Synthesis of Biphenyl Tyrosine Via Cross-Coupling Suzuki-Miyaura Reaction Using Aryltrifluoroborate Salts

We reported a fast and easy method for obtaining biarylic units from tyrosine derivatives via Suzuki-Miyaura cross-coupling using a variety of substituted and unsubstituted potassium aryland heteroaryltrifluoroborate salts. The scope of the methodology was also extended to the formation of bis-tyrosine linked dipeptide bonds, leading to biologically interesting compounds. Some biarylic units were obtained as free amino acids through the one step cleavage reaction in good yields.


Introduction
The biaryl subunit is an important structural motif that can be found in many compounds including natural products and polymers, 1 especially modified aromatic amino acids that are present in various pharmaceuticals, which are currently under development or have already been introduced to the market. 2aturally found in fungal cell wall proteins, 3 dityrosine is found as a dimer formed by 3,3'-biaryl bond formation, as well as a biaryl ether moiety in which the tyrosine units are linked through an oxygen atom.This latter is present in isotrityrosine and pulcherosine, which have been isolated from bacteria 4 and plants. 5Biaryl subunits are routinely obtained by employing palladium as a catalyst via Suzuki-Miyaura cross-coupling, as reported recently by Monteiro and co-workers 6 in the synthesis of pharmaceutical compounds, although Bedford et al. 1 reported the rhodium catalyzed direct ortho-arylation of 2-tert-butyl protected tyrosine using aryl bromide substrates.One known biaryl compound is the 3-arylated tyrosine moiety present in the antibacterial arylomycin A 2 (Figure 1), isolated from Streptomyces strain TU6075. 7,8Roberts et al. 9 exploited the latter compound by Suzuki-Miyaura reaction for the formation of the biaryl unit.Barluenga and co-workers 10 performed in one-pot mode the iodination and arylation of 3,5-diiodotyrosine side chains of biologically active peptides by a Suzuki-Miyaura cross-coupling reaction.
There are some biologically important tyrosine fragments, including monocyclic peptides such as K-13, noncompetitive inhibitor of angiotensin I-converting enzyme and a weak inhibitor of aminopeptidase B, 11 isolated from Micromonospora halophytica ssp.exilisia and synthetically exploited by Ma and co-workers, 12 bicyclic bouvardins, an antitumor agent, complex polycyclic antibiotics such as chloropeptins, an anti-human immunodeficiency virus (HIV) drug 13 and teicoplanin, used in the prophylaxis and treatment of serious infections caused by Gram-positive bacteria. 14The molecular structure and interesting biological activity make these compounds attractive synthetic targets.

