Total Syntheses of Natural Metallophores Staphylopine and

Nov 8, 2017 - Staphylopine has proven to play a central role in the competition for nutrient metals between host and microbes,(12) which would become ...
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Cite This: J. Org. Chem. 2017, 82, 13643−13648

Total Syntheses of Natural Metallophores Staphylopine and Aspergillomarasmine A Jian Zhang,‡,§ Sanshan Wang,†,§ Yingjie Bai,† Qianqian Guo,† Jiang Zhou,† and Xiaoguang Lei*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Department of Chemical Biology, College of Chemistry and Molecular Engineering, Synthetic and Functional Biomolecules Center and Peking-Tsinghua Center for Life Sciences, Peking University, Beijing 100871, China ‡ School of Life Sciences, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Staphylopine was discovered and functionally evaluated as a novel type of metallophore that Staphylococcus aureus employs to acquire multiple divalent transition metals. Aspergillomarasmine A (AMA), with a similar structure to staphylopine, was recently identified as an inhibitor of metallo-β-lactamases NDM-1 and VIM-2. Herein, we report a unified approach using Mitsunobu reaction as a key step to accomplish the concise and efficient total syntheses of staphylopine and AMA. We also elucidate the similar broad-spectrum metal chelation properties between staphylopine and AMA.

T

he increase and spread of antimicrobial resistance in bacteria is now a global public health problem.1 In contrast, only few new antibiotics have been introduced since the 1970s.2 We may be on the verge of the post-antibiotic era, where many procedures which we take for granted today, such as simple surgical operations or cancer immunosuppression therapies, may become impossible due to antimicrobial resistance.3 If current resistance rates increase as they have in recent years, it is projected that the consequences of unchecked antimicrobial resistance will cause an estimated 10 million deaths by 2050.4 Therefore, it is a pressing need to develop new means to overcome drug resistance. Transition-metal ions are essential nutrients for all organisms, and microbial pathogens have to acquire metal nutrients from the host to replicate and cause infections.5 In response, the host can initiate metal-withholding mechanisms to starve microbes, a process often termed “nutritional immunity”.6,7 The machineries employed by both the host and microbes to compete for metal nutrients can provide inspiration for new therapeutic strategies to prevent or treat infectious diseases, for example, the use of siderophores as drug delivery agents called the “Trojan horse” strategy.8 The most general model for bacterial metal acquisition using metallophores is affiliated with uptake of iron: one common strategy is the biosynthesis of high-affinity ferric siderophores.9 Structurally, the most common siderophores can be divided into three main classes: catecholates, hydroxamates, and carboxylates, such as enterobactin (3), ferrichrome (4), and rhizoferrin (5), respectively (Figure 1).8d These sidephores are usually involved in virulence and may also be used for the transport of other metals besides iron.10 Nicotianamine (6) is the first identified phytosiderophores, a family of molecules that share equivalent function to siderophores with variant chemical structures (Figure 1).11 In 2016, Arnoux and co-workers © 2017 American Chemical Society

Figure 1. Staphylopine (1), aspergillomarasmine A (2), and functionally related metallophore-type nature products.

reported the seminal discovery of an operon in Staphylococcus aureus that was required for the biosynthesis and trafficking of a new broad-spectrum metallophore: staphylopine (1), related to plant nicotianamine (6).12 Importantly, staphylopine's (1) production is not limited to S. aureus. Homologous biosynthetic genes are also encoded by other microbial pathogens, such as Received: September 15, 2017 Published: November 8, 2017 13643

DOI: 10.1021/acs.joc.7b02342 J. Org. Chem. 2017, 82, 13643−13648

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The Journal of Organic Chemistry the Gram-negative bacteria Pseudomonas aeruginosa. In 2015, Choi and co-workers concluded that P. aeruginosa secretes a metallophore of unknown structure in airway mucus and suggested that this metallophore was staphylopine or a staphylopine-like molecule.13 Staphylopine has proven to play a central role in the competition for nutrient metals between host and microbes,12 which would become a promising lead compound for the development of new antibiotics conjugate to overcome drug resistance, especially in the Gram-negative bacteria. However, no synthetic route has been reported, which hindered the further therapeutic exploration of staphylopine. Herein, we report the first total synthesis of staphylopine, using the Mitsunobu reaction as a key strategy to construct the full skeleton. In addition, this strategy also allows us to accomplish a more concise total synthesis of the New Delhi metallo-βlactamase-1 (NDM-1) natural product inhibitor aspergillomarasmine A (AMA)14 (2) in only 6 steps comparing with the previously reported syntheses in 15,15a 9,15b or 715c steps, respectively. Remarkably, we also further demonstrate that staphylopine and AMA share similar broad-spectrum metal chelation properties and could both serve as effective metallophores. Inspired by the proposed biosynthesis of staphylopine,12 our retrosynthetic analysis is outlined in Scheme 1. Staphylopine

derivative 13 (Scheme 2). Benzyl esterification of the carboxyl group in 13 using classical condensation reagents such as DCC Scheme 2. 8-Step Total Synthesis of Staphylopine (1)

