mole for hydrolysis reactions to around 30 kcal per mole for racemization... 

Aminals, compounds having two amino groups bound to the same carbon atom, are represented in many medicinal agents having versatile therapeutic action, such as proteinase inhibitors and neurotensins. Antilla and coworkers developed an en antioselective synthesis of protected aminals from the amidation reaction of N Boc imines with a series of sulfonamides catalyzed by chiral phosphoric acids (Scheme 3.54a) [111]. In this novel enantioselective transformation, phosphoric acid 9 exhibited excellent catalytic activity and enantioselectivity in addition to N Boc aromatic imines. The enantioenriched aminal products were stable upon storage neither decomposition nor racemization was observed in solution over several days. The same research group reported the enantioselective amidation reaction of N Boc aromatic imines with phthalimide or its derivatives (Scheme 3.54b) [112]. 

The various decomposition reactions outlined in the preceeding section would remove amino acid from the buffered solutions, and if these reactions were significant compared with the k values, Equation 9 would have to be modified. Since the racemization was studied under anaerobic conditions, the decarboxylation and deamination reactions are the only important amino acid decomposition reactions. With the exception of the deamination of aspartic acid, the decomposition rates of the other amino acids studied were negligible compared with the racemization rate of the amino acid. Integration of Equation 9 yields... 

The synthetic uses of the acid-promoted decomposition of a-diazoketones have been reviewed.A comparative study of methods for the synthesis of optically pure N-protected a-aminoaldehydes concludes that borane-THF is the reagent of choice for the reduction of N-protected amino-acids to the amino-alcohol without racemization, and that pyridinium dichromate is satisfactory for the subsequent oxidation to the aldehyde. Since the reaction between amines and a-haloketones is sometimes a poor method for the preparation of a-alkylaminoketones, the far less basic imidates have been developed as effective nucleophilic components in this reaction [equation (51)]. a-Aminoketones can also be prepared via Wittig rearrangement of iminoethers [equation (52)], ° or by a-aminomethylation of aldehydes [equation (53)]. ... 

Protected 6-amino-hexahydro-l,7-dioxopyrazolo[l,2-4]pyrazole-2-carboxylic acid 274 is available by a thermolytic decomposition of diazo compound 273 via the Wolff rearrangement. The starting compound is simply available by alkylation of racemic 272 with the corresponding bromoacetoacetate and subsequent diazo transfer reaction (Scheme 35) <1996TL4891>. 

---