Diastereomeric Products Research Articles

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

The racemization of an N-methylamino-acid residue during peptide-bond formation and 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 as triethylamine hydrochloride or p-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-1,2-dihydroquinoline and N,N′-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 as 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 the p-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.

Asymmetric induction IV. Dependence of some diastereomeric product ratios on ΔΔH ‡ and ΔΔS ‡

Asymmetric induction IV. Dependence of some diastereomeric product ratios on ΔΔH ‡ and ΔΔS ‡

The reaction of dimethyl (2-hydroxy-5-nitrobenzyl)sulfonium salts with tryptophan ethyl ester

Reaction of the water-soluble reporter group, dimethyl (2-hydroxy-5-nitrobenzyl)-sulfonium bromide, with l-tryptophan ethyl ester in aqueous solutions at pH 4.7 produces two diastereomeric monosubstitution products derived from alkylation of position 3 of the indole nucleus, followed by cyclization through the nucleophilic α-amino group. Comparison of these products with those formed during reaction of 2-hydroxy-5-nitrobenzyl bromide with tryptophan ethyl ester shows that they appear in the same characteristic ratio, 1.4:1. However, the monosubstitution products are formed in nearly quantitative yield when tryptophan ethyl ester is treated with an equimolar quantity of the sulfonium reagent, so that significantly less of the by-products are formed than in the case of the hydroxynitrobenzyl bromide. These findings confirm the applicability of dimethyl(2-hydroxy-5-nitrobenzyl) sulfonium salts in introducing the 2-hydroxy-5-nitrobenzyl reporter grouping into the indole nucleus of tryptophyl residues in proteins.

Study of the mechanism of reactions of primary and secondary organomagnesium compounds with alkyl halides

1. The effect of solvent and concentration of organomagnesium compounds on the rate of the Wurtz reaction was studied. 2. Primary organomagnesium compounds react with the alkyl halides corresponding to them two to five times more slowly than the secondary compounds. 3. The rates of formation of diastereomeric Wurtz reaction products are identical with one another and do not depend on the character of the solvent and concentration of reagent. 4. A mechanism is proposed for the Wurtz reaction which consists of reaction of the alkyl halide with a polarized solvated monomeric species with formation of an acyclic intermediate complex and conversion of this complex to reaction products in a four-membered transition state.

OXIDATION OF ALKENES BY MERCURIC SALTS

The α-oxymercurials of cyclohexene, 2-methyl-1-phenylpropene-1, and the geoisomers of stilbene have been shown to be the intermediates in the oxidation of these alkenes by mercuric nitrate and other salts, usually in methanol. These intermediates are stable in the oxidation environment and their alkoxy groups appear intact in the oxidation product; indeed organomercurials without vicinal alkoxy groups are likewise oxidized. The reaction products from alkenes in methanol are 1,2-dimethoxyethanes, 1,1-dimethoxyethanes, and (with mercuric nitrate) 1-methoxy-2-nitroxyethanes, formation of the latter being most rapid. The oxidation is stereospecific, each geoisomer yielding its characteristic dia-stereomeric products. It is catalyzed by acids such as nitric acid but not by nitrate ion, though neither affect the ratio of products. This over-all second-order reaction, first-order in organomercurial and first-order in mercuric salt, becomes over-all first-order with mercuric acetate when it is catalyzed by boron fluoride. Combination of the catalyst with the organomercurial must be rate-controlling. The similarity of the oxidation and the acid decomposition reactions of organomercurials is discussed.