Results and Discussion
For our preliminary studies, we used commercially available 3-iodotyrosine without the prior protection 1 or 2 directly for Suzuki coupling, but the product was not observed.Therefore, tyrosine was esterified with methanol in the presence of thionyl chloride and the amino group protected with tert-butyloxycarbonyl group (Boc) 3 (Scheme 1). 15,16he study of the optimal reaction conditions was carried out using the coupling between phenyltrifluoroborate (4o) and N-tert-butyloxycarbonyl-3-iodotyrosine methyl ester (3) in MeOH with K 2 CO 3 ; a yield of 73% of 5o was observed as a model study (Table 1, entry 1).
Using K 2 CO 3 as the base, other palladium catalysts were tested, resulting in the desired products; however, these were obtained in low to moderate yields (Table 1, entries 2-5 and 9).Only trietylamine was used as a different base, but this led to the cross-coupled product in 20% yield (Table 1, entry 8) and no improvement was achieved by replacing conventional heating with microwave (Table 1, entry 6).When MeOH was replaced with toluene, the yield of 5o was obtained in trace amounts only (Table 1, entry 7).The catalytic amount of Pd(OAc) 2 was 0.05 mmol.Under basic conditions and an alcoholic environment, the saponification of the methyl ester was not observed, as described by Stefani et al. 17 With the optimized reaction conditions in hand, the substrate scope of this Pd-catalyzed cross-coupling reaction was investigated.Different substituents on the aromatic ring of aryltrifluoroborate salts were tested, and the products were generally afforded in low to good yields (Table 2).
Aryltrifluoroborates bearing electron-withdrawing groups such as CN, CHO, and Cl substituents, all in the paraposition, were suitable substrates, giving the corresponding cross-coupled products in low to moderate yields ranging from 17% to 50% (Table 2, entries 1, 5 and 10).Aryltrifluoroborate bearing the CO 2 H group did not yield the desired product under the reaction conditions (Table 2, entry 12), 43% of start material was intact recovered, we believed that the remaining material decomposed.Other aspects observed as the presence of homocoupling product from organotrifluoroborate salt and a possible lability of tyrosine under the cross coupling conditions prevented the formation of desired products in better yields.
On the other hand, aryltrifluoroborate bearing electrondonor groups as CH 3 O, CH 3 , C 2 H 5 S, tert-butyl, OH and NH 2 undergo the reaction leading to the cross-coupled products with yields ranging from 19% to 79% (Table 2, entries 2-4, 6-9, 14 and 16).The reaction seems to be sensitive to steric hindrance, as noted in the case of the methyl group when present in the ortho, meta and para position on the aromatic ring, leading to diminishing yields such as 33, 47 and 62%, respectively (Table 2, entries 6-8).Neutral aromatic rings, such as benzene and naphthalene, gave   Table 2. Substrate scope of cross-coupling reaction of aryltrifluoroborates to iodo-tyrosine (cont.)moderate to good yields (Table 2, entries 11 and 15, 50% and 73%, respectively) and heterocyclic thiophene gave the highest yield in the series, 89% (Table 2, entry 13).The next step was the direct conversion of the N-Bocamino ester products into the correspondent free amino acid as hydrochloride salts 6a-e (Table 3) through the onestep cleavage reaction with 4.5 mol L −1 HCl at 70 °C for 3 h 18 (Scheme 2, route A).Alternatively, the N-Boc-amino ester 5o was first converted into free amino ester with trifluoroacetic acid 19 in 82% yield, which could be used after a simple filtration through a short pad of silica gel.The crude product was reduced into the respective amino alcohol 7 in 55% yield using sodium borohydride 20 (Scheme 2, route B).
Next, we did the synthesis of dipeptides 21 that could be subjected to cross-coupling reaction.For this purpose, the compounds 3-phenyl-tyrosine methyl ester 8, obtained from selective amino deprotection of 5o in the presence of trifluoroacetic acid (TFA) in dichloromethane (DCM) and 3-iodo-N-tert-butyloxycarbonyltyrosine 9, were coupled using N-N'-diisopropylcarbodiimide (DIC) and 1-hydroxybenzotriazole (HOBt) in DCM for 15 h.After purification, the resulting dipeptide 10 was isolated in 52% yield.With the iodine present in the peptide fragment, this Similarly, the dipeptide 12 was achieved via peptide bond between compounds 2 as hydrochloride salt and 9 in 48% yield.Enticed by the possibility of bis cross-coupling at the same time, due to the presence of iodine atoms on the aromatic rings, we performed the cross-coupling reaction using two equivalents of potassium 4-methoxyphenyltrifluoroborate salt 4b, leading exclusively to the compound 13, from which, after flash chromatography, the product could be obtained with high purity in 61% yield (Scheme 4).Surprisingly, the formation of the single coupling product was not observed during the reaction.

Conclusions
In conclusion, we have demonstrated that the crosscoupling of 3-iodotyrosine with potassium heteroaryl-and aryltrifluoroborate salts can be carried out using Pd(OAc) 2 as the catalyst.Both activated and deactivated species of trifluoroborate salts can be coupled, proven to be suitable nucleophiles.Subsequently, the obtained biphenyl tyrosines were transformed into the corresponding free amino acid as hydrochloride salts.The coupled tyrosine was converted in dipeptides and the submitted to the cross-coupling reaction with heteroaryl-and aryltrifluoroboate salts leading to products in moderate to good yields.The structures of all products were characterized by 1 H nuclear magnetic resonance (NMR), 13 C NMR, and high resolution mass spectrometry (HRMS) analysis.

