Azide procedure

General coupling methods are as follows (1) azide procedure, (2) anhydrides, (3) active esters, and (4) coupling reagents. ... 

Therefore racemization 1s in the same range as that observed for the racemizatlon-resistant azide procedure. The coupling of Cbz-L-aspartyl(O-t-Bul-L-phenylalanine with valine methyl ester is reported to be very sensitive to racemization.14 The trlpeptlde was prepared as described above, and the crude product was hydrolyzed. GLC showed the presence of 2-3% D-... 

When the chiral, cyclic imide 1 is subjected to the azidation procedure outlined in detail in Section 7.1.1., only modest levels of asymmetric induction are observed for 2 2 (determined by H NMR). Moreover, 10% of diazo transfer occurs (not shown). 

Caution. Extreme caution must be exercised in working with azide procedures, including use of adequate safety shielding, ready availability of fire extinguishing equipment, and careful attention to directions. 

For the design of synthetic routes based on protected hydrazides the appropriate choice of the protecting groups for the a-amino, A -hydrazide and side-chain functionalities is of critical importance, although the possibility offered by the azide procedure to apply the strategy of minimum protection significantly facilitates the problem of an orthogonal protection scheme. In Table 1 different examples of such protection schemes are listed. 

The azide procedure for peptide synthesis and particularly for fragment condensations is considered to be a mainly racemization free method. This low racemization tendency of azides was explained by several theories, which have been reviewed.t l The most plausible cause of racemization is the formation of oxazoles (Scheme 3) and the related enolization. In presence of bases the a-carbon proton is readily abstracted to form an anionic oxazol-5(4//)-one resonance system.For the formation of the oxazol-5(4//)-one the influence of the substituent Y on the a-carbonyl is essential. Since the a-carbonyl group of amino acid azides are less activated and thus relatively insensitive to oxygen containing nucleophiles such as water and alcohols, oxazol-5(4//)-one formation is largely prevented. It was proposed that the soft electron shell of the azide shields the a-carbonyl atom, so that only strong nucleophiles can attack it.t 1 The reactivity towards amines can be explained in a manner analogous to the aminolysis of anchimerically assisted active esters.h 1... 

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 17 Synthetic Route to a Protected Tripeptide by the Azide Procedure ...

Of the fragment condensation procedures, the azide procedure was employed for each fragment condensation in order to avoid racemization and side reactions (over reaction). 

The azide procedure adopted for the synthesis of apamin by CSPPS included the preparation of 4- [4-(bromomethyl)-3-nitrobenzoyl]amino)(phenyl)methylphenyl-copo-ly(styrene-l%-divinylbenzene) (BrCH2-Nbb-resin) 1 which was used for the generation of the protected hydrazide peptide (Scheme 1). The Kaiser oxime resin can also be used for generation of hydrazide peptides. 

Numerous aminocyclopropanes have been prepared by the Curtius degradation (Scheme 1). The acyl azides (15) were obtained in many cases from the acid chloride (16) and sodium azide either in an organic solvent ( Dry Sodium Azide Procedure ) or in water ( Wet Sodium Azide Procedure ). A modified preparation was reported by... 

Adde peptide synthesis. Hofmann el aV used the Rudii er modification of the azide synthesis (see n-Butyl nitrite) except that they used r-butyl nitrite and hydrogen chloride in dimethylformamide. Comparison with the standard azide procedure showed the modified procedure to be superior in yield and in ease of operation. 

Coates and Bertram developed a 7r-route to usefully substituted compounds potentially convertible by the acyl azide procedure" to atisine derivatives. Iso-steviol (70) was converted to the unsaturated tosylate (71). Buffered formolysis... 

In tests devised to determine the amount of racemization to be expected in peptide syntheses in which the carboxyl component is a di-or higher peptide, this method ranks below the racemization-resistant azide procedure but above almost all other standard methods. Using a very sensitive and accurate isotope dilution assay, 1 % racemization was observed in the formation of the Anderson test peptide (Z-Gly-Phe-Gly-OEt) and 7% racemization was observed in the formation of the Young test peptide (Bz-Lou-Gly-OEt) under optimized conditions. (The Young test was designed to exaggerate racemization problems, thus permitting more accurate studies of the effects of reaction condition variations.)... 

Synthesis of oxytocin was soon attempted in several laboratories. In 1955, Boissonnas and his group [25] in Basel, Switzerland, and in 1956, Rudinger and his associates [26] in Prague, Czechoslovakia, reported overall yields of about 5%. This was substantially higher than the yield achieved in New York. In both instances mixed anhydrides were used for the preparation of segments and the classical azide procedure of Curtius [27] (p. 26) for their combination into the molecule of the hormone. Further improvement both in yield (about 10%) and in the quality of the blocked nonapeptide intermediate was reported in a modified... 

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