Synthesis of the Isodityrosine Moiety of Seongsanamide A–D and Its Derivatives

The concise and highly convergent synthesis of the isodityrosine unit of seongsanamide A–D and its derivatives bearing a diaryl ether moiety is described. In this work, the synthetic strategy features palladium-catalyzed C(sp3)–H functionalization and a Cu/ligand-catalyzed coupling reaction. We report a practical protocol for the palladium-catalyzed mono-arylation of β-methyl C(sp3)–H of an alanine derivative bearing a 2-thiomethylaniline auxiliary. The reaction is compatible with a variety of functional groups, providing practical access to numerous β-aryl-α-amino acids; these acids can be converted into various tyrosine and dihydroxyphenylalanine (DOPA) derivatives. Then, a CuI/N,N-dimethylglycine-catalyzed arylation of the already synthesized DOPA derivatives with aryl iodides is described for the synthesis of isodityrosine derivatives.


Introduction
In 2018, Choi and coworkers [1] reported the isolation of seongsanamide A-D from a bacterial culture broth of Bacillus safensis KCTC 12796BP, obtained from a marine sponge collected in water samples of Seongsan on Jeju Island. Seongsanamides are bicyclic depsipeptides with an isodityrosine residue and exhibit antiallergenic properties ( Figure 1). Furthermore, isodityrosine is a tyrosine dimer containing oxidatively coupled aromatic nuclei, and the tyrosine units are linked through a diaryl ether moiety [2]. A large class of biological cyclopeptides containing this structural unit exists in nature, with a wide range of pharmaceutical activities, such as seongsanamidesA-D, K-13 [3], OF4949-III [3], rubiyunnanin D [4], and bouvardin [5].

Introduction
In 2018, Choi and coworkers [1] reported the isolation of seongsanamide A-D f a bacterial culture broth of Bacillus safensis KCTC 12796BP, obtained from a ma sponge collected in water samples of Seongsan on Jeju Island. Seongsanamides are b clic depsipeptides with an isodityrosine residue and exhibit antiallergenic proper ( Figure 1). Furthermore, isodityrosine is a tyrosine dimer containing oxidatively coup aromatic nuclei, and the tyrosine units are linked through a diaryl ether moiety [2 large class of biological cyclopeptides containing this structural unit exists in nature, w a wide range of pharmaceutical activities, such as seongsanamidesA-D, K-13 OF4949-Ⅲ [3], rubiyunnanin D [4], and bouvardin [5]. In addition, diaryl ether (DE) is a functional scaffold that exists widely in both nat ural products and new drugs approved for the market [6]. Moreover, DE is always con sidered the fundamental fragment of a wide variety of medicinal and agrochemica agents as well as their bioisosteres ( Figure 2). Over the years, medicinal chemists have exploited the use of privileged structures inspired by natural products in drug discovery The introduction of functional groups, such as α-keto [7] amide and gem-dimethyl [8 moieties, into biologically active small molecules has emerged as an efficient way to ob tain clinically useful drugs. Due to their potential structural diversity, pharmaceutical value, and good drugga bility, isodityrosines and cyclopeptide derivatives have attracted increasing attention in the synthetic community. Numerous research laboratories have engaged in the tota synthesis of these natural products or analogs, including Doẗz benzannulation [9], SNAr reactions [10], and the Ullman [11][12][13][14][15][16] and Evans-Chan-Lam [3,17,18] coupling reactions However, the synthesis of isodityrosines and their derivatives usually begins with available natural amino acids, limiting the diversity of the synthesis of their derivatives and their applications in drug design.
To address these challenges, we aimed to establish a convenient and convergen synthetic route. In recent years, transition-metal-catalyzed C-H functionalization has provided general and practical access to various natural and unnatural aromatic amino acids [19][20][21][22][23]. Our retrosynthetic analysis of the isodityrosine moiety of seongsanamides is outlined in Scheme 1. We selected a strategy based on palladium-catalyzed C(sp 3 )-H functionalization and copper-catalyzed Ullmann coupling reactions. The iodityrosine moiety was prepared from key precursor 1 and 4-iodophenylalaninederivatives via a C-O coupling reaction. The DOPA derivative 1 was derived from intermediate 2 by re moving hydroxyl protecting groups, Dakin oxidation of aldehyde, or Baeyer-Villiger oxidation of acetyl group. Compound 2 was prepared by removing the directing group o 3, which resulted from the palladium-catalyzed monoarylation of β-methyl C(sp 3 )-H o an alanine derivative with aryl iodides using a directing group. The structural diversity of aryl iodides could facilitate the synthesis of more isodityrosine derivatives.  Due to their potential structural diversity, pharmaceutical value, and good druggability, isodityrosines and cyclopeptide derivatives have attracted increasing attention in the synthetic community. Numerous research laboratories have engaged in the total synthesis of these natural products or analogs, including Dötz benzannulation [9], S N Ar reactions [10], and the Ullman [11][12][13][14][15][16] and Evans-Chan-Lam [3,17,18] coupling reactions. However, the synthesis of isodityrosines and their derivatives usually begins with available natural amino acids, limiting the diversity of the synthesis of their derivatives and their applications in drug design.
