Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. 2019, 21, 147−151
Macrolactamization Approaches to Arylomycin Antibiotics Core Ngiap-Kie Lim,* Xin Linghu, Nicholas Wong, Haiming Zhang, C. Gregory Sowell, and Francis Gosselin Department of Small Molecule Process Chemistry, Genentech, Inc., 1 DNA Way, South San Francisco, California 94080, United States
Org. Lett. 2019.21:147-151. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/09/19. For personal use only.
S Supporting Information *
ABSTRACT: Two practical entries to arylomycin antibiotics core structures are investigated. In route A, the activation of LHpg for the key macrolactamization step is achieved in 89% yield in the presence of unprotected phenol and amine functionalities. Alternatively, a propanephosphonic acid anhydride (T3P)-promoted coupling between theL-Tyr and L-Ala moieties in route B led to a facile macrolactamization in 68% yield with a marked reduction in competing oligomerization.
T
he increasing use of antibiotics, both in human health and agriculture, has led to the rapid development and dissemination of resistance to commonly used classes of antibiotics.1 This has garnered much attention and prompted global health concerns. If existing treatment options remain unchanged, human mortality associated with microbial infections may reach 10 million globally by 2050, surpassing the number of cancer deaths.2 A more sustained effort is hence underway to discover novel classes as well as potentiators of existing classes of antibiotics to address this emerging resistance.3 The arylomycin class of antibiotics4 offers an untapped mechanism for potentially regulating bacterial physiology, biochemistry, and pathogenicity.5 The unique chemical inhibition of type I signal peptidase by arylomycins can play an important role in new, broad-spectrum antibiotic development.6 Progress on potential therapeutics based on arylomycins has been thwarted by rather inefficient access to the core macrocyclic structure 1. The key challenge to the synthesis of arylomycin and its analogues is the macrocyclic core (1), which contains a distinctive 2,2′-disubstituted biaryl-bridged peptide. There are very few practical approaches to the arylomycin core reported in the literature (Figure 1). For example, macrolactamization of a linear Ala-Hpg-Tyr precursor was previously explored but found to be unproductive, yielding complex mixtures of unidentified products even under high dilution conditions (Figure 1, route B).7 A synthetic strategy involving intramolecular Suzuki−Miyaura coupling8 could only be achieved with a limited choice of catalysts under high Pd loading (up to 20 mol %) and high dilution conditions (0.01− 0.07 M) to afford product in 20−54% yields (Figure 1, route C).7,9 Recently, a stoichiometric Cu-mediated oxidative coupling provided a direct access to the arylomycin core (Figure 1, route D).10 However, high dilution (0.01 M) and use of 2.0 © 2018 American Chemical Society
Figure 1. Synthetic approaches to arylomycin core.
equiv of Cu reagent were required to obtain macrocycle product in a modest 60% yield. To support our internal drug discovery effort in developing novel arylomycin antibiotics,6 we sought to identify a practical entry to the arylomycin core structure (1). Herein, we describe an alternate macrocyclization route that provides both a practical and flexible entry point to the arylomycin core (Figure 1, route A) and our effort leading to a highly improved macrolactamization of the linear Ala-Hpg-Tyr precursor (Figure 1, improved route B). This synthesis design provided orthogonal protection and allowed for flexible and selective functionalization of each desired position for SAR purposes.9d Our synthesis began with Pd-catalyzed Miyaura borylation of peptide fragment 211 employing 1 mol % of PdCl2(dppf), bis(neopentylglycolato)diboron,12 and KOAc in DMSO at 80 °C (Scheme 1). The borylation reaction smoothly afforded a 91% yield of compound 3.13 Received: November 10, 2018 Published: December 19, 2018 147
DOI: 10.1021/acs.orglett.8b03603 Org. Lett. 2019, 21, 147−151
Letter
Organic Letters Scheme 1. Assembly of Macrocyclization Precursor
Figure 3. Key impurities for route A.
