N-Methylamino Acids in Peptide Synthesis. IV. Racemization and Yields in Peptide-bond Formation

Abstract

The racemization of an N-methylamino-acid residue during peptide-bond formationand mixed-anhydride activation has been investigated using Ala-MeLeu-Gly and Ala-MeLeu as model peptides. The results were compared with those for Ala-Leu-Gly, Ala-Leu, and Ala-Pro. The extents of racemization were determined by analysis of the diastereomeric products of the reactions after deprotection, using an amino-acid analyzer. Extensive racemization was detected after the hydrolysis of the mixed anhydrides of Bz-MeLeu, Z-Ala-MeLeu, and Z-Ala-Leu, but not of Boc-Ala-Pro and Z-MeIle. Significant racemization (2.8-39%) was observed when Z-Ala-MeLeu was coupled with Gly-OBzl by various methods in the presence of salts such a s triethylamine hydrochloride orp-toluenesulfonate. Only coupling through the N-hydroxysuccinimide (HONSu) ester gave stereochemically pure product. In the absence of salt, less racemization was observed, but only couplings using N-ethoxycarbonyl-2-ethoxy-l,2-dihydroquinoline and N,N1-dicyclohexylcarbodiimide-HONSu gave essentially pure products. Polar solvents promoted racemization, but excess base did not. Chemical evidence that the racemization intermediate is an oxazolium-5-oxide has been obtained by trapping the intermediate a s an addition product (a pyrrole) in 85% yield.The yields obtained by various coupling methods have been determined for several model peptides. Couplings at the carboxyl group of an N-methylamino acid gave high yields only in the absence of salt, except for coupling by the HONSu ester method. Couplings to an N-methylamino group gave high yields, except for coupling by thep-nitrophenyl ester method. Couplings to Ala-MeLeu-OBu' gave higher yields than couplings to Ala-MeLeu-OBzl, presumably due to piperazine-dione formation by the latter. Une racemisation importante a CtC dCtectte apres hydrolyse des anhydrides mixtes de Bz-MeLeu, Z-Ala-MeLeu et Z-Ala-Leu mais pas pour Boc-Ala-Pro e t Z-MeIle. Une racemisation notables (2.b39%) a etC observCe lors du couplage du Z-Ala-MeLeu sur le Gly-OBzl par les diverses mithodes en prisence de sels tels que le chlorohydrate ou para toluene sulfonate de triethylamine. Seul le couplage par I'ester d e La N-hydroxy succinimide (HONSu) conduit 2 un produit d e puretC sttreochimique. En l'absence de sel, la racemisation est plus faible mais seuls les couplages par le NCthoxycarbonyl Cthoxy-2 dihydro-1,2 quinoleine et le complexe N,N'-dicyclohexylcarbodiimide-HONSu donnent des produits essentiellement purs. Les solvants polaires favorisent la racCmisation mais pas un exces d e base. L'intermCdiaire de racemisation oxyde-5 d'oxazolium a it6 rnis en evidence par piegeage sous forme de produits d'addition (un pyrrole) avec un rendement de 85%.Les rendements obtenus dans les divers couplages ont ete dCterminCs sur differents peptides modeles.

Synthesis of N‐Alkyl Amino Acids

“…The use of alkaline conditions in the formation of the N-methyl group and removal of the methyl ester causes varying degrees of undesired epimerization [32][33][34]. Therefore, a direct route to N-methyl amino acids 19 was accomplished without esterification by lowering the reaction temperature to 0 C. McDermott and Benoiton [35] found that reaction temperature was an important factor in avoiding the formation of methyl esters (Scheme 6.8) and also identified acidic reaction conditions other than basic conditions that caused epimerization of N-methyl amino acid-containing dipeptides.…”

Separation of enantiomers by capillary gas chromatography with chiral stationary phases

“…It is known that N-methylamino acids are particularly susceptible to racemization [53]. For this reason a facile procedure for configurational analysis and for determination of the degree of racemization is highly desirable.…”

Synthesis of Cyclosporine. Part II. Synthesis of Boc‐D‐Ala‐MeLeu‐MeLeu‐MeVal‐OH, a part of the peptide sequence of cyclosporine, by different strategic ways and synthesis of its isomers Boc‐D‐Ala‐MeLeu‐D‐MeLeu‐MeVal‐OH, Boc‐D‐MeLeu‐DMeVal‐OH, and Boc‐D‐Ala‐MeLeu‐MeLeu‐D‐MeVal‐OH as reference compounds

“…Davies& Mohammed 1131 with the aid of NMR-techniques came to the same conclusion studying the racemization in N-methylated amino-acid derivatives during peptide coupling in a model dipeptide system. They could as McDermott & Benoiton [12] confirm that the source of the racemization involving dicyclohexylcarbodiimide (DCCI) at room temperature is the oxazolonium intermediate i.…”