title: Mechanistic studies of malonic acid-mediated in situ acylation.
abstract: We have previously introduced an easy to perform, cost-effective and highly efficient acetylation technique for solid phase synthesis (SPPS). Malonic acid is used as a precursor and the reaction proceeds via a reactive ketene that acetylates the target amine. Here we present a detailed mechanistic study of the malonic acid-mediated acylation. The influence of reaction conditions, peptide sequence and reagents was systematically studied. Our results show that the methodology can be successfully applied to different types of peptides and nonpeptidic molecules irrespective of their structure, sequence, or conformation. Using alkyl, phenyl, and benzyl malonic acid, we synthesized various acyl peptides with almost quantitative yields. The ketenes obtained from the different malonic acid derived precursors were characterized by in situ (1) H-NMR. The reaction proceeded in short reaction times and resulted in excellent yields when using uronium-based coupling agents, DIPEA as a base, DMF/DMSO/NMP as solvents, Rink amide/Wang/Merrifield resins, temperature of 20°C, pH 8-12 and 5 min preactivation at inert atmosphere. The reaction was unaffected by Lewis acids, transition metal ions, surfactants, or salt. DFT studies support the kinetically favorable concerted mechanism for CO2 and ketene formation that leads to the thermodynamically stable acylated products. We conclude that the malonic acid-mediated acylation is a general method applicable to various target molecules.
authors.affiliation: Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Koushik
authors.initials: K
authors.lastname: Chandra
authors.affiliation: Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Johnny N
authors.initials: JN
authors.lastname: Naoum
authors.affiliation: Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.|The Fritz Haber Research Center, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Tapta Kanchan
authors.initials: TK
authors.lastname: Roy
authors.affiliation: Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Chaim
authors.initials: C
authors.lastname: Gilon
authors.affiliation: Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.|The Fritz Haber Research Center, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.|Department of Chemistry, University of California, Irvine, California, 92697.
authors.forename: R Benny
authors.initials: RB
authors.lastname: Gerber
authors.affiliation: Institute of Chemistry, The Hebrew University of Jerusalem, Safra Campus, Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Assaf
authors.initials: A
authors.lastname: Friedler
mesh_terms: D000215:Acylation; D008314:Malonates / Q000737:chemistry*; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry; D060327:Solid-Phase Synthesis Techniques / Q000191:economics
chemical_list: D008314:Malonates; D010455:Peptides; C030290:malonic acid
keywords: SPPS; acylation; ketene; malonic acid; mechanism; peptides