Hydroxylamine ester bonds

Figure 197 Cross-linkers containing an ester bond in their cross-bridge are susceptible to cleavage under alkaline conditions using hydroxylamine.

Carboxylic acids can be attached to these linkers using methods of ester bond formation such as carbodiimide/DMAP [23] and acid chloride/base. For the loading of N-protected-a-amino acids in particular, an array of different methods has been developed to minimize enantiomerizahon and dipeptide formation during the esterification reaction. These include the use of MSNT/N-methylimidazole [24], mixed anhydrides generated with 2,6-dichlorobenzoyl chloride [25], esters of 2,5-diphenyl-2,3-dihydro-3-oxo-4-hydroxythiophene [26] and acid fluorides [27]. Phenols and N-protected hydroxylamines have been immobilized using the Mitsunobu reaction [28, 29], The latter are particularly useful for the preparation of hydroxamates [29, 30],... 

The hydroxyl version of the Rink amide linker, known as the Rink acid resin (25), was developed as a tool for the preparation of protected peptide fragments [13]. The peptide-linker ester bond is labile to extremely weak acids, such as HOBt or acetic acid, allowing peptides bearing t-butyl-based side-chain protection to be cleaved intact. Conversion of the hydroxyl group into chloride [66] or trifluoroacetyl [67] provides linkers that have been used for immobilization of various nucleophiles, including alcohols, N-protected hydroxylamines, phenols, purines, amines, anilines and thiols [66-68], The stability of the cation derived from this tinker is such that even thiols and amines can be cleaved from this tinker with TFA (Figure 14.12). 

Figure 10.25 Twenty-nine specific toxicophores for mutagenicity as identified by Kazius el al. (Kazius, J-, et al. Derivation and validation of toxicophores for mutagenicity prediction. J. Med. Chem. 2005, 48, 312-320.) (A) Specific aromatic nitro, (B) specific aromatic amine, (C) aromatic nitroso, (D) alkyl nitrite, (E) nitrosamine, (F) epoxide, (G) aziridine, (H) azide, (I) diazo, (J) triazene, (K) aromatic azo, (L) unsubstituted heteroatom-bonded heteroatom, (M) aryl hydroxylamine, (N) alkyl halide, (O) acyl halide, (P) N- or 5-mustard, (Q) polycyclic aromatics, (R) bay-region, (S) K-region, (T) sulphonate-bonded C, (U) unsaturated aldehyde, (V) alkyl A-nitro, (W) diazonium, (X) p-propiolactone, (Y) unsubstituted a,p unsaturated alkoxy, (Z) l-aryl-2-monoalkyl hydrazine, (AA) aromatic methylamine, (AB) aryl hydroxylamine ester, and (AC) polycyclic planar system.

Alanine-lactic acid, /-alanine-a-hydroxyisobutyric acid and a-amino-isobutyric acid-lactic block copolymers were prepared by copolymerization of the respective a-amino acid A-carboxy anhydride and a-hydroxy-acid-anhydrosulfite in benzene in the presence of triethylamine. Specific cleavage of the ester bonds by hydroxylamine in the cold confirmed the AB block structure of short chain lengths. 

Fig. 45. Hypothetical arrangement of serine (or serine-containing peptide) in main polynucleotide chain of desoxyribonucleic acid (Bendich and Rosenkranz, 1962), On the segment of the structure an ester bond between an amino acid and the 3 -carbon atom of a nucleotide is illustrated. It is assumed that rupture of the chain by the action of hydroxylamine takes place at the site of this ester bond as a result a hydroxamate is formed.

It has been shown (see text) that hydroxylamine splits DNA into fragments retaining their double-stranded structure hence it may be postulated that amino acid ester bonds in both DNA chains are located one opposite the other. 

Figure 20.6 Available amine groups on an antibody molecule may be modified with the NHS ester end of SATA to produce amide bond derivatives containing terminal protected sulfhydryls. The acetylated thiols may be deprotected by treatment with hydroxylamine at alkaline pH. Reaction of the thiolated antibody with a maleimide-activated enzyme results in thioether crosslinks.

Intermolecular or intramolecular 1,4-addition of hydroxylamines as well as N- and 0-alkylhydroxylamines 39 to activated carbon-carbon double bonds (e.g. 40, equation 27) is widely used for preparation of both A-substituted, and A,A-disubstituted hydroxylamines. The addition proceeds regiospecifically. The most commonly utilized activating groups are ester , carboxyl , suRone , ketone and 2-pyridyl . Depending on reaction conditions, addition of hydroxylamines to a,/ -unsaturated ketones can be accompanied by formation of oximes . 

Pyrimidines can be formed in reactions involving multiple bond formations, and the reactions of this subgroup have a long history <1994HC(52)1>. A recent example is the synthesis of a 6-substituted uracil derivative 740 (Scheme 9), where an a,/3-unsaturated ester 737, A,0-bis(trimethylsilyl)hydroxylamine, phenyl chloroformate, and ammonia supplied the four components of C(4)-C(5)-C(6), N-1, C-2, and N-3, respectively <2000TL4307> ... 

Most recent investigations aimed specifically at the study of nitrenium ions have employed the heterolytic cleavage of various esters of hydroxylamines 46 (Fig. 13.26). ° This method has the advantage of not requiring the introduction of acids to promote the reaction. Also, because the N—O bond is stronger than the N—Cl, it is less likely to result in homolysis. With sterically unhindered esters (such as acetylhydroxylamines), complications can arise from acyl-transfer reactions, which generate the corresponding A-hydroxylamine Por this... 

The first term of the rate law requires acid-catalyzed decomposition of the conjugated acid of the ester. This term predominates only under strongly acidic conditions. It has not been investigated in detail, but the major product of the acid catalyzed reaction is the corresponding hydroxylamine. The second term predominates under neutral to mildly acidic conditions. This term is consistent with uncatalyzed heterolysis of the N—O bond of the neutral ester to generate a heteroaryinitrenium ion. " The rate law is more complicated than that for reactive esters of carbocyclic hydroxylamines or hydroxamic acids that show pH-independent decomposition over a wide pH range. The kinetic behavior of the heterocyclic esters is caused by protonation of a pyridyl or imidazolyl N under mildly acidic conditions. The protonated substrates are not subject to spontaneous uncatalyzed decomposition, so decreases under acidic conditions until acid-catalyzed... 

Oligonucleotides containing amine groups introduced by enzymatic or chemical means may be modified with SATA (Chapter 1, Section 4.1) to produce protected sulfhydryl derivatives. The NHS ester end of SATA reacts with a primary amine to form a stable amide bond. After modification, the acetyl protecting group can be removed as needed by treatment with hydroxylamine under mildly alkaline conditions (Fig. 401). The result is terminal sulfhydryl groups that can be used for subsequent labeling with thiol-reactive probes or activated-enzyme derivatives (Kumar and Malhotra, 1992). 

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