To address these challenges, we aimed to establish a convenient and convergent synthetic route. In recent years, transition-metal-catalyzed C-H functionalization has provided general and practical access to various natural and unnatural aromatic amino acids [19][20][21][22][23]. Our retrosynthetic analysis of the isodityrosine moiety of seongsanamides is outlined in Scheme 1. We selected a strategy based on palladium-catalyzed C(sp 3 )-H functionalization and copper-catalyzed Ullmann coupling reactions. The iodityrosine moiety was prepared from key precursor 1 and 4-iodophenylalaninederivatives via a C-O coupling reaction. The DOPA derivative 1 was derived from intermediate 2 by removing hydroxyl protecting groups, Dakin oxidation of aldehyde, or Baeyer-Villiger oxidation of acetyl group. Compound 2 was prepared by removing the directing group of 3, which resulted from the palladium-catalyzed monoarylation of β-methyl C(sp 3 )-H of an alanine derivative with aryl iodides using a directing group. The structural diversity of aryl iodides could facilitate the synthesis of more isodityrosine derivatives.
In addition, diaryl ether (DE) is a functional scaffold that exists widely in both natural products and new drugs approved for the market [6]. Moreover, DE is always considered the fundamental fragment of a wide variety of medicinal and agrochemical agents as well as their bioisosteres ( Figure 2). Over the years, medicinal chemists have exploited the use of privileged structures inspired by natural products in drug discovery. The introduction of functional groups, such as α-keto [7] amide and gem-dimethyl [8] moieties, into biologically active small molecules has emerged as an efficient way to obtain clinically useful drugs. Due to their potential structural diversity, pharmaceutical value, and good druggability, isodityrosines and cyclopeptide derivatives have attracted increasing attention in the synthetic community. Numerous research laboratories have engaged in the total synthesis of these natural products or analogs, including Doẗz benzannulation [9], SNAr reactions [10], and the Ullman [11][12][13][14][15][16] and Evans-Chan-Lam [3,17,18] coupling reactions. However, the synthesis of isodityrosines and their derivatives usually begins with available natural amino acids, limiting the diversity of the synthesis of their derivatives and their applications in drug design.
To address these challenges, we aimed to establish a convenient and convergent synthetic route. In recent years, transition-metal-catalyzed C-H functionalization has provided general and practical access to various natural and unnatural aromatic amino acids [19][20][21][22][23]. Our retrosynthetic analysis of the isodityrosine moiety of seongsanamides is outlined in Scheme 1. We selected a strategy based on palladium-catalyzed C(sp 3 )-H functionalization and copper-catalyzed Ullmann coupling reactions. The iodityrosine moiety was prepared from key precursor 1 and 4-iodophenylalaninederivatives via a C-O coupling reaction. The DOPA derivative 1 was derived from intermediate 2 by removing hydroxyl protecting groups, Dakin oxidation of aldehyde, or Baeyer-Villiger oxidation of acetyl group. Compound 2 was prepared by removing the directing group of 3, which resulted from the palladium-catalyzed monoarylation of β-methyl C(sp 3 )-H of an alanine derivative with aryl iodides using a directing group. The structural diversity of aryl iodides could facilitate the synthesis of more isodityrosine derivatives.
Initially, the selectivity for mono-versus diarylation needed to be controlled. We initiated our investigation with the selective methyl C(sp 3 )-H monoarylation of alanine derivative with aryl iodide (5a) as the model system (Scheme 2). In 2012, Daugulis reported that the selective β-monoarylation of alanine derivatives occurred under solventfree conditions in high yield using a 2-thiomethylaniline auxiliary [27]. In 2014, Bull reported that the 3-monoarylation of proline derivatives under solvent-free conditions with 8-aminoquinoline directing groups was also successful [26]. Previously, Shi reported that the use of coordinating solvents, such as dimethylamine (DMA) and dimethylpropyleneurea(DMPU), improved the selectivity in the arylation of alanine derivatives with aryl iodides [29].
