Solid phase synthesis and ligative processing of photocaged aziridine-2-carbonyl-terminated peptides

Abstract

a b s t r a c tA general synthesis of N-terminal aziridinyl-2-carbonyl (Azy) peptides has been developed aided by the photolabile o-nitrophenylethyl protecting group. This method enables the synthesis of unprotected Azy-terminated peptides incorporating ionizable groups using solid phase techniques followed by photorelease of the free N-terminal Azy moiety. The resulting Azy peptides undergo Cu(II)-mediated ligation with thioacids to give Azy-embedded peptides, providing a handle for site-specific modification of the peptide.Ó 2015 Published by Elsevier Ltd.The Cu(II)-promoted ligation of unprotected peptide thioacids 1 and aziridine-2-carbonyl (Azy) peptides 2 generates electrophilic N-acylated aziridines 3 that may be subjected to nucleophilic attack to give peptides corresponding to 4 (Scheme 1). 1 This aziridine-mediated ligation can be performed under denaturing aqueous conditions and has the potential, via the use of different nucleophiles, 2 to enable the site-specific modification of peptides and proteins. While access to C-terminal peptide 3 (and protein) 4 thioacids 1 is fairly well established, a general and reliable synthesis of Azy peptides 2 is not available. Relatively few examples of compounds that correspond to 2 have been reported in the literature. 5 The observations of Moroder and co-workers (Reference 5a) were particularly relevant in this regard wherein the propensity for unprotected Azy peptides 2 to decompose upon lyophilization or storage was duly noted. 6 This stability problem may be traced to the susceptibility of the aziridine moiety in 2 to undergo ringopening, a property that can be exacerbated by the presence of acidic functional groups in the peptide. The inability to prepare Azy peptides corresponding to 2 without imposing significant functional group restrictions would, of course, limit the utility of our aziridine-mediated ligation. We now report a general strategy for the synthesis of unprotected Azy peptides 2 and their in situ Cu(II)-promoted coupling to peptide thioacids 1.Since HPLC purification of 2 was out of the question due to its instability, we focused on a solid phase synthesis strategy (Scheme 2) that would allow for purification of the SPPS cleavage product 5 prior to ligation. This led us to search for an orthogonal protecting group (PG) that would not activate the aziridine ring thus enabling purification of 5 and that could be removed in situ to give a ligation-competent solution of 2. The known instability of both N-H and N-acyl aziridines complicated implementation http://dx.

Fully Synthetic Granulocyte Colony-Stimulating Factor Enabled by Isonitrile-Mediated Coupling of Large, Side-Chain-Unprotected Peptides

“…49i–k The utility of peptidyl aziridines in context with activated thioacids has also been realized. 50 Recent methods report mild conditions for acyl transfer through the unique activation of thioacids 51a and thioesters 51b with N , O -bis(trimethylsilyl)-acetamide. Lastly, thioacid activation with N-terminal dithio-carbamates 52a or N-terminal thioamides 52b highlights the utility of presumed 1,3-diacyl intermediates.…”

“…In recent years, a variety of other methods demonstrate the potential of thiocarboxylic and peptidyl C-terminal thiocarboxylic acids alike to serve as effective acyl donors. − The activation of thiocarboxylic acids under various oxidative conditions is well-studied. , Additionally, thiocarboxylic acid substrates provide access to various amide types upon engagement with acyl azides, azides, − isocyanates, 2-pyridyl thioethers, , thiols, and aryl sulfonamides. − The utility of peptidyl aziridines in context with activated thioacids has also been realized . Recent methods report mild conditions for acyl transfer through the unique activation of thioacids and thioesters with N , O -bis­(trimethylsilyl)­acetamide.…”