and EDCI16a,b all led to racemization, due to the relatively basic reaction conditions. Therefore, we attempted the use of acidic conditions to solve this problem. To our delight, desired product 14 could be obtained in 77% yield in the presence of D(+)-10-camphorsulfonic acid additive.16d The Fmoc group was deprotected smoothly with piperidine to yield free amine followed by protection with o-nosyl group to afford the key precursor 9 in excellent yield. The first C−N bond formation under Mitsunobu condition between compound 9 and the protected L-homoserine 8 (obtained from commercially available Boc-L-aspartic acid 1-benzyl ester in one step with 99% yield17) proceeded smoothly to furnish the desired diamine 15 in 92% yield without any racemization. Initially, we attempted to construct the framework of staphylopine directly by a Mitsunobu reaction between 15 and alcohol 7. Unfortunately, all attempts under various conditions failed, because the Boc-protected amide proton was not sufficiently acidic to facilitate Mitsunobu reaction.18 Therefore, it was necessary to replace the Boc group with a more electronwithdrawing group. However, the trityl group at the imidazole ring is acid sensitive which may generate an additional problem for protecting group maneuver. Gratifyingly, Boc could be selectively removed by using Riniker’s protocol (1 N HCl in 90% aqueous AcOH)19 without any deprotection of trityl group, and the product was further protected with o-nosyl to yield 16 in one-pot in 88% yield. With compound 16 in hand, after extensive reaction conditions screening (Table S1), the desired product 17 with staphylopine skeleton was obtained in moderate yield through the second Mitsunobu reaction between 16 and secondary alcohol 7, by using diisopropyl azodicarboxylate and triphenylphosphine. The relatively low conversion is likely due to the steric hindrance of secondary alcohol.20 To complete the synthesis of staphylopine, it was only necessary to remove the protecting groups globally. However, initial attempts to deprotect the o-nosyl group were unsuccessful, which was probably due to the steric hindrance of the nearby benzyl ester. Thus, we decided to revise the deprotection sequence, which began with the deprotection of benzyl group. In our previous synthesis of AMA, we had utilized a deprotection protocol

Scheme 1. Retrosynthetic Analysis of Staphylopine (1) and AMA (2)a

a

8−12 are natural amino acid derivatives.

could be derived from three major building blocks: D-lactate derivative 7, L-homoserine 8, and D-histidine derivative 9. Conceivably, the most significant synthetic challenge is the effective C−N bond formation given the presence of several reactive nitrogen atoms and stereogenic centers at α-carbonyl positions in the target molecule. We envisioned that the Mitsunobu reaction could serve as a suitable conjugation reaction. The mild reaction conditions would facilitate sequential transformations and prevent racemization. Moreover, the selection of suitable protecting groups would also be essential for the success of the synthesis, considering the presence of various polar and reactive functional groups. In terms of the retrosynthetic analysis of AMA, we also considered that Mitsunobu reaction might provide us a more efficient approach to construct the natural product framework than the previously established reductive amination strategy.15a Consequently, we decided to use serine derivative 10, aziridine 11,15b and protected L-aspartic acid derivative 12 as building blocks (Scheme 1). Our synthesis of staphylopine commenced with the preparation of fragment 9 from commercially available D-His 13644

DOI: 10.1021/acs.joc.7b02342 J. Org. Chem. 2017, 82, 13643−13648

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The Journal of Organic Chemistry

synthesis of AMA (2) was accomplished in only 6 steps in 28% yield from commercially available 18. The spectroscopic and physical properties of our synthesized AMA and natural AMA are fully identical. Because AMA and staphylopine are structurally related natural products, we decided to further investigate their metal chelation properties systematically. Stability constants for AMA-metal complexes were estimated by metal competition experiments using ESI-MS method that was previously applied for the studies of staphylopine’s metal binding properties.12,22 Interestingly, the complex dissociation constants obtained from our experiments indicated strong coordination properties of AMA toward bivalent metal ions. Moreover, AMA showed the same order of metal affinity as staphylopine (similar pKd and affinity order: Cu2+ > Ni2+ > Co2+ > Zn2+ > Fe2+; Table 1),12 which further suggested AMA could also serve as a broadspectrum metallophore.

involving CF3SO3H and anisole in dichloromethane for the one-pot removal of both tert-butyl and Cbz groups.15a,21 Here, the benzyl and trityl group could also be removed successfully in one pot with this protocol. Finally, the o-nosyl group was deprotected with ethyldiisopropylamine and thiophenol in N,N-dimethylformamide to afford sodium salts of staphylopine in 8 steps starting from commercially available 13. To further confirm the structure of staphylopine, hydrochloride salts of staphylopine were prepared. To our delight, synthetic staphylopine exhibited NMR spectra indistinguishable from the reported data.12 Furthermore, a 1H NMR spectrum that was obtained after mixing our synthetic staphylopine with natural staphylopine purchased from Toronto Research Chemicals12 (in a 1:1 ratio) showed only one set of peaks (see the Supporting Information for detail). Collectively, these results unambiguously confirmed the assigned structure of staphylopine. With the established synthetic route to staphylopine, we then set out to develop a more efficient synthesis of the structurally related natural product AMA. We developed a more efficient route to the key aziridine 11 in only 2 steps. Starting with commercially available O-benzyl serine 18 (Scheme 3), the

