Note pubs.acs.org/joc
Cite This: J. Org. Chem. 2019, 84, 1035−1041
Bringing Macrolactamization Full Circle: Self-Cleaving Head-to-Tail Macrocyclization of Unprotected Peptides via Mild N‑Acyl Urea Activation Christine A. Arbour,† Kayla J. Belavek,† Rooha Tariq,† Subha Mukherjee,‡ Janine K. Tom,§ Albert Isidro-Llobet,∥ Michael E. Kopach,⊥ and Jennifer L. Stockdill*,† †
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States Bristol-Myers Squibb, Chemical and Synthetic Development, New Brunswick, New Jersey 08903, United States § Amgen, Inc., Pivotal Drug Substance Process Development, Thousand Oaks, California 91320, United States ∥ GlaxoSmithKline, Medicines Research Centre, Stevenage SG1 2NY, U.K. ⊥ Eli Lilly and Company, Indianapolis, Indiana 46285, United States
J. Org. Chem. 2019.84:1035-1041. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/18/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: We establish herein conditions for the cyclization of unprotected N-acyl urea-linked peptides to form macrocyclic peptides mediated by N-terminal cysteine. We report a detailed investigation of the parameters of the reaction, including variation of the reaction conditions, the C-terminal residue, and the macrocycle size. C-Terminal epimerization was not observed. The synthesis of macrocyclic targets ranging from tetrapeptides to the disulfide-linked 14-mer, sunflower trypsin inhibitor 1 are demonstrated. For most substrates, hydrolysis and head-to-tail dimer formation are avoided.
M
reactivity observed for aniline using each linker.8b,9a Thus, we hypothesized that the MeNbz group would be selectively displaced by an N-terminal Cys thiol in the presence of fully unprotected peptides containing nucleophilic side chain groups capable of head-to-side chain cyclization. We envisioned that we could capitalize on the pseudodilution effect to avoid intermolecular reactions and dilute reaction conditions.10 By performing the TFA cleavage on resin, we expected to reduce steric interactions from protecting groups and harness favorable conformational preferences induced by the primary amino acid sequence, while providing a product with good solubility properties. Finally, the self-cleaving macrocyclization should simplify, if not avoid, purification of the final macrocycle.11 With these considerations in mind, we established the general approach outlined in Scheme 1. We employ a dual linker strategy combining the hydroxymethyl benzoic acid (HMBA)12 linker and the N-methyl-benzimidazolinone (MeNbz) linker (1),13 facilitating reaction monitoring14,15 and enabling resin reuse. On-resin side-chain protecting group removal provides resin-bound unprotected peptide, and upon neutralization or treatment with mild base, the thiol of the C-
acrocyclic peptides have gained prevalence as pharmaceutical leads for “undruggable” biological targets because their large surface areas, minimal conformational flexibility, and multiple side chain moieties facilitate selective binding to shallow protein surfaces, metabolic stability,1 and oral bioavailability of a small molecule target,2 and their membrane permeability can be modulated via chemical modification to target intracellular proteins.3 Because of these exceptional properties, efficient methods for the synthesis of macrocyclic peptides are in constant demand.4 We present herein a broadly applicable, fully on-resin method for macrocyclization of unprotected peptides. This approach harnesses the mild activating nature of N-acyl ureas to achieve selectivity for Cys-mediated macrocyclization in the presence of other nucleophilic side chains. The method is compatible with structurally diverse C-terminal amino acids and is effective for challenging substrates 5 such as tetrapeptides and pentapeptides.6 We showcase this approach via the total syntheses of cyclic peptide natural product sunflower trypsin inhibitor 1 (SFTI-1) via two routes.7 We recently established that side-chain nucleophiles were effective in displacing the N-acyl urea (MeNbz) group to form head-to-side chain macrocycles.8 Related methods were recently published using a benzotriazole (Bt) activation strategy.9 The Bt group appears to be more activating toward nucleophilic attack than the MeNbz group based on the © 2018 American Chemical Society
Received: September 18, 2018 Published: December 19, 2018 1035
DOI: 10.1021/acs.joc.8b02418 J. Org. Chem. 2019, 84, 1035−1041
Note
The Journal of Organic Chemistry
