Tryptophan alkylation side reaction

Besides HF and TFA, the two extremes of acid treatment generally employed in the final deprotection step, alternative acids have been proposed, such as MsOH and TfOH, particularly in the context of arginine protection (Table 2). These acids, however, are known to generate serious side reactions such as aspartimide formationt and an N O shift at serine or threonine residues.Moreover, independent of the type of arylsulfonyl group employed, cations originating from the acid-mediated cleavage lead to alkylation and arylsulfonylation of reactive side chains, as present in tryptophan and tyrosine residues. Additionally, sulfation of hydroxy groups is even observed, as well as... 

The indole nucleus in the tryptophan side chain is unreactive enough to be left without protection. Its NH group is not readily acylated but can suffer alkylation to a small but not always negligible extent and some investigators prefer to counteract this side reaction by the application of a masking group. So far only the formyl group found acceptance, perhaps because it is smoothly introduced by the treatment of tryptophan with HCl in formic acid ... 

Electrophilic substitution of aromatic nuclei in tyrosine and tryptophan side chains has frequently been reported in connection with acidolytic removal of blocking groups. C-Benzylation and tert.butylation of the tyrosine side chain and N-alkylation of the indole nucleus in tryptophan are often attributed to the alkyl cations generated in the reaction. This common side reaction is caused, however, mainly by the alkylating agents formed in the process, such as benzyl bromide or tert.butyl trifluoroacetate. The same is true for the S-alkylation of the methionine side chain. Conversion of the thioether to a sulfonium salt can... 

This side reaction is relatively innocuous because the by-product is irreversibly bound to the polymer and only the yield is affected not the purity of the synthetic peptide. More disturbing is the succinimide ring formation at aspartyl residues exposed to HF. Alkylation of the indole ring in tryptophan, the phenolic side chain in tyrosine and the sulfur atom in methionine must be suppressed by the addition of scavengers. The often appUed anisole is less than unequivocal in this role it can be the source of methyl groups which convert the methionine thioether to a tertiary sulfonium derivative. The acid stable thioanisole seems to be a better scavenger. 

Tryptophan offers an indole side chain that can be used for ligation chemistry. A water-compatible rhodium carbene can be added to the indole ring (19) [105,139]. The reactive species is generated in situ by a conjugated diazo compound by a rhodium catalyst like rhodium(II) acetate [63,139,149]. The reaction takes place in the two- and three-position of indole. Thus, a mixture of N-alkylated and C-alkylated product is obtained. It is necessary to add hydroxylamine hydrochloride as an additive to bind to the distal rhodium carbenoid complex. The usage of this salt lowers the pH value below 3.5 and therefore limits the scope of this methodology. As a side reaction, the carbene inserts into the O-H bond of water (Table 6). 

Lewis acids such as zinc triflate[16] and BF3[17] have been used to effect the reaction of indole with jV-proiected aziridine-2-carboxylate esters. These alkylations by aziridines constitute a potential method for the enantioselective introduction of tryptophan side-chains in a single step. (See Chapter 13 for other methods of synthesis of tryptophans.)... 

Af -2,2-Bis(ethoxycarbonyl)vinyl-protected amino acids are prepared by reaction of commercially available diethyl 2-(ethoxymethylene)malonate (127) with the respective amino acid in methanolic KOH. This rapid reaction is complete within 5 minutes and leads to the potassium salts. Subsequent acidification with 1M HCl yields the amino acid derivative in 75-90% yield.f This intermediate enamine-type N-protection is of particular interest in chemistry to be performed on the carboxy groups of the amino acids such as esterification with alkyl bromides in the presence of a base. Since cleavage of the enamine entity is achieved by treatment with bromine in chloroform at room temperature, it cannot be used for amino acids sensitive to halogenation such as tyrosine, tryptophan, and methionine (Scheme 61). Based on the experience gained with the enamine-type protection the Al-2-(4,4-dimethyl-2,6-dioxocyclohexylidene)ethyl (Dde) and N-2-(4,4-dimethyl-2,6-dioxocyclohex-ylidene)isovaleryl derivatives were developed as specific side-chain protecting groups (see Section 2.1.2.2.5.2). 

If highly substituted aUylic esters are used, even two adjacent quaternary centers can be generated in one step (134). These examples also show that this method is not restricted to amino acids with aUphatic or aromatic side chains, but can also be applied to derivatives of functionalized amino acids such as tryptophan (132) or lysine (133). These examples had been chosen, because especially the tryptophan and lysine derivatives are critical substrates for a-alkylation reactions [72]. 

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