Umpolung AlaB Reagents for the Synthesis of Non-Proteogenic Amino Acids, Peptides and Proteins

Abstract

Non-proteogenic amino acids and functionalized peptides are important motifs in modern drug discovery. Here we report that Ala^B can serve as universal building blocks in the synthesis of a diverse collection of modified amino acids, peptides, and proteins. First, we develop the synthesis of Ala^B from redox-active esters of aspartic acid resulting in a series of β-boronoalanine derivatives. Next, we show that Ala^B can be integrated into automated oligopeptide solid-phase synthesis. Ala^B is compatible with common transformations used in preparative peptide chemistry such as native chemical ligation and radical desulfurization as showcased by total synthesis of Ala^B -containing ubiquitin. Furthermore, Ala^B reagents participate in Pd-catalyzed reactions, including C-C cross-couplings and macrocyclizations. Taken together, Ala^B synthons are practical reagents to access modified peptides, proteins, and in the synthesis of cyclic/stapled peptides.

Keywords: AlaB; Boronoalanine; Macrocyclization; Non-Proteogenic Amino Acids; Peptides.

Scheme 1.

Ala^M as synthetic reagents for peptide and protein modifications.

Scheme 2.

Synthesis of Ala^BPai and Ala^BMIDA reagents. [a] Refers to conditions A. [b] Refers to conditions B.

Scheme 3.

Elaboration of Ala^B into peptides and proteins.^a [a] LC conditions: Agilent EC-C18 Poroshell, 40 °C, 1 mL/min; 5–65% MeCN/H₂O with 0.05 % TFA over 1 h. Abbreviations: DIPEA=diisopropylethylamine; Gnd=guanidine; GSH=glutathione reduced; HBTU=(2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate; MPAA=mercaptophenylacetic acid; SPPS=solid-phase peptide synthesis; TCEP=(tris(2-carboxyethyl)phosphine; TFA=trifluoroacetic acid; TIPSH=triisopropylsilane; VA-044=2,2’-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride.

Scheme 4.

Scope of Suzuki cross-coupling with Ala^B. [a] Refers to 34A series. [b] Refers to 34B series. [c] K₂CO₃ at 60 °C; [d] PdCl₂dppf (30 mol%), 72 h.

Scheme 5.

Macrocyclization of Ala^B oligopeptides. ^aPdCl₂dppf (10 mol%), Ag₂O (2 equiv.) K₂CO₃ (3 equiv.) THF (50 mM). ^bMacrocycle derived from 35 (Boc-AlaB^Pai-AA₂-AA₃-Phe(m-Br)-OMe). ^cMacrocycle derived from 36 (Boc-Phe(m-Br)-AA₂-AA₃-AlaB^Pai-OMe).