Initially, the selectivity for mono-versus diarylation needed to be controlled. We initiated our investigation with the selective methyl C(sp 3 )-H monoarylation of alanine derivative with aryl iodide (5a) as the model system (Scheme 2). In 2012, Daugulis reported that the selective β-monoarylation of alanine derivatives occurred under solvent-free conditions in high yield using a 2-thiomethylaniline auxiliary [27]. In 2014, Bull reported that the 3-monoarylation of proline derivatives under solvent-free conditions with 8-aminoquinoline directing groups was also successful [26]. Previously, Shi reported that the use of coordinating solvents, such as dimethylamine (DMA) and dimethylpropyleneurea(DMPU), improved the selectivity in the arylation of alanine derivatives with aryl iodides [29].
Due to these studies, we selected solvent-free conditions using palladium (II) acetate (Pd(OAc)2, 20 mol%), silver acetate (AgOAc, 1.5 equiv), and DMPU (5.0 equiv) as additives. In accordance with reported results, the arylation of 4b bearing an 8-aminoquinoline auxiliary mainly produced diarylated product 3ab when the reaction was conducted at 150 °C, and 4a bearing a 2-thiomethylaniline auxiliary mainly produced monoarylated product 3a (Table 1, entries 1 and 2). The optimization of this transformation continued with 4a. We aimed to maximize the yield of 3a and the selectivity of monoarylation. We found that lowering the reaction temperature to 100 °C improved both the yield and selectivity of the reaction (48% yield, mono/di 12:1; Table 1, entry 3). Further screening showed that running the reaction at 80 °C increased both mono-selectivity and yield (62% yield, Table 1, entry 4). However, no satisfactory result was obtained when the reaction was run at 60 °C (42% yield, Table 1, entry 5). Due to these studies, we selected solvent-free conditions using palladium (II) acetate (Pd(OAc) 2 , 20 mol%), silver acetate (AgOAc, 1.5 equiv), and DMPU (5.0 equiv) as additives. In accordance with reported results, the arylation of 4b bearing an 8-aminoquinoline auxiliary mainly produced diarylated product 3ab when the reaction was conducted at 150 • C, and 4a bearing a 2-thiomethylaniline auxiliary mainly produced monoarylated product 3a (Table 1, entries 1 and 2). The optimization of this transformation continued with 4a. We aimed to maximize the yield of 3a and the selectivity of monoarylation. We found that lowering the reaction temperature to 100 • C improved both the yield and selectivity of the reaction (48% yield, mono/di 12:1; Table 1, entry 3). Further screening showed that running the reaction at 80 • C increased both mono-selectivity and yield (62% yield, Table 1, entry 4). However, no satisfactory result was obtained when the reaction was run at 60 • C (42% yield, Table 1, entry 5). Additional optimization experiments showed that the more strongly coordinating 8-aminoquinoline auxiliary still provided a predominantly diarylated product at 80 • C (mono/di 1:1.7; Table 1, entry 6). Interestingly, the palladium-catalyzed C(sp 3 )-H arylation of Weinreb amides 4c bearing an N-methoxyamide auxiliary was also successful, with a 36% yield (Table 1, entry 7). Notably, the reaction was also successful on a larger scale, and the yields were similar to those obtained on a smaller scale ( Table 1, entries 8 and 9).
Based on our access to a wide range of phenylalanine derivatives, we subsequently examined the removal of the directing group. The sequential transformation of formyl, acetyl, and alkoxy groups to hydroxyl groups provided various tyrosine and DOPA derivatives that could be used to produce more isodityrosine derivatives via the Ullmann coupling re- action. The directing group of 3a was easily removed by treatment with H 2 SO 4 in methanol (MeOH) to produce the corresponding methyl ester 2a with a 77% yield (Scheme 4). The treatment of aldehyde 2a with m-CPBA produced the formate ester, which was immediately converted into the corresponding phenol 1 (74% yield for two steps).