Table 1. Putative Metal-Chelation Structures and Comparison of the Estimated pKd Values for Staphylopine (1) and AMA (2)a

Scheme 3. 6-Step Total Synthesis of AMA (2)

pKd metal ions 2+

Cu Ni2+ Co2+ Zn2+ Fe2+

staphylopine12

AMA

19.0 16.4 15.1 15.0 12.3

18.9 17.4 15.1 14.9 13.0

a

Putative chemical structures of staphylopine-metal complex (A) and AMA-metal complex (B). Comparison of the estimated pKd values for staphylopine (1) and AMA (2) using competition experiments with five different metal species by ESI-MS.12 pKd represents the dissociation equilibrium constants of coordination compounds.

amine moiety was protected with o-nosyl group in quantitative yield, followed by an intramolecular Mitsunobu reaction to generate the aziridine 11 in 92% yield. The aziridine was treated with L-aspartic acid derivative 12 to afford the ring opening product 20. Then we attempted to couple the N-Bocbenzyl-serine and compound 20 under several Mitsunobu conditions, but unfortunately we only observed the intramolecular elimination byproduct generated from N-Boc-benzylserine (Table S2).20e When the protecting group was change to a trityl group, the desired product 21 was detected. Fortunately, after screening many conditions (Table S3),21 we found that the optimal condition was diethyl azodicarboxylate and triphenylphosphine in toluene, which could afford the desired compound 21 in 53% yield (75% based on recovered starting material). The moderate yield of Mitsunobu reaction is mainly caused by the stereohinder effects generated by the adjacent functional groups. The o-nosyl group was removed with potassium carbonate and thiophenol in N,N-dimethylformamide to afford 22. Finally, the benzyl, tert-butyl, and trityl group were removed successfully in one pot with our previously established protocol15a to furnish AMA in 90% yield. The total

In summary, we have accomplished the first total synthesis of staphylopine and a more concise total synthesis of AMA in 8 and 6 steps, respectively. The key synthetic strategy relies on Mitsunobu reaction to form the C−N bond efficiently. The lessons learned from our synthetic endeavors regarding the protecting group maneuvers should generate more insights to amino acid containing natural product synthesis. Our synthesis features a convergent, flexible, and stereocontrolled route that is amenable to efficiently prepare analogues for both staphylopine and AMA. Moreover, we have further demonstrated that AMA has the same order of metal affinity as staphylopine, indicating for the first time that AMA could also serve as an effective broad-spectrum metallophore. The efficient chemical synthesis of this family of natural product will provide a significant support to facilitate the mode of action studies in order to uncover the unknown mechanisms on how these novel metallophores transport metal ions for bacteria as well as to develop effective antibiotics conjugate to overcome drug resistance crisis. 13645

DOI: 10.1021/acs.joc.7b02342 J. Org. Chem. 2017, 82, 13643−13648

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The Journal of Organic Chemistry

