Unexpected aspartimude formation during coupling reactions using Asp(OAl) in solid-phase peptide synthesis

Abstract

Succinimide ring closure is a well-documented side reaction in the synthesis of certain Asp-containing peptides. This side reaction is typically acid-or base-catalyzed, and its occurrence during coupling reactions has not been previously noted. This unforeseen manifestation of aspartimide formation was detected while exploring a new strategy for side-chain to side-chain lactam formation on a solid support to synthesize cyclo[D-Asp2,DapS]dynorphin A-(1-11) amide. The availability of allyl protecting groups, which provide an additional level of orthogonality in solid-phase peptide synthesis, was very appealing for use in preparing this conformationally constrained analogue. We found that the allyl ester (OA1) was not sufficient protection from this side reaction in this susceptible D-Asp2-Gly 3 sequence. Remarkably, the aspartimide formation appeared to occur during the coupling reaction in the absence of base if excess coupling reagent was present.

The aspartimide problem persists: Fluorenylmethyloxycarbonyl‐solid‐phase peptide synthesis (Fmoc‐SPPS) chain termination due to formation of N‐terminal piperazine‐2,5‐diones

“…In the series, the extent of cyclization is β ‐PG = (OAll) > (OtBu) > (OMpe) (ie, coupling of α ‐carboxy of Asp) and even remarkably higher with α ‐OtBu or α ‐OAll protection (ie, coupling of β ‐carboxy of Asp). A similar trend is also known for Asi formation . Recently, the OMpe protecting group was surpassed by β ‐tri‐alkylmethyl esters (alkyl = ethyl/propyl/butyl) for Asi suppression .…”

Impact of the amino acid sequence on the conformation of side chain lactam‐bridged octapeptides

“…The HPLC and MS analysis of the crude products revealed different levels of success in the syntheses of the cyclopeptides: For example, attempts to cyclize the linear precursors of 2 and 6 failed, as the first was prone to dimerization, whereas the second remained mainly in the linear form. Moreover, in accordance with the high propensity of Asp(Oall) to form an aspartimide upon incorporation into a peptide chain, especially under basic conditions [69,70], we also observed the formation of an aspartimide during the assembly of some linear sequences, particularly of the linear precursors of cyclopeptides 7 and 9, both containing the motif Asp(Oall)-Ala. Nevertheless, we were able to purify all cyclized peptides with a homogeneity ≥90%, with the exception of cyclopeptide 7, whose homogeneity was~70% ( Figures S1-S3).…”

Synthesis and Biological Activity of Argiotoxin 636 and Analogues: Selective Antagonists for Ionotropic Glutamate Receptors

“…Extended reaction times were required for complete conversion in the first coupling reaction, and it was necessary to protect the amide side chain of asparagine for efficient coupling, to avoid a competing cyclization. [11] The polyamine chain was prepared was prepared by a Fukuyama-Mitsunobu alkylation strategy [12] with amino alcohols as building blocks. Intermediate 11 was transformed efficiently into the Teoc-protected amine 12 by treatment with N-Teoc-protected 3-hydroxypropylamine in the presence of 1,1'-azodicarbonyldipiperidine (ADDP) and tributylphosphane as redox reagents (Scheme 2).…”