Hydroxylamine linkages

Hydroxamic acids are an important class of compounds targeted as potential therapeutic agents. A-Fmoc-aminooxy-2-chlorotrityl polystyrene resin 61 allowed the synthesis and subsequent cleavage under mild conditions of both peptidyl and small molecule hydroxamic acids (Fig. 14) [70]. An alternative hydroxylamine linkage 62 was prepared from trityl chloride resin and tV-hydroxyphthalimide followed by treatment with hydrazine at room temperature (Scheme 30) [71]. A series of hydroxamic acids were prepared by the addition of substituted succinic anhydrides to the resin followed by coupling with a variety of amines, and cleavage with HCOOH-THF(l 3). 

Cycloaddition of nitrones to 5,6-dideoxy-l,2-0-isopropylidene-a-D-jcy/o-hex-5-enofuranose gave mainly or exclusively the cw-substituted isoxazolines 49. The hydroxylamine-linkage of the esperamicin trisaccharide 51 was constructed by 0-glycosylation of the nitrone 50 followed by deprotection (Scheme 11). ... 

Figure 20.13 The thiolation reagent SATA can be used to create sulfhydryl groups on Fab fragments. After deprotection of the acetylated thiol of SATA with hydroxylamine, conjugation with a maleimide-activated enzyme can take place, producing thioether linkages.

When the dye is formed from tetraphenylhydrazine, diphenyl-hydroxylamine (pp. 182, 341) is first produced by hydrolysis at the N—N-linkage. 

Since the thioester linkage is susceptible to nucleophilic attack but stable to TEA treatment during solid-phase peptide synthesis (using standard Boc protection), Weigel and colleagues " have envisioned that hydroxylamine derivatives could directly cleave resin-bound peptide thioesters 201 or 202 to form the corresponding peptide hydroxamates 203 (Scheme 88). 

The linkage Asp-Gly can often be cleaved specifically by treatment with hydroxylamine at high pH.195 Procedures for specific cleavage of tryptophanyl bonds have been de-... 

The teichoic acid which had not been subjected to ion-exchange chromatography contained D-alanine in characteristic, labile, ester linkage. It contained insufficient glucose to accommodate all of the alanine ester residues (alanine phosphorus ratio, 0.89 1), and the kinetics of the reaction with hydroxylamine indicated that all of the alanine is attached to the available hydroxyl groups at C-2 of glycerol residues. 

The teichoic acid shows an infrared absorption band at 1751 cm.-1, characteristic of carboxylic ester groups, which is not observed in samples from which the D-alanine residues have been removed. Removal of the u-alanine was readily effected with ammonia or hydroxylamine, when D-alaninamide or D-alanine hydroxamate were formed. The kinetics of the reaction with hydroxylamine reveal the high reactivity of its D-alanine ester linkages, which, like those in most other teichoic acids, are activated by the presence of a neighboring phosphate group. That the D-alanine residue is attached directly to the ribitol residues, instead of to the d-glucosyl substituents, was also shown by oxidation with periodate under controlled conditions of pH, when it was found that the D-alanine residues protect the ribitol residues from oxidation. Under the same conditions, all of the ribitol residues were oxidized in a sample of teichoic acid from which the D-alanine had been removed, and it is concluded that the ester groups are attached to C-2 or C-3 of the ribitol residues. 

Enmein was converted to 20-hydroxykaur-6-en-15a-pyranylether (382), which was oxidized with chromium trioxide in pyridine to afford the aldehyde 383. The latter was converted with hydroxylamine to the oxime 384. The nitrone 385 was prepared by treatment of 384 with bromine azide. Photolysis of 385 gave the desired compound 381 in 46% yield. This intermediate possesses several useful functionalities (e.g., carbinolamine ether linkage), which may be of interest for synthesis of C20-diterpenoid alkaloids after minor changes in this scheme. 

The structure must somehow accommodate the various unusual covalent bonds (e.g., hydroxylamine-sensitive ester-type bonds, 7-car-boxylglutamyl and e-aminolysyl peptide linkages, see Section V) which have been reported in collagen and gelatin and which may possibly be involved in interchain cross-linking. 

The estimation of ester-like linkages using hydroxylamine or hydrazine, which have since been the methods most widely used for ester determination in collagen, was first carried out by Gallop et al. (1959). 

The basis of the method for the determination of ester linkages by hydroxylamine is set out in the following scheme. 

The protein is treated with hydroxylamine which splits any ester linkages forming a hydroxamic acid. The amount of hydroxamic acid present may be determined colorimetrically by (a) foimation of a ferric complex, (6) oxidation to nitrous acid which is then reacted with sulfanilic acid and a-naphthylamine to give a diazo dye, or (c) hydrolyzing off the hydroxylamine which then reacts with indole. In methods (6) and (c) excess hydroxylamine must be removed before the determination. This is normally achieved by dialysis. 

The use of the term ester-like is deliberate, as it is known that hydroxylamine will react with many other groupings to form hydroxamic acids. Among these are imides, anhydrides, acid chlorides, amides, and certain peptide linkages (Yale, 1943 Gallop el al., 1959 Williams et al.,... 

Nikkari and Kulonen (1962) tried to relate the impaired stability of lathyritic collagen to the hydroxylamine-sensitive linkages. They found values of 0.5-1.5 moles/10 gm for both lathyritic and normal collagen, but in view of the threefold range quoted and the lack of analytical data it is difficult to assess the value of this work. These values are close to those determined by Bello (1960) and later Bello and Bello (1963). The latter authors also noted a fall of molecular weight to 32,000 during the reaction,... 

Determination of Ester-Like Linkages in Collagen and Gelatin Using Hydroxylamine... 

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