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 synthesizecyclo[D-Asp2, Dap5]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 (OAl) was not sufficient protection from this side reaction in this susceptible D-Asp2-Gly3 sequence. Remarkably, the aspartimide formation appeared to occur during the coupling reaction in the absence of base if excess coupling reagent was present.
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