Experimental
All reactions were carried out under a nitrogen atmosphere; all compounds were characterized by 1 H NMR, 13 C NMR and electrospray ionization-mass spectrometry (ESI-MS).Copies of the 1 H, and 13 C spectra can be found at the end of the Supplementary Information (SI) section.NMR spectra were recorded on a 300 MHz instrument.All 1 H NMR experiments are reported in δ units, parts per million (ppm), and were measured relative to the signals for tetramethylsilane (TMS) (0.00 ppm).All 13 C NMR spectra are reported in ppm relative to deuteronchloroform (77.23 ppm), unless otherwise stated, and all were obtained with 1 H decoupling.For HRMS, previously lyophilized samples were dissolved in methanol and deposited into the 96 well plate of the SIL-20A autosampler for ESI-MS analysis in an IT-TOF mass spectrometer system (Shimadzu).Typically, 0.1 µL sample aliquots were injected and infused into the instrument in 50% acetonitrile, containing 0.5% formic acid under a constant flow rate of 0.2 mL min −1 .Instrument control, data acquisition and processing were performed by the LCMS Solution suite (Shimadzu).ESI conditions: source temperature 200 °C, cone voltage 4.5 kV, detector voltage 1.57 kV, nebulizing gas flow 1.5 L min −1 .Solvents and reagents were of analytical grade or the highest grade commercially available and were used without further purification.Palladium catalysts, 3-iodotyrosine and potassium aryltrifluroborate salts (4a-p) were purchased from Sigma-Aldrich and used as received.

General amino alcohol procedure (7)
A solution of 5 (128 mg, 0.5 mmol) and TFA (1.5 mL) in DCM (3 mL) was stirred until complete consume of start material, monitored by TLC.After, NaHCO 3 saturated solution was added slowly, the mixture was extracted with DCM, the organic layer was collected, dried with MgSO 4 , filtered and the solvent was removed under vacuum.A MeOH (4 mL) solution of 8 was cooled to 0 °C, and NaBH 4 (8 equivalent) was added in one portion.The resulting solution was stirred under ambient temperature for 24 h.After the reaction was complete (monitored by TLC), the organic solvents were evaporated under vacuum.Water (5 mL) was added, and after phase separation, the corresponding aqueous phase was extracted with EtOAc (3 × 3 mL).The combined organic extracts were washed with brine, dried (MgSO 4 ), and then concentrated under high vacuum.The crude product was purified directly by silica gel column chromatography 17

Synthesis of dipeptides 10
A solution of 9 (203 mg, 0.5 mmol) in dry DCM was cooled to 0 °C and 1-hydroxybenzotriazole (1.1 equivalent) was added, followed by N-N'-diisopropylcarbodiimide (1.2 equivalent).After 1 h, the mixture was allowed to warm to room temperature then compound 5o dissolved in DCM was added.The reaction mixture was stirred for 15 h.The urea was filtered out, the supernatant was dried to get the crude compound. 18Purification by silica flash chromatography (eluting with ethyl acetate/hexane 5:5).
N-tert-Butyloxycarbonyl-di-tyrosine methyl ester (10) A solution of 9 (203 mg, 0.5 mmol) in dry DCM was cooled to 0 °C and 1-hydroxybenzotriazole (1.1 equivalent) was added, followed by N-N'-diisopropylcarbodiimide (1.2 equivalent).After 1 h, the mixture was allowed to warm to room temperature, then compound 2 dissolved in DMF was added, followed by triethylamine (TEA) (1.2 equivalent).The reaction mixture was stirred for 15 h.The precipitated dicyclohexylurea was filtered off and washed with DCM.The filtrated was evaporated under high vacuum, the crude compound was acidified under ice cold condition with 1 mol L −1 HCl to pH 2-3 and extracted with DCM, the organic layer was collected, dried with MgSO 4 , filtered and the solvent was removed under vacuum. 5urification by silica flash chromatography (eluting with ethyl acetate/hexane 5:5).

12 Table 2 . 12 a
Substrate scope of cross-coupling reaction of aryltrifluoroborates to iodo-tyrosineYields refer to isolated pure products.

12 a
Yields refer to isolated pure products.Vol.26, No. 4, 2015   offered the opportunity of a new cross-coupling reaction showing the versatility to form different biarylic subunits in peptides fragments.To evidence this fact, dipeptide 10 was reacted with organotrifluoroborate salt 4m under Pd catalysis to give the compound 11 in 42% yield (Scheme 3).
to isolated pure products.Compounds 6a to 6e were obtained as hydrochloride salts.
to isolated pure products.Compounds 6a to 6e were obtained as hydrochloride salts.

Table 1 .
Survey for palladium-catalyzed coupling of aryltrifluoroborates with 3-iodotyrosine a

Table 2 .
Substrate scope of cross-coupling reaction of aryltrifluoroborates to iodo-tyrosine

Table 3 .
Free amino acids a