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triphenylphosphine (2.07 mL, 7.88 mmol, 3 equiv) and cooled to 0 °C, and then diisopropyl azodicarboxylate (1.55 mL, 7.88 mmol, 3 equiv) was added. The reaction mixture was warmed to rt and stirred for 1 h. The mixture was cooled to 0 °C and filtered, which was washed with cooled THF. The combined organic layers were evaporated under reduced pressure, and the residue was subjected to column of silica gel (PE/EtOAc = 3:1 to 1/1) to afford the diamino derivatives 15 (2.33g, 2.42 mmol, 92%) as a white solid; mp = 48−51 °C; 1H NMR (400 MHz, CDCl3) δ 7.90 (dd, J = 7.9, 1.0 Hz, 1H), 7.53−7.28 (m, 21H), 7.17−7.07 (m, 8H), 6.71 (s, 1H), 6.18 (d, J = 7.8 Hz, 1H), 5.13 (d, J = 3.1 Hz, 2H), 5.04 (dd, J = 10.5, 3.9 Hz, 1H), 4.88 (dd, J = 51.6, 12.1 Hz, 2H), 4.22−4.11(m, 1H), 3.66−3.53 (m, 1H), 3.40−3.25 (m, 2H), 3.04 (dd, J = 15.5, 10.9 Hz, 1H), 2.36−2.19 (m, 1H), 1.87−1.76 (m, 1H), 1.35 (s, 9H); 13C NMR (101 MHz, CDCl3) δ171.9, 170.0, 155.7, 147.9, 142.2, 138.1, 135.6, 135.4, 134.8, 133.3, 132.2, 131.2, 130.9, 129.7, 128.49, 128.45, 128.3, 128.2, 128.1, 128.04, 127.95, 123.7, 120.1, 79.6, 67.1, 66.9, 60.8, 52.2, 43.5, 33.1, 29.6, 28.2; IR (neat) νmax 1739 (CO), 1367 (N−O), 1161(C−O) cm−1; HRMS (ESI): [M + H]+ calculated for C54H53N5O10S: 964.3586, found: 964.3568; [α]19 D +14 (c 1.0, CHCl3). Benzyl (S)-4-((N-((R)-1-(Benzyloxy)-1-oxo-3-(1-trityl-1H-imidazol4-yl)propan-2-yl)-4-nitrophenyl)sulfonamido)-2-((4-nitrophenyl)sulfonamido)butanoate (16). Compound 15 (2.2 g, 2.3 mmol, 1 equiv) was treated with 1 N HCl in 90% aqueous HOAc (13 mL), and the mixture was stirred at rt for 15 min under Ar. Then concentrated in vacuo to afford the crude primary amine, which was directly used without further purification. The crude mixture was dissolved in EtOAc (13 mL), and to this solution was added saturated NaHCO3 (13 mL) and 2-nitrobenzenesulfonyl chloride (NsCl 0.84 g, 3.8 mmol, 1.6 equiv) at 0 °C. The reaction mixture was warmed to rt and stirred for 12 h. The resulting solution was extracted with EtOAc (20 mL × 3), and the combined organic extracts were washed with brine (4 mL × 2), dried over anhydrous Na2SO4 and the filtrate was concentrated in vacuo. The residue was subjected to column of silica gel (PE/EtOAc = 3:1 to 1/1) to afford the diamino derivative 16 (2.12g, 2.02 mmol, 88%) as a white solid; mp = 80−83 °C; 1H NMR (400 MHz, CDCl3) δ 9.25−8.89 (br, 1H), 7.97 (dd, J = 7.6, 1.5 Hz, 1H), 7.90 (dd, J = 7.8, 1.1 Hz, 1H), 7.71−7.29 (m, 20H), 7.24−7.07 (m, 12H), 6.78 (s, 1H), 5.04−4.68 (m, 5H), 4.18 (d, J = 9.8 Hz, 1H), 3.77−3.64 (m, 1H), 3.51−3.20 (m, 3H), 2.52−2.35 (m, 1H), 1.91−1.78 (m, 1H); 13C NMR (101 MHz, CDCl3) δ 170.5, 169.8, 147.9, 147.5, 142.1, 137.6, 135.4, 134.82, 134.81, 134.4, 133.4, 133.0, 132.2, 132.1, 132.0, 131.7, 131.2, 130.8, 130.4, 129.8, 128.5, 128.5, 128.43, 128.36, 128.3, 128.1, 128.0, 124.9, 123.7, 120.3, 75.6, 67.4, 67.0, 62.1, 55.5, 42.5, 33.7, 28.9; IR (neat) νmax 1740 (CO), 1371 (N−O), 1163 (C−O) cm−1; HRMS (ESI): [M + H]+ calculated for C55H49N6O12S2: 1049.2844, found: 1049.2843; [α]19 D −2.4 (c 1.0, CHCl3). Benzyl (S)-4-((N-((R)-1-(Benzyloxy)-1-oxo-3-(1-trityl-1H-imidazol4-yl)propan-2-yl)-4-nitrophenyl)sulfonamido)-2-((N-((S)-1-(benzyloxy)-1-oxopropan-2-yl)-4-nitrophenyl)sulfonamido)butanoate (17). Diamino derivatives 16 (300 mg, 0.286 mmol, 1 equiv) and protected (R)-lactate 7 (154m g, 0.860 mmol, 3 equiv) were dissolved in anhydrous THF (5 mL), and to this solution was added triphenylphosphine (300 mg, 1.14 mmol, 4 equiv) and cooled to 0 °C and then diisopropyl azodicarboxylate (224 mg, 1.14 mmol, 4 equiv) was added. The reaction mixture was warmed to rt and stirred for 16 h. The mixture was cooled to 0 °C and filtered, which was washed with cooled THF. Then the solvent was removed in vacuo, and the resulting mixture was purified by reverse-phase column to afford the compound 17 (118 mg 0.097 mmol 34%) as a white solid; mp = 85−90 °C; 1H NMR (400 MHz, CDCl3) δ 8.09 (d, J = 7.9 Hz, 1H), 8.00 (d, J = 8.0 Hz, 1H), 7.49−7.28 (m, 25H), 7.13 (m, 11H), 6.72 (s, 1H), 5.10 (dd, J = 10.4, 5.0 Hz, 1H), 5.08−4.92 (m, 4H), 4.87 (q, J = 12.1 Hz, 2H), 4.63 (q, J = 7.2 Hz, 1H), 4.55 (dd, J = 7.7, 5.7 Hz, 1H), 3.73−3.58 (m, 1H), 3.54−3.43 (m, 1H), 3.37 (dd, J = 15.4, 4.9 Hz, 1H), 3.00 (dd, J = 15.3, 10.6 Hz, 1H), 2.55−2.41 (m, 1H), 2.25−2.13 (m, 1H), 1.43 (d, J = 7.3 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ171.0, 170.4, 170.0, 148.2, 148.0, 142.3, 138.1, 136.0, 135.2, 135.0, 134.7, 133.4, 133.1, 132.9, 132.4, 131.5, 131.4, 131.2, 129.8, 128.6, 128.5, 128.4, 128.34, 128.30, 128.25, 128.2, 128.0, 127.9, 123.8,

EXPERIMENTAL SECTION 1

General Information. H NMR spectra were recorded on a Bruker 400 MHz spectrometer at ambient temperature with CDCl3 as the solvent unless otherwise stated. 13C NMR spectra were recorded on a Bruker 101 MHz spectrometer (with complete proton decoupling) at ambient temperature. 1H NMR spectra of synthetic staphylopine were recorded on a Bruker AVANCE 600 MHz spectrometer (BBO probe) at ambient temperature. 13C NMR spectra of synthetic staphylopine were recorded on a Bruker AVANCE 150 MHz spectrometer (BBO probe) (with complete proton decoupling) at ambient temperature. Chemical shifts are reported in parts per million relative to chloroform (1H, δ 7.26 ppm; 13C, δ 77.00 ppm) and deuterium oxide (1H, δ 4.79 ppm). Data for 1H NMR are reported as follows: chemical shift, integration, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet) and coupling constants. Infrared spectra were recorded on a Thermo Fisher FT-IR200 spectrophotometer. High-resolution mass spectra were obtained at Peking University Mass Spectrometry Laboratory using a Bruker Fourier Transform Ion Cyclotron Resonance Mass Spectrometer Solarix XR. Optical rotations were recorded on an AUTOPOL III digital polarimeter at 589 nm and are recorded as [α]23 D (concentration in grams/100 mL solvent). The samples were analyzed by HPLC/MS on a Waters Auto Purification LC/MS system (3100 Mass Detector, 2545 Binary Gradient Module, 2767 Sample Manager, and 2998 Photodiode Array (PDA) Detector). The system was equipped with a Waters C18 5 m Sun Fire separation column (150*4.6 mm), equilibrated with HPLC grade water (solvent A) and HPLC grade methanol (solvent B) with a flow rate of 0.3 mL/min at rt. Preparative HPLC-MS on a Waters Auto Purification LC/MS system (3100 Mass Detector, 2545 Binary Gradient Module, 515 HPLC pump, 2767 Sample Manager, and 2998 Photodiode Array (PDA) Detector). The system was equipped with a Waters C18 5 μm X-bridge separation column (150*19 mm). Analytical thin-layer chromatography was performed using 0.25 mm silica gel 60-F plates. Flash chromatography was performed using 200−400 mesh silica gel. Yields refer to chromatographically and spectroscopically pure materials, unless otherwise stated. All reagents were used as supplied by commerical sources. Methylene chloride was distilled from calcium hydride; tetrahydrofuran was distilled from sodium/benzophenone ketyl prior to use. All reactions were carried out in oven-dried glassware under an argon atmosphere unless otherwise noted. Natural staphylopine was purchased from a chemical supplier. Benzyl Nα-(((9H-Fluoren-9-yl)methoxy)carbonyl)-Nτ-trityl-D-histidinate (14). A solution of acid 13 (2.48 g, 4.00 mmol, 1 equiv), benzyl alcohol (DCC, 1.49 g, 7.20 mmol, 1.8 equiv), 1,3dicyclohexylcarbodiimide (CSA, 1.21 g, 5.20 mmol, 1.3 equiv), and D(+)-10-camphorsulfonic acid (504 mg, 6.00 mmol, 1.5 equiv) in dichloromethane (40 mL) was stirred at rt for 2 h. After the addition of a Na2CO3 (504 mg, 6 mmol), the reaction mixture was stirred for additional 1 h, and diethyl ether and anhydrous Na2SO4 were added to the reaction, then the mixture was filtered with Celite, which was washed with EtOAc. The combined organic layers were evaporated under reduced pressure, and the residue was subjected to column of silica gel (PE/EtOAc = 3/1 to 1/1) to afford compound 14 (2.2 g, 3.1 mmol, 77%) as a white solid; mp = 91−94 °C; 1H NMR (400 MHz, CDCl3) δ 7.76 (d, J = 7.5 Hz, 2H), 7.62 (t, J = 6.7 Hz, 2H), 7.42−7.30 (m, 14H), 7.25−7.21 (m, 5H), 7.12−7.08 (m, 6H), 6.62 (d, J = 8.2 Hz, 1H), 6.51 (s, 1H), 5.05 (q, J = 12.2 Hz, 2H), 4.73−4.62 (m, 1H), 4.40−4.22 (m, 3H), 3.15−3.02 (m, 2H); 13C NMR (101 MHz, CDCl3) δ 171.4, 156.2, 144.1, 143.9, 142.2, 141.2, 138.9, 136.3, 135.4, 129.7, 128.5, 128.2, 128.0, 127.6, 127.0, 125.4, 125.3, 119.9, 119.6, 75.2, 67.2, 66.8, 54.3, 47.1, 30.0; IR (neat) νmax 1743 (CO), 1620 (CN), 1243 (C−N),1168 (C−O) cm−1; HRMS (ESI): [M + H]+ calculated for C47H40N3O4: 710.3013, found: 710.3017; [α]19 D +76 (c 1.0, CHCl3). Benzyl (S)-4-((N-((R)-1-(Benzyloxy)-1-oxo-3-(1-trityl-1H-imidazol4- y l ) pr o p an - 2 - y l ) - 4 - n i t r op h e n y l )s u l f on a m i d o )- 2 - ( ( te rt butoxycarbonyl)amino)butanoate (15). 9 (1.77 g, 2.63 mmol, 1 equiv) and alcohol 8 (1.22 g, 3.94 mmol, 1.5 equiv) were dissolved in anhydrous THF (26 mL), and to this solution was added 13646

