Amino adds esters, formation

Formation of Amides. When condensed with ammonia or amines, amino add esters form acid amides ... 

Figure 3. Correlation between the initial velocities of the papain-catalyzed aminolysis of ethyl hippurate by amino add esters (abscissa) and the initial velocities (extent after 2 hr) of their incorporation during the plastein formation from an ovalbumin hydrolysate by papain (ordinate). Open circles ethyl esters. Filled drcles n-hexyl esters. Unusual abbreviations Abu = a-aminobutyric acid, Nva = norvaline.

Process economics dictate the recycling of the unwanted isomer. Path A in Figure A8.2 illustrates that racemisation of the D-N-benzylidene amino add amide is fadle and can be carried out under very mild reaction conditions. After removal of die benzaldehyde die D,L-amino add amide can be recyded 100% conversion to the L-amino add is theoretically possible. Another method for racemisation and recycling of the L-amino add (path B, Figure A8.2) comprises the conversion of the L-amino add into die ester in the presence of concentrated add, followed by addition of ammonia, resulting in the formation of the amide. Addition of benzaldehyde and racemisation by OH- (pH =13) gives the D,L-amino add amide. In this way 100% conversion to die D-amino add is possible. 

These are major disadvantage of the esterase resolution process. Since die optimum pH of die enzymic reaction is generally on the alkaline side, die esters used as substrates are non-enzymatically hydrolysed and die optical purity of die L-amino adds obtained is generally low. Also the substrate has to be protected at the amino group in most cases in order to prevent formation of diketopiperasines. The esterase method is not attractive in practice and to the best of our knowledge is not used on an industrial scale. 

Acidic proteinoid potentiates the active structure of lysine-rich proteinoid participating in forming microspheres in neutral buffer. Physical surface effects and providing micro condition in the microspheres could be surmised. Activation of amino acids generally requires acidic condition. Amino acids are activated by ATP and Mg2+ at pH 4-5 32 33). Aminoacyl adenylate anhydride and ester is formed preferentially from amino acid and adenylate imidazolide at pH 6.0J7). On the other hand, polycondensation of activated amino acids undergoes at pH values higher than 7. Peptides are formed from aminoacyl adenylate in basic buffer (the optimum pH is 10 for alanyl adenylate 40) from amino acid adenylate phosphoramidate and imidazole at pH 7.0 from N-(aminoacyl)-imidazole at pH 6-9 43). In this context, acidic and basic environments may be provided inside and/or on the surface of the microspheres composed of acidic and basic proteinoids in neutral buffer. Acidic micro condition suitable for the activation of amino acids and basic micro environment favorable for peptide formation from activated amino add may be provided. 

The procedure described is essentially that of Shioiri and Yamada.4 Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis.5 It has been used for racemlzation-free peptide syntheses,4 6 7 thiol ester synthesis, a modified Curtius reaction,6 9 10 an esterification of a-substituted carboxylic add,11 formation of diketopiperazines, an alkyl azide synthesis,13 phosphorylation of alcohols and amines,14 and polymerization of amino acids and peptides.15 Furthermore, diphenyl phosphorazidate acts as a nltrene source3 and as a 1,3-dipole.16 17 An example In the ring contraction of cyclic ketones to form cycloalkanecarboxyllc acids 1s presented 1n the next procedure, this volume. 

Peptide synthesis is made possible by the use of selective protecting groups. An N-protected amino add with a free carboxyl group is coupled to an 0-protected amino acid with a free amino group in the presence of dicyclo-hexylcarbodiimide (DCC). Amide formation occurs, the protecting groups are removed, and the sequence is repeated. Amines are usually protected as their tert-butoxycarbonyl (BOO derivatives, and acids are protected as esters. This synthetic sequence is often carried out by the Merrifield solid-phase technique, in which the peptide is esterified to an insoluble polymeric support. 

Hydrochlorides of enamines add as iminium salts to dialkyl phosphites with formation of a-amino phosphonic esters.132... 

The classical method for making rerf-butyl esters involves mineral acid-catalysed addition of the carboxylic add to isobutene but it is a rather harsh procedure for use in any but the most insensitive of substrates [Scheme 6.33]. °" 2 Moreover, the method is hazardous because a sealed apparatus is needed to prevent evaporation of the volatile isobutene. A simpler procedure (Scheme 6.34] involves use of rm-butyl alcohol in the presence of a heterogeneous acid catalyst — concentrated sulfuric add dispersed on powdered anhydrous magnesium sulfate. No internal pressure is developed during the reaction and the method is successful for various aromatic, aliphatic, olefinic, heteroaromatic, and protected amino adds. Also primary and secondary alcohols can be converted into the corresponding /er/-butyl ethers using essentially the same procedure (with the exception of alcohols particularly prone to carbonium ion formation (e.g. p-... 

Procedure A [136] formation of S — )-N- trifluoroacetylj-prolyl amino acid esters Esterify the amino acids (5 mg) with thionyl chloride/methanol (1 9)(5 ml) under reflux for 2 h and remove the excess reagent in vacuo. Dissolve tfie residue in S( — )-Ai(trifluoroacetyl)prolyl chloride (0.017 M in chloroform, 2 ml) and add triethylamine (0.1 ml). Allow to stand for 15 min at room temperature and partition against distilled water. Separate the organic phase, dry over sodium sulphate and analyse by gas chromatography. 

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