of the peptide on resin during aqueous treatment (entry 6). Our interests were piqued by a recent report from Olsen and co-workers wherein thiodepsipeptides were synthesized via displacement of on-resin MeNbz by an internal cysteine in a mixture of aqueous phosphate buffer (0.2 M Na2HPO4, pH 6.8) and MeCN at 50 °C.25 Treatment of a 1:1 mixture of phosphate buffer and MeCN at pH ranges of 6.5−7.2 resulted in excellent reactivity and selectivity (entry 7). We confirmed that no detectable epimerization of the C-terminal residue occurs during the cyclization at pH 7.2 (samples from Table 1, entry 7 (4, L-Ala) and Table 2, entry 7 (8g, D-Ala)).26 During the preparation of this manuscript, a study from Olsen and coworkers applied their previously reported internal thiolactone conditions (0.2 M Na2HPO4 buffer (pH 6.8), MeCN, 50 °C) to the synthesis of head-to-tail macrocyclic peptides.27 Their reported protocol is similar to our own with the exception of the reaction temperature and the post-TFA cleavage rinse method (discussed below). We did not observe dimers resulting from undesired intermolecular head-to-tail dimer formation for any of the conditions in entries 5−7 in Table 1.28 Following optimization of the Cys-mediated cyclization, we sought to confirm the need for a thiol-containing residue in the N-terminal position using the optimal phosphate buffer/ MeCN conditions. We selected two substrates for our studies (Table S2).6 These activated, deprotected substrates were subjected to a 1:1 v/v mixture of phosphate buffer (pH 7.2) and MeCN for 5 h. Based on our previous studies of solutionphase side-chain macrolactamization and diversification of Cterminal MeNbz peptides,8 we expected that nonbranched Nterminal primary amines would participate in macrocyclization reactions. However, the extent to which this pathway would compete with MeNbz hydrolysis remained unclear.8 Cyclization with N-terminal Gly29 and Lys nucleophiles were the only substrates that provided a viable macrolactam product. Other N-terminal substrates tested under these conditions resulted in mainly a hydrolyzed peptide (Table S2). The level of hydrolysis experienced by several substrates suggested that the reaction might simply be too slow to outcompete hydrolysis in an aqueous environment. For these substrates, 10% Hü nig’s base in N-butylpyrrolidinone (NBP) has
Scheme 1. Strategy for Fully On-Resin Synthesis of Peptide Macrolactams
terminal Cys residue will displace the MeNbz group, and subsequent S → N acyl transfer will afford the cyclic peptide (2).16 The initial reaction screening process was performed with the substrate CFLFA-MeDbz-G-HMBA bound to the polyethyleneglycol-based ChemMatrix resin (3, Table 1).17 The cyclic O-acyl serine analog (SFLFA) of this peptide undergoes O → N acyl transfer under organic conditions (piperidine/DMF), but suffers significant hydrolysis in pH 7.4 phosphate buffer.18 Thus, this substrate represented a viable, but not overly simplistic, starting point. For the industrial synthesis of macrocyclic peptides, it is ideal to avoid substances of very high concern (SVHC) as solvents.19 With this consideration in mind, we focused on solvents ranked as “green” and “amber”,19b which have fewer environmental and health concerns than many traditional solvents such as CH2Cl2 and NMP. Despite their efficacy in related reports,20 the latter proved unproductive when employed in early optimization studies (see Table S1).15 We observed good conversion (91%) to the desired cyclic lactam21 with minimal formation of 5 upon treatment with 10% Hünig’s base in 4-formylmorpholine (Table 1, entry 1). The reactivity and selectivity were further improved when the reaction was conducted in the amidecontaining solvents N-butylpyrrolidinone (NBP, entry 2)19,22 and N,N-dimethyldecanamide (entry 3), and with 2,4,6collidine (entry 4).19b,23 We next investigated a variety of aqueous reaction conditions. To ensure solubility of the product macrocycles, we initially investigated a traditional native chemical ligation (NCL) buffer (entry 5); however, a mixture of undesired products was observed.15,24 In pH 7.2 phosphate buffer, the conversion was 69%, and lactam 4 remained physically trapped in the resin, requiring extensive rinsing with MeCN, presumably due to hydrophobic collapse
Table 1. Optimization of Self-Cleaving Macrocyclization via Cys-Mediated Displacement of an N-Acyl Urea
entrya
base
solvent (pH)
time (h)
conversion (%)b
ratio 4:5:6c
d
DIEA DIEA DIEA DIEA − − −
4-formylmorpholine NBP N,N-dimethyldecanamide 2,4,6-collidine NCL Buffer Na2HPO4 (7.2) (1:1) Na2HPO4/MeCN (6.57.2)
5 5 5 5 5 5 5
91 >99 >99 >99 >99 69 >99
98:2:99:99
39 (8) 58 61c(11)
93 41 >99
>99:
