Amino acids deprotection

The hexahydropyrazinones 179 were subjected to further peptide coupling with various AT-protected amino acids, deprotection and repeated cyclization to give bicyclic dipeptides with a spirocyclopropaneoctahydropyrazinopyrazine skeleton of type 182 (Scheme 54). Compounds of the types 182 and 179 represent potentially useful classes of geometrically defined peptidomimetics. For example, the skeleton of 179 has been found in the hydrolysis products of the naturally occurring lysomarasmine [90]. 

Resin (polystyrene-based resin, 1% divinylbenzene, 100-200 mesh) substitution and amino acid deprotection were carried out in disposable... 

The reaction of phosgene with benzyl alcohol yields benzyl chloroformate (carbobenzoxy chloride), CICO2CH2C6H5, useful in protecting (Ege, 1999) amino groups of amino acids. (Deprotection is accomplished with H2/Pd/C.)... 

While these results clearly show, that the triazole ring is formed upon click reaction and that it is stable under amino acid deprotection conditions, further measurements are necessary to quantify the yield of the deprotection reaction. 

The build-up of the bicylic peptidomimetics (18), follows the same sequence as that used for the monocyclic compounds followed by, coupling of the piperazinone 33 with a Boc-protected amino acid, deprotection and Aermally induced cyclization (Figure 10). With chiral amino acids used in the second step, in most cases separation of the two diastereomers by chromatography was possible (Table III). 

Then N-Boc-O-benzylserine is coupled to the free amino group with DCC. This concludes one cycle (N° -deprotection, neutralization, coupling) in solid-phase synthesis. All three steps can be driven to very high total yields (< 99.5%) since excesses of Boc-amino acids and DCC (about fourfold) in CHjClj can be used and since side-reactions which lead to soluble products do not lower the yield of condensation product. One side-reaction in DCC-promoted condensations leads to N-acylated ureas. These products will remain in solution and not reaa with the polymer-bound amine. At the end of the reaction time, the polymer is filtered off and washed. The times consumed for 99% completion of condensation vary from 5 min for small amino acids to several hours for a bulky amino acid, e.g. Boc-Ile, with other bulky amino acids on a resin. A new cycle can begin without any workup problems (R.B. Merrifield, 1969 B.W. Erickson, 1976 M. Bodanszky, 1976). 

In each step of the usual C-to-N peptide synthesis the N-protecting group of the newly coupled amino acid must be selectively removed under conditions that leave all side-chain pro-teaing groups of the peptide intact. The most common protecting groups of side-chains (p. 229) are all stable towards 50% trifluoroacetic acid in dichloromethane, and this reagent is most commonly used for N -deprotection. Only /ert-butyl esters and carbamates ( = Boc) are solvolyzed in this mixture. 

Another protecting group of amines is 1-isopropylallyloxycarbonyl, which can be deprotected by decarboxylation and a /3-elimination reaction of the (tt-l-isopropylallyl)palladium intermediate under neutral conditions, generating CO2 and 4-methyl-1,3-pentadiene. The method can be applied to the amino acid 674 and peptides without racemization[437]. 

The A/-carboxyl group is lost duting the reaction, and no additional deprotection step is requited (104). Benzene reacts with A/-carboxyglyciae anhydride to give aminomethyl phenyl ketone however, it does not react with other A/-carboxy-a-amino acid anhydrides (105). 

These methodologies have been reviewed (22). In both methods, synthesis involves assembly of protected peptide chains, deprotection, purification, and characterization. However, the soHd-phase method, pioneered by Merrifield, dominates the field of peptide chemistry (23). In SPPS, the C-terminal amino acid of the desired peptide is attached to a polymeric soHd support. The addition of amino acids (qv) requires a number of relatively simple steps that are easily automated. Therefore, SPPS contains a number of advantages compared to the solution approach, including fewer solubiUty problems, use of less specialized chemistry, potential for automation, and requirement of relatively less skilled operators (22). Additionally, intermediates are not isolated and purified, and therefore the steps can be carried out more rapidly. Moreover, the SPPS method has been shown to proceed without racemization, whereas in fragment synthesis there is always a potential for racemization. Solution synthesis provides peptides of relatively higher purity however, the addition of hplc methodologies allows for pure peptide products from SPPS as well. 

This procedure is restricted mainly to aminodicarboxyhc acids or diaminocarboxyhc acids. In the case of neutral amino acids, the amino group or carboxyl group must be protected, eg, by A/-acylation, esterification, or amidation. This protection of the racemic amino acid and deprotection of the separated enantiomers add stages to the overall process. Furthermore, this procedure requires a stoichiometric quantity of the resolving agent, which is then difficult to recover efficiendy. Practical examples of resolution by this method have been pubUshed (50,51). 