Based on our access to a wide range of phenylalanine derivatives, we subsequently examined the removal of the directing group. The sequential transformation of formyl acetyl, and alkoxy groups to hydroxyl groups provided various tyrosine and DOPA derivatives that could be used to produce more isodityrosine derivatives via the Ullmann coupling reaction. The directing group of 3a was easily removed by treatment with H2SO in methanol (MeOH) to produce the corresponding methyl ester 2awith a 77% yield (Scheme 4). The treatment of aldehyde 2a with m-CPBA produced the formate ester which was immediately converted into the corresponding phenol 1 (74% yield for two steps). Subsequently, we focused on the second key step, the copper-catalyzed Ullmann coupling reaction of DOPA derivative 1 (Scheme 5, see Supplementary Materials). The development of useful methods via the Cu/ligand-catalyzed arylation of phenols for assembling diaryl ethers was accomplished [30][31][32][33][34]. Initially, we selected the coupling of phenylalanine-derived aryl iodides (1.5 equiv), with 1 as a model reaction to optimize the reaction conditions. The coupling reaction was tested in the presence of CuI (1.0 equiv) and Cs2CO3 (4.5 equiv) when the reaction was carried out in 1,4-dioxane (0. 1 M) under N at 90 °C for 20 h. We tested several ligands that are known to promote copper-catalyzed coupling reactions. Interestingly, we found that using commercially available N,N-dimethylglycine as a ligand produced the corresponding products. Moreover, several phenylalanine-derived aryl iodides with different protecting groups at the C-terminus were compatible with the reaction conditions (6a, 49%; 6b, 50%; 6c, 44%) Notably, in isodityrosine derivatives 6a-d, all amino and carboxylate groups with different protections enabled the selective manipulation of the individual functionalities.
With the optimized reaction conditions, we next examined an extensive range of ary iodides to produce a 3-aryloxyphenylalanine derivative bearing a diaryl ether moiety. A variety of functional groups of aryl iodides are known to tolerate this reaction condition including alkoxyl, formyl, acetyl, nitro, carbonyl, iodo, and cyano groups. Both electron-rich and electron-deficient aryl iodides underwent this reaction to produce the corresponding diaryl ethers in good yields. A high yield was provided by 4-iodobenzaldehyde (6e, 88%), whereas 3-iodobenzaldehyde provided a 52% yield under standard conditions. Furthermore, aromatics with aldehyde and methoxyl groups were also compatible, with reduced yields (6g, 30%). Notably, the reaction of 1,4-diiodobenzene with phenol 1produced the coupling product 6j in 64% yield, which was a 3-(4-iodoaryloxy)-phenylalanine derivative and provided a handle for potentia further functionalization. Subsequently, we focused on the second key step, the copper-catalyzed Ullmann coupling reaction of DOPA derivative 1 (Scheme 5, see Supplementary Materials). The development of useful methods via the Cu/ligand-catalyzed arylation of phenols for assembling diaryl ethers was accomplished [30][31][32][33][34]. Initially, we selected the coupling of phenylalanine-derived aryl iodides (1.5 equiv), with 1 as a model reaction to optimize the reaction conditions. The coupling reaction was tested in the presence of CuI (1.0 equiv) and Cs 2 CO 3 (4.5 equiv) when the reaction was carried out in 1,4-dioxane (0. 1 M) under N 2 at 90 • C for 20 h. We tested several ligands that are known to promote coppercatalyzed coupling reactions. Interestingly, we found that using commercially available N,N-dimethylglycine as a ligand produced the corresponding products. Moreover, several phenylalanine-derived aryl iodides with different protecting groups at the C-terminus were compatible with the reaction conditions (6a, 49%; 6b, 50%; 6c, 44%). Notably, in isodityrosine derivatives 6a-d, all amino and carboxylate groups with different protections enabled the selective manipulation of the individual functionalities.

Scheme 5.
Coupling of the aryl iodides with DOPA derivative 1.
In conclusion, we have developed a convenient and highly convergent synthesis of the isodityrosine unit of seongsanamide A-D and its derivatives bearing a diaryl ether moiety. The synthetic sequence is based on a palladium-catalyzed C(sp 3 )-H functionalization and a Cu/ligand-catalyzed coupling reaction. Initially, we developed a Pd-catalyzed mono-arylation of β-methyl C(sp 3 )-H of an alanine derivative using the 2-thiomethylaniline directing group. A wide range of aryl iodides could be applied in this protocol to provide various aromatic α-amino acid compounds; these compounds could be used for the synthesis of various tyrosine and DOPA derivatives. Subsequently, the CuI/N,N-dimethylglycine-catalyzed coupling of DOPA derivatives with aryl iodides was accomplished to prepare diaryl ethers and the synthesis of isodityrosine derivatives. Our convergent synthetic method facilitates a concise way for the efficient preparation of various DE-based analogs for drug development efforts. The specific introduction of DE-based units into biologically active small molecules and a study of the impact on the With the optimized reaction conditions, we next examined an extensive range of aryl iodides to produce a 3-aryloxyphenylalanine derivative bearing a diaryl ether moiety. A variety of functional groups of aryl iodides are known to tolerate this reaction condition, including alkoxyl, formyl, acetyl, nitro, carbonyl, iodo, and cyano groups. Both electron-rich and electron-deficient aryl iodides underwent this reaction to produce the corresponding diaryl ethers in good yields. A high yield was provided by 4-iodobenzaldehyde (6e, 88%), whereas 3-iodobenzaldehyde provided a 52% yield under standard conditions. Furthermore, aromatics with aldehyde and methoxyl groups were also compatible, with reduced yields (6g, 30%). Notably, the reaction of 1,4-diiodobenzene with phenol 1 produced the coupling product 6j in 64% yield, which was a 3-(4-iodoaryloxy)-phenylalanine derivative and provided a handle for potential further functionalization.