DOI: 10.1021/acs.joc.7b02342 J. Org. Chem. 2017, 82, 13643−13648

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The Journal of Organic Chemistry

atmosphere. The mixture was cooled to 0 °C and stirred for 10 min. DEAD (140 μL, 0.897 mmol) was added dropwise over a period of 30 min, and the reaction was allowed to warm to rt and stirred for 8 h. Then the mixture was cooled to 0 °C, compound 9 (390 mg, 0.897 mmol), triphenylphosphine (236 mg, 0.897 mmol), and DEAD (140 μL, 0.897 mmol) were added again, and the mixture was allowed to warm to rt and stirred for another 8 h. Then the solvent was removed in vacuo, and the resulting mixture was purified by reverse-phase column to afford the desired product 21 (337 mg, 53%) as a colorless oil and compound 20(118 mg, 30%) was recovered. 1H NMR (400 MHz, CDCl3) δ 7.95−7.89 (m, 1H), 7.48−7.41 (m, 8H), 7.32 (m, 7H), 7.25−7.11 (m, 13H), 4.92 (dd, J = 27.8, 12.0 Hz, 2H), 4.68 (t, J = 7.0 Hz, 1H), 4.53 (q, J = 11.9 Hz, 2H), 3.86 (dd, J = 16.4, 11.6 Hz, 1H), 3.69−3.56 (m, 2H), 3.27 (t, J = 6.4 Hz, 1H), 3.06−2.87 (m, 3H), 2.24 (dd, J = 15.9, 6.7 Hz, 1H), 2.07 (dd, J = 15.5, 5.6 Hz, 2H), 1.40 (d, J = 23.8 Hz, 18H). 13C NMR (101 MHz, CDCl3) δ172.4, 170.2, 168.9, 147.8, 145.6, 135.2, 134.9, 133.4, 132.9, 131.5, 131.2, 129.3, 128.8, 128.7, 128.59, 128.56, 128.4, 128.2, 128.0, 126.6, 123.9, 80.6, 71.1, 67.6, 67.3, 61.4, 58.1, 56.6, 51.8, 47.7, 39.0, 28.1, 28.0. IR (neat) νmax: 3317 (N−H), 1731 (CO), 1545 (NO), 1147 (C−N) cm−1; HRMS (ESI): [M + H]+ calculated for C57H63N4O12S: 1027.4171; Found: 1027.4157; [α]22 D −77 (c 0.10, CHCl3). Di-tert-butyl ((S)-3-(Benzyloxy)-2-(((S)-3-(benzyloxy)-3-oxo-2(tritylamino)propyl)amino)-3-oxopropyl)-L-aspartate (22). To a solution of compound 21 (100 mg, 0.097 mmol, 1 equiv) in DMF (4 mL) were added K2CO3 (268 mg, 1.95 mmol, 20 equiv) and PhSH (99 μL, 0.97 mmol, 10 equiv) successively under an argon atmosphere, then the reaction mixture was stirred at rt for another 2 h. Then the reaction mixture was concentrated in vacuo, resolved in ethyl acetate (5 mL), and washed by brine (5 mL). The combined organic layers were dried over anhydrous Na2SO4, and the filtrate was concentrated in vacuo. The residue was purified by silica gel column chromatography (PE/EtOAc = 9/1 to 7/1) to afford compound 22 (73 mg, 92%) as a light yellow oil; 1H NMR (400 MHz, CDCl3) δ 7.54−7.46 (m, 6H), 7.37−7.27 (m, 8H), 7.25−7.12 (m, 11H), 5.20− 5.08 (m, 2H), 4.68 (d, J = 12.3 Hz, 1H), 4.49 (d, J = 12.2 Hz, 1H), 3.56−3.47 (br, 1H), 3.47−3.37 (br, 1H), 3.37−3.25 (br, 1H), 2.99 (dd, J = 11.6, 5.7 Hz, 1H), 2.95−2.79 (br, 2H), 2.78−2.64 (br, 1H), 2.63−2.46 (br, 2H), 2.45−2.34 (br, 1H), 1.41 (d, J = 5.7 Hz, 18H). 13 C NMR (100 MHz, CDCl3) δ174.1, 173.7, 172.6, 170.1, 146.0, 135.6, 128.9, 128.6, 128.4, 128.3, 128.2, 128.2, 128.1, 128.0, 127.9, 127.3, 126.4, 81.3, 80.8, 77.3, 70.9, 66.5, 62.0, 58.6, 56.8, 51.9, 50.0, 39.5, 32.0, 30.3, 29.7, 29.4, 28.1, 28.1, 22.7, 14.2. IR (neat) νmax: 3320 (N−H), 1730 (CO), 1149 (C−N), 1029 (C−O) cm−1; HRMS (ESI): [M + H]+ calculated for C51H59N3O8: 842.4382; Found: 842.4375; [α]22 D −22 (c 0.10, CHCl3). AMA (2). To a solution of compound 22 (44 mg, 0.052 mmol, 1 equiv) in CH2Cl2 (5 mL) were added anisole (102 μL, 0.936 mmol, 18 equiv) and trifluoromethanesulfonic acid (74 μL, 0.83 mmol, 16 equiv) at 0 °C. The reaction mixture was stirred at 0 °C for 0.5 h, then warmed to rt, and stirred for 1.5 h. The reaction was quenched with a solution of NaHCO3 (96 mg, 1.1 mmol, 21 equiv) in water (8 mL) at 0 °C and stirred for an additional 1 h. The mixture was washed with CH2Cl2 (10 mL × 3), and the water phase was concentrated in vacuo. The resulting solid was purified by gel column to afford the sodium salt of (S,S,S)-AMA (19.5 mg, 90%) as a white solid, mp = 246−256 °C. The compound showed identical physical and spectroscopic properties to those reported previously.15a 1H NMR (400 MHz, D2O) δ 3.81 (dd, J = 5.9, 3.8 Hz, 1H), 3.78 (dd, J = 9.0, 3.4 Hz, 1H), 3.41 (dd, J = 9.6, 4.1 Hz, 1H), 3.22 (dd, J = 12.9, 4.5 Hz, 2H), 3.06 (dd, J = 12.6, 9.9 Hz, 1H), 2.90 (dd, J = 13.4, 3.9 Hz, 1H), 2.80 (dd, J = 17.2, 3.9 Hz, 1H), 2.65 (dd, J = 17.1, 9.2 Hz, 1H). 13C NMR (100 MHz, D2O) δ 176.7, 176.6, 172.44, 172.36, 59.1, 58.6, 53.8, 47.2, 46.3, 35.0. IR (neat) νmax 3386(O−H), 1592 (CO), 1069 (C−O) cm−1; HRMS (ESI): [M-H]− calculated for C10H16N3O8: 306.0931; Found: 306.0942; [α]23 D −51 (c 0.80, phosphate buffer pH = 7).