Table 1. Optimization of Macrolactamization Conditions for Route Aa
entry
Using microscale high-throughput experimentation (HTE) screening of the Suzuki−Miyaura coupling of boronate 3 and iodide 4 (see Figure S1 in the Supporting Information), we identified several catalysts to effect this sterically challenging C− C bond formation. Catalysts such as P(t-Bu)3Pd(crotyl)Cl,14 CataCXium A Pd G3,15 PdCl2[P(t-Bu)2Ph]2,16 PCy3 Pd G3,15 PEPPSI-IPr,17 and Pd(Amphos)Cl216 were all effective to generate the Suzuki−Miyaura coupling product 5 when the reaction employed K2HPO4 as base in MeOH (Figure 2) at 60
1 2 3 4 5 6 7 8 9
coupling reagent
yield (%)b
solvent/base
direct dosing of 6 T3P DMF/DIPEA HATU DMF/DIPEA EDCI/HOBt DMF/DIPEA PyBOP DMF/DIPEA slow dosing of 6 over 4 h PyBOP NMP/NMM EDCI/K-Oxyma NMP/n.a. TBTUc NMP/NMM CMPIc,d NMP/NMM DPPC/Oxymad NMP/NMM
10 13 19 26 73 73 82(80)e 76 86(89)e
8 (%) 13 11 27 42 3 6 2 3 2
a
Conditions: (a) 6 (141 mg, 0.20 mmol), coupling reagent (0.30 mmol, 1.5 equiv), and base (0.60 mmol, 3.0 equiv) in solvent (2.0 mL, 0.1 M concentration) at 20 °C; (b) 6 (141 mg, 0.20 mmol) in NMP (1.0 mL) added slowly over 4 h into solution of coupling reagent (0.30 mmol, 1.5 equiv) and base (0.60 mmol, 3.0 equiv) in NMP (1.0 mL) (final effective concentration = 0.1 M) at 20 °C. b HPLC assay yield. cYield after NH3/MeOH treatment. dCoupling reagent (2.0 equiv) and base (6.0 equiv) were employed. eIsolated yield for reaction at 4 mmol scale in 0.1 M concentration.
HOBt and PyBOP (entries 3 and 4) produced mainly the desired product 120 and cyclic dimer 8.21 Dimer formation was minimized through further reaction dilution. When the reaction was performed at lower concentration (0.005 vs 0.1 M) with PyBOP as the coupling reagent, macrocycle 1 was obtained in 72% yield. This is a stark contrast to the literature report for the alternate macrolactamization approach (route B), in which yield was very low (10%) even with dilution up to 0.0005 M.7 Optimization of reaction solvent and base showed NMP/ NMM to be optimal. Furthermore, slow dosing of 6 over 4 h was found to simulate a high dilution environment to disfavor dimer formation, allowing more practical reaction concentration (0.1 M) to be utilized (entry 5). Finally, a broader reagent evaluation revealed an unexpectedly versatile macrocyclization operable under wide ranging conditions. In addition to PyBOP, we found that EDCI/K-Oxyma, TBTU, and CMPI (entries 6−8) could produce an equally high-yielding (>70%) coupling reaction.21 However, the activation of carboxylic acid was not always selective over neighboring unprotected phenol particularly under TBTU and CMPI conditions, resulting in concurrent formation of either phenoxy uronium or pyridinium adduct of 1 (up to 35% based on HPLC analysis). A postreaction treatment with NH3 in MeOH was successful in regenerating desired target
Figure 2. Subset of HTE catalyst screening on Suzuki−Miyaura coupling of 3 and 4.
°C. Further validation and optimization led to the optimized conditions which employ 1 mol % of Pd(Amphos)Cl2 as the catalyst and 1.5 equiv of Na2CO3 as the base in MeOH at 60 °C (Scheme 1). Coupling product 5 was initially isolated as an amorphous solid. Fortunately, we identified a DBU salt of 5, which purged the two major impurities derived from homocoupling and protodeborylation of 3 to