Benzyl carbamates are readily cleaved under strongly acidic conditions HBr, AcOH 50% CF3COOH (25°, 14 days, partially cleaved) - 70% HF, pyridine CF3S03H FSOaH, or CHjSO.H.- In cleaving benzyl carbamates from peptides, 0.5 M 4-(methylmercapto)phenol in CF3CO2H has been recommended to suppress Bn additions to aromatic amino acids. To achieve deprotection via an Sn2 mechanism that also reduces the problem of Bn addition, HF-Me2S-p-cresol (25 65 10, v/v) has been recommended for peptide deprotection. 

This ester was developed to impart greater hydrophilicity in C-terminal peptides that contain large hydrophobic amino acids, since the velocity of deprotection with enzymes often was reduced to nearly useless levels. Efficient cleavage is achieved with the lipase from R. niveus (pH 7, 37°, 16 h, H2O, acetone, 78-91% yield)... 

Preparation of PhAcOZ amino acids proceeds from the chloroformate, and cleavage is accomplished enzymatically with penicillin G acylase (pH 7 phosphate buffer, 25°, NaHS03, 40-88% yield). In a related approach, the 4-ace-toxy derivative is used, but in this case deprotection is achieved using the lipase, acetyl esterase, from oranges (pH 7, NaCl buffer, 45°, 57-70% yield). 

This group was developed for the protection of amino acids. It is formed from 4-ethoxy-l,l,l-trifluoro-3-buten-2-one in aqueous sodium hydroxide (70-94% yield). Primary amino acids form the Z-enamines, whereas secondary amines such as proline form the -enamines. Deprotection is achieved with 1-6 N aqueous HCl in dioxane at rt. ... 

Q, 9/3,9n/3)]-9-(Benzyloxycarbonylamino)-6-oxoperhydropyrido[2,l-Z)][l, 3]thiazine-4-carboxylic acid was obtained from the methyl ester by treatment with 2 N LiOH in MeOH at 0°C for 4.5 h. The carboxyl group was coupled with amino esters. The 9-(benzyloxycarbonylamino) group was deprotected by treatment with a 1 1 mixture of TFA and CH2CI2 at room temperature and the amino group was acylated with an amino acid (97MIP4, 98USP5710129). 

Deprotection of 2-(rerr-butoxycarbonylamino)-3-(4-oxo-4/f-pyrido[l, 2-n]pyrimidin-3-yl)propionate 190 with cone. HCl afforded the amino acid 191 in quantitative yield (95TL7503, 97JCS(P1)1297). 

The mixture of deprotected amino acid derivatives in solution was then immobilized onto a polymeric solid support, typically activated 5-)xm macroporous poly(hydroxyethyl methacrylate-co-ethylene dimethacrylate) beads, to afford the chiral stationary phases with a multiplicity of selectors. Although the use of columns... 

The cycle of deprotection, coupling, and washing is repeated as many times as desired to add amino acid units to the growing chain. 

Robotic peptide synthesizers are now used to automatically repeat the coupling, washing, and deprotection steps with different amino acids. Each step occurs in high yield, and mechanical losses are minimized because the peptide intermediates are never removed from the insoluble polymer until the final step. Using this procedure, up to 25 to 30 mg of a peptide with 20 amino acids can be routinely prepared. 

Enantiomerically pure of-dibenzylamino-/V-tosylimines 2 arc accessible from amino acids. Since they are not suitable for storage it is advantageous to prepare them in situ from the corresponding aldehydes 1 and A-sulfmyl-4-toluenesulfonamide immediately before use. Addition of Grignard reagents affords the protected 1,2-diamines 3 in good yields (57-95%) and diastereoselectivities (d.r. 85 15 >95 5)8. Deprotection is achieved without racenuzation by reductive methods, see 4-6. 

These amino acids were initially synthesized by asymmetric aminomethylation of optically pure (R)- and (S)-N-Acyl-4-phenyhnethyl)oxazolidin-2-ones 52 through TiCVenolates (Evans methodology [135]) with (benzoylamino)methylchloride or benzyl N-(methoxymethyl)carbamate [66, 97-99, 104]. Hydrolytic removal of the auxiliary yielded the N-protected (benzoyl or Z) amino acid 54. Deprotection afforded the free amino acid which was converted to the required Boc- or Fmoc-pro-tected derivatives (Scheme 2.7). 

This route has been widely exploited because of the availability of a-amino azomethine compoimds from natural (S)-a-amino acids, through the corresponding a-amino aldehydes, which are configurationally stable provided that the amino function is suitably protected. Moreover, some a-amino acids are available with the R configuration and a number of enzymatic and chemical transformations have been described for the preparation of optically active unnatural a-amino acids. Overall, the route suffers from the additional steps required for protection/deprotection of the amino function and, in the case of hydrazones and nitrones, cleavage of the N - N or N - O bond. 

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