In conclusion, we have developed a convenient and highly convergent synthesis of the isodityrosine unit of seongsanamide A-D and its derivatives bearing a diaryl ether moiety. The synthetic sequence is based on a palladium-catalyzed C(sp 3 )-H functionalization and a Cu/ligand-catalyzed coupling reaction. Initially, we developed a Pd-catalyzed monoarylation of β-methyl C(sp 3 )-H of an alanine derivative using the 2-thiomethylaniline directing group. A wide range of aryl iodides could be applied in this protocol to provide various aromatic α-amino acid compounds; these compounds could be used for the synthesis of various tyrosine and DOPA derivatives. Subsequently, the CuI/N,N-dimethylglycinecatalyzed coupling of DOPA derivatives with aryl iodides was accomplished to prepare diaryl ethers and the synthesis of isodityrosine derivatives. Our convergent synthetic method facilitates a concise way for the efficient preparation of various DE-based analogs for drug development efforts. The specific introduction of DE-based units into biologically active small molecules and a study of the impact on the physicochemical properties and potential biological activities are currently being performed.

General Experimental Methods
Chemicals were acquired from commercial sources and used as received. Nuclear magnetic resonance (NMR) spectra were recorded on Bruker Ascend-400 and Bruker Ascend-500 spectrometers at the following spectrometer frequencies. Multiplicities are assigned as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and app (apparent).The exact mass was obtained using a time-of-flight (TOF) detector on Agilent 6530-Q-TOF. Thin layer chromatography (TLC) was carried out with 0.2 mm thick silica gel plates. Visualization was accomplished by using UV light. Column chromatography was performed on silica gel (200-300 mesh).

Synthesis of the Compounds 3a and 1
Methyl (S)-3-(4-methoxy-3-formylphenyl)-2-phthalimidopropionate (2a). 3a (3.0 g, 6.3 mmol), dry methanol (600 mL), conc. H 2 SO 4 (5.0 mL) were added to a 500-mL round bottom flask. The mixture was stirred at 100 • C for 24 h under N 2 . After cooling to room temperature, the resulting solution was evaporated, and the residue was diluted with ethyl acetate (200 mL) and brine (100 mL).The organic layer was separated and washed successively with water, saturated aqueous NaHCO 3 and brine, dried over anhydrous Na 2 SO 4 , and concentrated in vacuo. The organic layer was purified via column chromatography in a petroleum ether/ethyl acetate mixture of2:1 to produce corresponding product2a (1.8 g, 77%) as a solid. 1  Methyl (S)-3-(4-methoxy-3-hydroxy-phenyl)-2-phthalimidopropionate (1). 2a (750 mg, 2.0 mmol), dichloromethane (50 mL), and meta-chloroperoxybenzoic acid(m-CPBA,85% purity, 770 mg, 3.8 mmol) were added to a 250-mL round bottom flask. The mixture was stirred at room temperature overnight. The resulting solution was diluted with dichloromethane(DCM, 300 mL), washed successively with saturated aqueous Na 2 S 2 O 3 and brine, dried over anhydrous Na 2 SO 4 , and concentrated in vacuo. The crude intermediate was used for the next step without further purification. A mixture of the concentrated residueandNaHCO 3 (3.4 g, 40 mmol) in tetrahydrofuran (THF,40 mL) and water (20 mL) was stirred at room temperature overnight. Then, the resulting solution was evaporated, and the residue was adjusted to pH<1 with 2 M HCl. The mixture was extracted with ethyl acetate (200 mL). The organic phase was washed with brine, dried over anhydrous Na 2 SO 4 , and concentrated in vacuo. This mixture was purified via column chromatography in a petroleum ether/ethyl acetate mixture of3:1 to produce corresponding product1 (540mg, 74% yield for two steps) as a solid. 1