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. 2023 Mar 15;145(10):5684-5695.
doi: 10.1021/jacs.2c11451. Epub 2023 Feb 28.

Diversifying Amino Acids and Peptides via Deaminative Reductive Cross-Couplings Leveraging High-Throughput Experimentation

J Cameron Twitty  1 , Yun Hong  1 , Bria Garcia  1 , Stephanie Tsang  1 , Jennie Liao  1 , Danielle M Schultz  2 , Jennifer Hanisak  3 , Susan L Zultanski  2 , Amelie Dion  2 , Dipannita Kalyani  3 , Mary P Watson  1
Affiliations

Diversifying Amino Acids and Peptides via Deaminative Reductive Cross-Couplings Leveraging High-Throughput Experimentation

J Cameron Twitty et al. J Am Chem Soc. .

Abstract

A deaminative reductive coupling of amino acid pyridinium salts with aryl bromides has been developed to enable efficient synthesis of noncanonical amino acids and diversification of peptides. This method transforms natural, commercially available lysine, ornithine, diaminobutanoic acid, and diaminopropanoic acid to aryl alanines and homologated derivatives with varying chain lengths. Attractive features include ability to transverse scales, tolerance of pharma-relevant (hetero)aryls and biorthogonal functional groups, and the applicability beyond monomeric amino acids to short and macrocyclic peptide substrates. The success of this work relied on high-throughput experimentation to identify complementary reaction conditions that proved critical for achieving the coupling of a broad scope of aryl bromides with a range of amino acid and peptide substrates including macrocyclic peptides.

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Figures

Scheme 1.
Scheme 1.
State of the Art in Aryl Alanines and Homologated Derivatives
Scheme 2.
Scheme 2.. Initial Aryl Bromide Scopea
a Conditions: 3a (0.50 mmol), Ar–Br (1.2 equiv), NiBr2·diglyme (10 mol %), 2,2’-bypyridine (bipy) (12 mol %), TDAE (2.0 equiv), NaCl (1.0 equiv), DMPU (0.1 M), 80 °C, 24 h. Isolated yields, unless otherwise noted. b Yield determine by 1H NMR using an internal standard.
Scheme 3.
Scheme 3.. Ligand/Reductant Interdependencea
a Conditions: 3a (10 μmol), Ar–Br (1.5 equiv), NiBr2·DME (10 mol %), ligand (12 mol %), reductant (2.0 equiv), TBAI, DMA (0.1 M), 80 °C, 24 h. Numbers in each box represent the experimental LCAP. Reaction success was determined by LCAP (LC area %) of the desired product from the UPLC-MS analysis of the crude reaction mixture. Conditional formatting was applied to highlight the range of product LCAPs (see the key above).
Scheme 4.
Scheme 4.. HTE Investigation of Aryl Bromide Scope using TDAE as Reductanta
a For microscale experiments, numbers represent the experimental product LCAP for reactions performed on 10 μmol scale. For selected examples, numbers at the bottom of each box represents isolated yields of the products on 0.5 mmol scale. Conditional formatting was applied to product LCAP (see the key above). Microscale conditions: 3a (10 μmol), Ar–Br (1.5 equiv), NiBr2·DME (10 mol %), L1 or L3 (12 mol %), reductant (2.0 equiv), TBAI (0 or 1.0 equiv), DMA (0.1 M), 80 °C, 24 h. (A) L3, TDAE, TBAI. (B) L1, Mn0, TBAI, (C) L1, Zn0. For 0.5-mmol experiments, isolated yields (IY) are reported. 0.5-mmol conditions: 3a (0.50 mmol), Ar–Br (1.2 equiv), NiBr2·diglyme (10 mol %), L1 or L3 (12 mol %), reductant (2.0 equiv), NaCl (0 or 1.0 equiv), DMPU (0.1 M), 80 °C, 24 h. (D) L3, TDAE, NaCl. (E) L1, Mn0.
Scheme 5.
Scheme 5.. Interdependence of Aryl Bromide and Reaction Conditionsa
a Aryl bromide numbers on the x-axis are the same as those in Scheme 4. Experimental product LCAP are reported for reactions performed on 10 μmol scale. Conditions: 3a (10 μmol), Ar–Br (1.5 equiv), NiBr2·DME (10 mol %), L1 or L3 (12 mol %), reductant (2.0 equiv), TBAI (0 or 1.0 equiv), DMA (0.1 M), 80 °C, 24 h. (A) L3, TDAE, TBAI. (B) L1, Mn0, TBAI, (C) L1, Zn0.
Scheme 6.
Scheme 6.. Comparison of Reaction Performance at Nanoscale vs. Microscalea
a Experimental LCAP for reactions performed on 100 nmol or 10 μmol scale. Nanoscale conditions: 3a (0.1 μmol), Ar–Br (1.5 equiv), NiBr2·DME (10 mol %), 4,4’-tBubipy (12 mol %), TDAE (2.0 equiv), TBAI (1.0 equiv), DMA (0.1M), 80 °C, 24 h. For microscale conditions, see Schemes 4 and 5, Conditions A.
Scheme 7.
Scheme 7.. Interdependence of Pyridinium Salt and Reaction Conditionsa
a Conditions: 3a-h (10 μmol), Ar–Br (1.5 equiv), NiBr2·DME (10 mol %), ligand (12 mol %), reductant (2.0 equiv), TBAI (0 or 1.0 equiv), DMA (0.1M), 80 °C, 24 h. For TDAE, bipy (L3) used as ligand. For Mn and Zn, pyridine-2,6-bis(carboximidamide) dihydrochloride (L1) used as ligand. Conditional formatting was applied to highlight the range of product LCAP detected (see the key above).
Scheme 8.
Scheme 8.. Optimal Conditions for Pyridinium/Bromide Pairsa
a Conditions: 3a (10 μmol), Ar–Br (1.5 equiv), NiBr2·DME (10 mol %), L1 or L3 (12 mol %), reductant (2.0 equiv), TBAI (0 or 1.0 equiv), DMA (0.1 M), 80 °C, 24 h. (A) L3, TDAE, TBAI. (B) L1, Mn0, TBAI, (C) L1, Zn0. Conditional formatting was applied to highlight the range of product LCAP detected (see the key above).
Scheme 9.
Scheme 9.. Scope of Pyridinium Saltsa
a Condition A: Pyridinium 3 (0.50 mmol), Ar–Br (1.2 equiv), NiBr2·diglyme (10 mol %), bipy (12 mol %), TDAE (2.0 equiv), NaCl (1.0 equiv), DMPU (0.1 M), 80 °C, 24 h. Condition B: Pyridinium 3 (0.50 mmol), Ar–Br (1.2 equiv), NiBr2·diglyme (10 mol%), L1 (12 mol%), Mn0 (2.0 equiv), DMPU (0.1M) 80 °C, 24 h. Isolated yields using Condition A except that Condition B was used to prepare 36. For 34–36, average yield of duplicate experiments (±5%). Ee’s determined by SFC using a chiral stationary phase.
Scheme 10.
Scheme 10.
Solid-Phase Peptide Synthesis (SPPS) and Cross-Coupling
Scheme 11.
Scheme 11.
Application to synthesis of PCSK9 inhibitor analog
See this image and copyright information in PMC

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