123.5, 119.9, 75.2, 67.5, 67.4, 67.3, 60.6, 57.0, 55.1, 44.0, 31.7, 29.3, 16.3; IR (neat) νmax 1740 (CO), 1372 (N−O), 1155 (C−O) cm−1; HRMS (ESI): [M + H]+ calculated for C65H59N6O14S2: 1211.3525, found: 1211.3536; [α]19 D −35 (c 0.50, CHCl3). Staphylopine Sodium Salt (1). To a solution of compound 17 (73.0 mg, 0.060 mmol, 1 equiv) in DCM (3 mL) was added anisole (61.0 μL, 0.560 mmol, 9 equiv) and trifluoromethanesulfonic acid (50.0 μL, 0.560 mmol, 9 equiv) at 0 °C. The reaction mixture was stirred at 0 °C for 0.5 h, then warmed to rt, and stirred for 1 h. The reaction was quenched with a solution of NaHCO3 (55 mg, 0.66 mmol) in water (5 mL) at 0 °C and stirred for an additional 1 h. The mixture was washed with DCM (10 mL × 3), and the water phase was concentrated in vacuo and directly used for the next step without further purification. The crude mixture was dissolved in dry DMF (3 mL), and to this solution was added thiophenol (124 μL, 1.20 mmol, 20 equiv) and diisopropylethylamine (DIPEA 160 μL, 0.960 mmol, 16 equiv). The reaction was carried out for 10 h at rt before water (10 mL) was added and washed with DCM (10 mL × 3). Then the water phase was concentrated in vacuo. The resulting solid was purified by gel column to afford staphylopine sodium salt (16 mg, 0.042 mmol, 70%) as a white solid; mp >270 °C; 1H NMR (400 MHz, D2O) δ 7.89 (s, 1H), 7.08 (s, 1H), 3.82 (t, J = 6.1 Hz, 1H), 3.62−3.49 (m, 2H), 3.27−3.03 (m, 4H), 2.21−2.05 (m, 2H), 1.44 (d, J = 7.1 Hz, 3H); 13C NMR (101 MHz, D2O) δ 176.8, 175.1, 174.8, 135.7, 117.1, 62.1, 60.8, 58.0, 44.4, 27.7, 27.5, 16.7; IR (neat) νmax 1620 (CO), 1135 (C−O) cm−1; HRMS (ESI): [M + H]+ calculated for C13H21N4O6: 329.1456, found: 329.1459; [α]19 D +7.6 (c 0.20, H2O). Benzyl (S)-1-((4-Nitrophenyl)sulfonyl)aziridine-2-carboxylate (11). The compound 19 (5.85g, 15.4 mmol) and triphenylphosphine (3.28g, 16.2 mmol) were dissolved in anhydrous THF (50 mL) under an argon atmosphere, then DEAD (2.52 mL, 16.2 mmol) dissolved in THF (50 mL) was added dropwise over a period of 30 min at 0 °C, and then the reaction was allowed to stir at same temperature for further 30 min. The solution was concentrated in vacuo, and the residue was directly purified by silica gel column chromatography (PE/ EtOAc = 9/1 to 7/1) to afford the aziridine 11 as a light yellow oil (5.01g, 94%); 1H NMR (400 MHz, CDCl3): δ 8.30−8.21 (m, 1H), 7.84−7.71 (m, 3H), 7.43−7.29 (m, 5H), 5.22 (s, 2H), 3.66 (dd, J = 7.1, 4.4 Hz, 1H), 3.08 (d, J = 7.1 Hz, 1H), 2.81 (d, J = 4.4 Hz, 1H); 13 C NMR (101 MHz, CDCl3): δ 166.5, 134.9, 134.8, 132.6, 131.6, 128.7, 128.7, 128.4, 124.7, 67.9, 37.8, 34.2; IR (neat) νmax: 1745 (C O), 1541 (NO), 1364(N−O), 1283 (C−N) cm−1; HRMS (ESI): [M+NH3]+ calculated for C16H18N3O6S:380.0911, Found:380.0906; [α]22 D −55 (c 1.0, CHCl3). Di-tert-butyl ((S)-3-(Benzyloxy)-2-((4-nitrophenyl)sulfonamido)3-oxopropyl)-L-aspartate (20). The aziridine 11 (804 mg, 2.22 mmol, 1 equiv) was dissolved in THF (25 mL) and was added Laspartic acid di-tert-butyl ester 12 (1.09 g, 4.44 mmol, 2 equiv), followed by TEA (62 μL, 0.444 mmol) at 0 °C. The reaction was allowed to stir for 36 h at rt. After completion of the reaction, the solvent was removed in vacuo, then the residue was dissolved in ethyl acetate (20 mL) and washed with brine (2 × 20 mL), dried over anhydrous Na2SO4, and the filtrate was concentrated in vacuo. The crude product was purified by using silica gel column chromatography (PE/EtOAc = 9/1 to 5/1) to afford the product 20 as yellow oil (941 mg, 70%); 1H NMR (400 MHz, CDCl3): δ 8.05−7.97 (m, 1H), 7.78− 7.82 (m, 1H), 7.65−7.54 (m, 2H), 7.36−7.29 (m, 3H), 7.22−7.15 (m, 2H), 6.94−6.77 (m, 1H), 4.94 (q, J = 12.2 Hz, 2H), 4.37−4.24 (br, 1H), 3.50−3.30 (m, 2H), 2.92−2.79 (m, 1H), 2.68−2.55 (m, 1H), 2.53−2.41 (m, 1H),δ 1.37 (d, J = 5.1 Hz, 18H); 13C NMR (101 MHz, CDCl3) δ 170.2, 169.9, 147.6, 134.9, 133.3, 132.6, 130.4, 128.6, 128.5, 128.2, 125.3, 81.4, 67.4, 58.9, 57.2, 50.2, 39.3, 28.1, 28.0; IR (neat) νmax: 3331 (N−H), 1726 (CO), 1541 (NO), 1366 (N−O), 1147 (C−N) cm−1; HRMS (ESI): [M + H]+ calculated for C28H37N3O10S: 608.2272; Found: 608.2269; [α]22 D −118 (c 1.00, CHCl3). Di-tert-butyl ((S)-3-(Benzyloxy)-2-((N-((S)-3-(benzyloxy)-3-oxo-2(tritylamino)propyl)-4-nitrophenyl)sulfonamido)-3-oxopropyl)-L-aspartate (21). Compound 20 (390 mg, 0.897 mmol) and compound 9 (363 mg, 0.598 mmol) were combined with triphenylphosphine (236 mg, 0.897 mmol) in anhydrous toluene (5 mL) under an argon 13647

DOI: 10.1021/acs.joc.7b02342 J. Org. Chem. 2017, 82, 13643−13648

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02342. Supplemental methods, tables and relevant 1H, 13C, and HRMS spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaoguang Lei: 0000-0002-0380-8035 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Houhua Li, Dr. Benke Hong, Dr. Alexander Jones, Dr. Rongwen Yang, Dr. Jing Zhang, and Dr. Xiaohui Liu for helpful discussions as well as Prof. Changwen Jin and Prof. Hongwei Li (Peking University) for the assistance with 600 MHz NMR analysis. We also thank Prof. Xiaoran He for HRMS-MS analysis. Financial support from the National Key Research and Development Program of China (2017YFA0505200), National High Technology Project 973 (2015CB856200), and NNSFC (21472010, 21521003, 21561142002 and 21625201) are gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.joc.7b02342 J. Org. Chem. 2017, 82, 13643−13648