Carboxy activation

Table 2 shows a list of collagen model peptides which have teen prepared. Many efforts have been made to prevent racemization. The polycondensation reaction seemed to be more sensitive to racemization than the coupling steps preparing the monomeric tripeptide. Therefore, the sequence of the monomer was selected with Gly or Pro at the C-terminal chain end, because racemization is mostly favored at the carboxy-activated amino acid, and these amino acids cannot racemize. 

COOH (asparagine, glutamine) -nh2 Covalent after carboxy activation -> amide... 

The 2-benzoxazolinone moiety is also effective for carboxy-activation as a comparable leaving group. In contrast, the saturated 2-oxazolidinone skeleton fails to show such a high leaving ability. Regioselective acylation of the primary... 

The side-chain protected peptide was first synthesized on 4-nitrobenzophenone oxime resin utilizing Boc chemistry (see Section 12.4.1.2). Briefly, the first amino acid residue was attached to the resin using DIC activation. Subsequent couplings of residues to the resin-bound peptide were accomplished employing the BOP reagent for carboxy activation of the a-amino acids avoiding preneutralization of the resin-bound peptides for reasons discussed above. A six-fold excess of amino acid is utilized for the third... 

The NCA 138 represents the amino-protected and carboxy-activated form of poly-oxamic acid 140, the hydroxylic amino acid portion of the antifungal family of polyoxins 139, Fig. 8. Other polyolic NCAs such as 141,142, and 143, Fig. 9, have also been prepared from the corresponding a-hydroxy p-lactams with equal success [123, 124]. 

Scheme 2 Most Common Carboxy-Activated Amino Acid Derivatives Used in Peptide Synthesisl " - ...

The intrinsic consequences of such strong activation of the carboxy group toward aminolysis are (1) the increased acidity of the C -proton which favors enolization giving 16 and (2) the facile ring closure of the carboxy-activated amino acid or peptide component to oxazol-5(4//)-ones 17 by base catalysis. Both mechanisms lead to loss of stereochemical integrity, i.e. racemization or epimerization as illustrated in Schemes 6 and (for the correct use of the terms racemization or epimerization see Vol. E 22b, Section 7.4). 

Since amide bond formation is a bimolecular reaction which involves a nucleophUic attack of the amino group on the activated carboxy group (Scheme 1), the kinetics are strongly affected by the type of carboxy activation, by steric effects exerted by the involved reaction partners, as well as by the solvents and catalysts employed (for comprehensive reviews, see refst ). Moreover, the concentration of the reactants plays a critical role and excess of one of the two components serves to drive the reaction to completion. 

Monoacyl-type protecting groups are not recommendable for N -protection, since racemi-zation of the related amino acid derivatives during carboxy activation for their coupling represents a serious problem. Consequently, Ai-acyl derivatives such as formyl or tri-fluoroacetyl have been used mainly for side-chain protection. Nevertheless, some new N -derivatives, which address this problem, have been proposed. With the quinone-derived acyl-type protecting group 3-methyl-3-(2,4,4-trimethyl-3,6-dioxocyclohexa-l,4-dienyl)butyryl (74) (Scheme 39) racemization is almost totally suppressed, and it is readily cleaved with sodium dithionite with formation of the chromanone byproduct The Al -(alkyldisulfa-... 

A step-by-step peptide synthesis from the N- to the C-terminus is not possible with chemical methods as it risks partial epimerization due to the repeated carboxy activation procedures, In constrast, the stereo- and regiospecificity of serine and cysteine proteases ensures integrity of the stereogenic center and allows ecological reaction conditions without side-chain protection. Scheme 4 shows the synthesis scheme using clostripain and chymo-trypsin as catalysts.The second coupling reaction was carried out by enzyme catalysis in a frozen aqueous system (see Section 4.2.3.1). 

A more important cause of enantiomerisation stems (Scheme 7.10) from the cyclisation of carboxy-activated derivatives of -/V-acylamino acids including peptides (7.55) to form a 5(4)-oxazolone (7.56) concurrently with the formation of a coupled product (7.55—>7.60) Enolisation of the 5(4H)-oxazolone (7.56 = 7.57 7.58) destroys the chirality and the rate of enantiomerisation depends on the... 

We then turned to an alternative approach involving carboxy activation of amino acid 145, readily obtained by Staudinger reaction of 142. Again, it was extremely difficult to separate triphenylphosphine oxide from 145, but fortunately, the crude material was quite suitable for use in the cyclization step. 

X = Temporary amine protecting group Y = Permanent side-chain protecting group A = carboxy activating group... 

Fmoc amino acid fluorides, highly carboxy-activated amino acid derivatives suited both for solution syntheses and for SPPS. These species are especially recommended for SPPS of complicated longer peptides, and also mainly for the coupling of steri-cally hindered amino acid building blocks. Fmoc amino acid fluorides are usually stable crystalline derivatives and can be synthesized from the Fmoc amino acids with DAST, cyanur fluoride, or the TFFH reagent [L. A. Carpino et al., J. Org. Chem. 1986, 51, 3732 L. A. Carpino et al, J. Am. Chem. Soc. 1990, 112, 9651 L. A. Carpino et al., J. Am. Chem. Soc. 1995, 117, 5401]. 

In a study of micelles on cyclization reactions, iV-hexadecyl-2-chloropyridinium iodide was used as an amphiphilic carboxy activating agent for lactonization and lactamization procedures (94JOC(59)415>. Ferricyanide oxidation (Decker oxidation) of 1-substituted polyarylpyridinium salts (2,4,6-triaryl, 2,3,4,6-tetraaryl, and pentaaryl) have been found to provide a general approach to synthesis of substituted pyrroles (94H(37)1347>. 

Carboxy-activated amlnoaclds have been coupled with unprotected glycosylamines to prepare both anomers of N-(L-3-aspartoyl)-D-... 

The stepwise introduction of N -protected amino acids in solid phase synthesis normally involves in situ carboxy activation of the incoming amino acid or the use of pre-formed activated amino acid derivatives. In order to drive the acylation to completion an excess of activated amino acid derivative is utilized, typically 2-10 times the resin functionality. The excess used depends on the nature of the activated species and on the resin loading and void volume of the reaction vessel employed. The most important consideration is to maintain as high an elective concentration of reagents as possible. In small-scale synthesis and with low-functionality resins, large excesses may need to be used as the dead volume of such systems tends to be quite high conversely with resins of high functionality lower excesses can be utilized. 

For covalently bonded immobilizates functionalized polymer structures have to be designed for the preparation of functionalized polymeric films on electrodes ( functionalized polymer-covered electrodes )- Some 3-functionalized thiophene derivatives have been prepared, bearing e.g. carboxy, activated ester, hydroxy, amino groups, ready for reaction with bioorganic compounds via amino, carboxy or hydroxy groups of the biomolecules as well known in peptide chemistry. 

Peptide bonds are formed by using carboxy activation... 

With the ability to protect either end of the amino add, we can synthesize peptides selectively by coupling an amino-protected unit with a carboxy-protected one. Because the protecting groups are sensitive to acid and base, the peptide bond must be formed under the mildest possible conditions. Spedal carboxy-activating reagents are used. 

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