Benzophenone semicarbazone protection strategy for synthesis of aza‐glycine containing aza‐peptides

Abstract

Aza-glycine has been incorporated into peptide mimics as a tool for studying the active conformation and characterizing structure-function relationships for activity. Side reactions, such as intramolecular cyclizations to form hydantoins and oxadiazalones, have, however, inhibited efforts to make activated aza-Gly residues in solution using carbamate protection. Herein, we describe efficient incorporation of aza-glycine into aza-peptides using diphenyl hydrazone protection. Hydrazone acylation with p-nitrobenzyl chloroformate provided the protected aza-Gly activated ester, which was used to acylate a set of amino ester and amino acids to provide aza-Gly-Xaa aza-dipeptide fragments for peptide synthesis. Removal of the hydrazone protection was performed under acidic conditions to provide the hydrochloride salt of the aza-Gly residue for subsequent elongation of the aza-peptide chain using standard coupling conditions. A proof of concept for the use of benzophenone protection has been established by the synthesis of an aza-peptide analog of a potent activator of caspase 9 in cancer cells.

Synthesis and Evaluation of Azapeptide Prostaglandin F2α Receptor Modulators in Pursuit of Inhibitors of Preterm Labor

“…10, Scheme 1). Aza-glycinyl proline tert-butyl ester 4 was synthesized as reported [6] and treated with potassium tert-butoxide and 4-methyl benzyl bromide to provide aza-(4-Me)Phe analog 5 in 84% yield [5,6]. Benzhydrylidene removal using hydroxylamine hydrochloride in pyridine [6], acylation of the resulting semicarbazide with phenylacetyl chloride, and tert-butyl ester removal in a TFA/DCM solution afforded the N-terminal aza-dipeptide 8.…”

Solution‐phase submonomer diversification of aza‐dipeptide building blocks and their application in aza‐peptide and aza‐DKP synthesis

“…Hydrazone protection has recently been used to prepare activated aza‐Gly analogs without oxadiazolone formation28, 29. Moreover, the alkylation of supported aza‐Gly peptide has been used to make aza‐peptide on solid phase29.…”

“…The prerequisite to synthesize the N ‐alkyl hydrazine prior to incorporation into aza‐peptide has limited the diversity of the aza‐residue side chain, because of the inherent difficulty to differentiate selectively the two nitrogen of the hydrazine moiety27. Alternatively, alkylation of a suitably N ‐protected aza‐glycine moiety surmounts the issues associated with hydrazine synthesis28, 29. Although N ‐Fmoc‐aza‐glycine acid chloride 4 could not be introduced into aza‐peptide in solution, likely because of competitive formation of oxadiazalone 6 (Figure 2)23, aza‐Gly peptides were synthesized using the activated methylidenecarbazate 20 from mixing benzophenone hydrazone 17 and p ‐nitrophenyl chloroformate 18 (Scheme )28, 29.…”

“…Alternatively, alkylation of a suitably N ‐protected aza‐glycine moiety surmounts the issues associated with hydrazine synthesis28, 29. Although N ‐Fmoc‐aza‐glycine acid chloride 4 could not be introduced into aza‐peptide in solution, likely because of competitive formation of oxadiazalone 6 (Figure 2)23, aza‐Gly peptides were synthesized using the activated methylidenecarbazate 20 from mixing benzophenone hydrazone 17 and p ‐nitrophenyl chloroformate 18 (Scheme )28, 29. Subsequent alkylation of solid‐phase bound benzophenone semicarbazone‐protected aza‐Gly peptide, semicarbazone deprotection, peptide elongation and cleavage has provided a submonomer approach for aza‐peptide synthesis29.…”

Peptide Coupling Challenges to Aza-Pipecolyl Smac Mimetic

“…A variety of coupling condition was examined to achieve acylation, albeit with minimal success. For example, various coupling conditions failed to afford the Boc protected aza-tripeptide amide 7: TBTU/HOBT [10], EDCl/HOBT [11], and DIC/AtOH [12]. Moreover, attempts were unsuccessful using the mixed anhydride generated from Boc-Ala, iso-butyl chloroformate and N-methylmorpholine [13,14].…”