Peptide azides

Kisfaludy, O Nyeki. Racemization during peptide azide coupling. Acta Chim (Budapest) 72, 75, 1972. 

Indeed, there were those who described the azide coupling method as racemization-free. [15l However, this viewpoint proved to be overly optimistic. In 1970, Sieber reported that during a synthesis of calcitonin M by the azide method, significant epimerization occurred during two of the segment condensation steps in one of these reactions 40% of the epimerized product was observed. 16 There is a crucial detail in the experimental procedure here. The workers used tert-butyl nitrite to convert a peptide hydrazide into a peptide azide, but did not isolate the azide as was typical for research at that time. Instead, they neutralized the active intermediate in situ with DIPEA and added the amino segment for acylation. This demonstrates another important theme in the control of epimerization, the presence of a tertiary amine in the reaction mixture, even if only as a neutralization equivalent, can result in the formation of epimerized products. Indeed, most observations of racemization during... 

Semicarbazones of peptide aldehydes are stable to hydrogenolysis and various coupling procedures (azide and mixed anhydride). Deprotection of the Z-protected amino acid semicarbazones, such as Z-Phe-H semicarbazone, by catalytic dehydrogenation gives the deprotected derivatives in good yields,these can be coupled to peptide azides or peptide acids using the mixed anhydride procedure. The semicarbazone is readily deprotected with 37% formaldehyde/HCl to give the peptide aldehyde. 

Scheme 1 Different Routes to Peptide Hydrazides and Peptide Azides...

More recently another modification for the preparation of peptide azides was introduced by Alfeeva et al-f l using tetrabutylammonium nitrite as auxiliary reagent. In contrast to the alkyl nitrites which are relatively unstable and therefore have to be purified prior to use by distillation, tetrabutylammonium nitrite is a crystalline and stable compound, which is soluble in anhydrous dipolar aprotic solvents. Moreover, in this procedure the acidity of the reaction mixture is adjusted with anhydrous p-toluenesulfonic acid instead of HCl in anhydrous organic solvents. These conditions are experimentally convenient and more easily controlled than those of the Honzl-Rudinger method. Comparative model reactions performed with ferf-butyl nitrite and tetrabutylammonium nitrite produced nearly identical peptide yields. To date, there are no reports of the condensation of larger fragments and peptide cyclization by this azide procedure. 

Scheme 8 Preparation of Peptide Azides by the DPPA Methodl ...

To enhance the reaction rate of azides independently of their method of preparation and to prevent related side reactions, in situ conversion of peptide azides into their corresponding A-hydroxysuccinimide esters was introduced by Hirschmann etal.h l This approach was further extended by Wang etald by the use ethyl 1-hydroxy-1//-1,2,3-triazole-4-carbox-ylate (HOCt) or 7-aza-l,2,3-benzotriazol-l-ol (HOAt) as additives, to enhance yields and reaction rates while maintaining the advantages of minimal side chain protection. In this study it was shown that HOBt is not suitable for the TAEC method, although previous reports indicated that HOBt could catalyze cyclization reactions, which were carried out by the azide method. ]... 

The disadvantage of slow coupling of peptide azides 99 has been circumvented by trans-esterifying them into the 7-azabenzotriazolyl or 4-carboxy-l,2,4-triazolyl esters 100 simultaneously with their generation from the hydrazides 98 (Scheme 29)J l Azides 99 are formed by reaction of hydrazides 98 with tertiary butyl nitrite in the presence of substituted hydroxylamines that serve as acids. The products of aminolysis by peptides 101 are N -protected peptides 102. 

Triethylamine s, a. under N-bromosuccinimide Synthesis of peptides Azide method... 

In synthetic target molecules esters, lactones, amides, and lactams are the most common carboxylic acid derivatives. In order to synthesize them from carboxylic acids one has generally to produce an activated acid derivative, and an enormous variety of activating reagents is known, mostly developed for peptide syntheses (M. Bodanszky, 1976). In actual syntheses of complex esters and amides, however, only a small selection of these remedies is used, and we shall mention only generally applicable methods. The classic means of activating carboxyl groups arc the acyl azide method of Curtius and the acyl chloride method of Emil Fischer. 

The phenolic hydroxyl group of tyrosine, the imidazole moiety of histidine, and the amide groups of asparagine and glutamine are often not protected in peptide synthesis, since it is usually unnecessary. The protection of the hydroxyl group in serine and threonine (O-acetylation or O-benzylation) is not needed in the azide condensation procedure but may become important when other activation methods are used. 

The major disadvantage of solid-phase peptide synthesis is the fact that ail the by-products attached to the resin can only be removed at the final stages of synthesis. Another problem is the relatively low local concentration of peptide which can be obtained on the polymer, and this limits the turnover of all other educts. Preparation of large quantities (> 1 g) is therefore difficult. Thirdly, the racemization-safe methods for acid activation, e.g. with azides, are too mild (= slow) for solid-phase synthesis. For these reasons the convenient Menifield procedures are quite generally used for syntheses of small peptides, whereas for larger polypeptides many research groups adhere to classic solution methods and purification after each condensation step (F.M. Finn, 1976). 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

HONZL RUDINGER Peptide Synthesis Peptide synthesis by coupling ot acyl azides with amino esters... 

Hydrazides have been used in penicillin and peptide syntheses. In the latter syntheses they are converted by nitrous acid to azides to facilitate coupling. 

The procedure described is essentially that of Shioiri and Yamada. Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis. It has been used for racemlzatlon-free peptide syntheses, thiol ester synthesis, a modified Curtius reaction, an esterification of a-substituted carboxylic acld, formation of diketoplperazines, alkyl azide synthesis, phosphorylation of alcohols and amines,and polymerization of amino acids and peptides. - Furthermore, diphenyl phosphorazidate acts as a nitrene source and as a 1,3-dipole.An example in the ring contraction of cyclic ketones to form cycloalkanecarboxylic acids is presented in the next procedure, this volume. 

Cyclic GMP is made from GTP by the enzyme gua-nylyl cyclase, which exists in soluble and membrane-bound forms. Each of these isozymes has unique physiologic properties. The atriopeptins, a family of peptides produced in cardiac atrial tissues, cause natriuresis, diuresis, vasodilation, and inhibition of aldosterone secretion. These peptides (eg, atrial natriuretic factor) bind to and activate the membrane-bound form of guanylyl cyclase. This results in an increase of cGMP by as much as 50-fold in some cases, and this is thought to mediate the effects mentioned above. Other evidence links cGMP to vasodilation. A series of compounds, including nitroprusside, nitroglycerin, nitric oxide, sodium nitrite, and sodium azide, all cause smooth muscle re-... 

Nilsson, B.L., Kiessling, L.L., and Raines, R.T. (2001) High-yielding Staudinger ligation of a phosphi-nothioester and azide to form a peptide. Org. Lett. 3, 9-12. 

Xu, J., DeGraw, A.J., Duckworth, B.P., Lenevich, S., Tann, C.-M., Henson, E.C., Gruber, S.J., Barany, G., and Distefano, M.D. (2006) Synthesis and reactivity of 6,7-dihydrogeranylazides reagents for primary azide incorporation into peptides and subsequent Staudinger ligation. Chem. Biol. Drug Des. 68, 85-96. 

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