An Electrochemical Approach to Designer Peptide α-Amides Inspired by α-Amidating Monooxygenase Enzymes

Abstract

Designer C-terminal peptide amides are accessed in an efficient and epimerization-free approach by pairing an electrochemical oxidative decarboxylation with a tandem hydrolysis/reduction pathway. Resembling Nature’s dual enzymatic approach to bioactive primary α-amides, this method delivers secondary and tertiary amides bearing high-value functional motifs, including isotope labels and handles for bioconjugation. The protocol leverages the inherent reactivity of C-terminal carboxylates, is compatible with the vast majority of proteinogenic functional groups, and proceeds in the absence of epimerization, thus addressing major limitations associated with conventional coupling-based approaches. The utility of the method is exemplified through the synthesis of natural product acidiphilamide A via a key diastereoselective reduction, as well as bioactive peptides and associated analogues, including an anti-HIV lead peptide and blockbuster cancer therapeutic leuprolide.

Synthesis of Peptide N-Acylpyrroles via Anodically Generated N,O-Acetals

“…Following the deprotection of the methyl ester, the peptide acid was dissolved in MeOH and electrolyzed at constant current in an undivided cell equipped with carbon electrodes to afford N , O -acetal 15 in 88% yield (over 2 steps from 14 ) as a mixture of diastereomers (Table 1 ). 18 Based on our early observations (see Scheme 1 ) 6b and drawing on literature precedent, 17 we reasoned that the aromatization reaction would be promoted by acidic conditions, including in the presence of TFA. Pleasingly, the addition of neat TFA to the electrolyzed peptide 15 proceeded in 20% isolated yield to deliver Cbz-Ala-pyrrole ( 16 ; Table 1 , entry 1).…”

“…Incorporation of this key building block into a peptide sequence was accomplished using the standard coupling protocols on Rink amide resin. Following cleavage from the resin and deprotection of the Hyp methyl ester, electrolysis of the side-chain Hyp carboxylate was carried out using a slightly modified approach, 6b namely through the addition of acetic acid. After aromatization, this two-step protocol successfully delivered side-chain-modified peptide 22 in 32% yield.…”

“…Under the acidic reduction conditions (TFA, Et 3 SiH), the electrochemically generated N , O -acetal was competitively aromatized to deliver the C-terminal N -acylpyrrole 2 rather than the desired amide 4 . 6b Notably, the N -acylpyrrole motif has long been an attractive synthetic target due to its high versatility, for example as an acylating agent or a carbonyl equivalent, and for its potential applications in medicinal chemistry. 8 The signature electronic characteristics of N -acylpyrroles stem in part from the unique nature of the pyrrolic amide motif, which has a much lower amidicity 9 – a measure of amide bond strength – compared with other aliphatic or primary amides, owing to the reduced propensity of the pyrrole nitrogen lone pair to delocalize into the adjacent carbonyl group.…”

“…The acetal is capable of engaging with various nucleophiles (electron-rich aromatics, 6a hydride-based reductants, water, etc. 6b ) to form valuable, C-terminally modified peptides. This method avoids direct activation of the C-terminal carboxylate, and therefore circumvents the risk of α-epimerization associated with conventional coupling strategies.…”

“…Notably, in prior work, we have comprehensively explored the functional group compatibility of the electrochemical oxidative decarboxylation in the presence of the majority of amino acid side-chain functionalities, hinting at the potential for broad applicability of the method in complex peptide substrates. 6b Accordingly, with a reliable strategy for the generation of the key peptide N , O -acetal intermediates already in hand, we began our investigations by optimizing the conditions for aromatization. To this end, dipeptide Cbz-Ala-Hyp-OMe ( 14 ) was prepared as a model system (see Supporting Information for details).…”