Synthesis of Boronocysteine

Abstract

Herein we report the first synthesis of protected boronocysteine. The target compound was prepared via copper-catalysed diastereoselective nucleophilic borylation of a sulfinimine. After deprotection to give the amine as the hydrochloride salt, four boronocysteine amide derivatives were prepared through reaction with a variety of different active acylating agents.

Key words

amino acids - boron - copper - imines - peptides

α-Aminoboronic acids have attracted considerable attention as analogues of amino acids with potential applications in medicinal chemistry.[1] Bortezomib (Velcade®), a peptide analogue incorporating an α-aminoboronic acid used in cancer treatment, became the first drug on the market containing a boron atom (Figure [1]). Subsequently, other α-aminoboronic acid derivatives have reached the market including ixazomib and delanzomib. The synthesis of α-aminoboronic acids has proved to be especially challenging, however, as free α-aminoboronate compounds readily undergo rearrangement to N-boryl compounds leading to protodeboronation.[2]

Figure 1 Medicinally useful α-aminoboronic acid derivatives

Scheme 1 Synthetic approaches to α-aminoboronic acid derivatives

Nevertheless, Matteson was able to carry out the first synthesis of an α-aminoboronic acid derivative in 1966 starting from iodomethylmercuric iodide (Scheme [1]).[3] He was subsequently able to access a wider range of α-aminoboronic acids via rearrangement reactions of boronate esters using dichloromethyllithium.[4] Alternative approaches to α-aminoboronic acid derivatives have included copper-catalysed nucleophilic borylation of imines,[5] alkylation of α-sulfenyl boronates followed by nucleophilic displacement of the sulfur,[6] Curtius rearrangement of α-borylcarboxylic acids,[7] and lithiation/borylation of protected amine derivatives.[8] Notably, whilst a wide range of α-aminoboronic acid derivatives have been reported,[4] [5] [6] [7] [8] [9] there are relatively few examples containing heteroatomic functional groups on the side chain.[10] For an ongoing project involving the study of peptides containing C-terminal cysteine derivatives,[11] we required access to the boron analogue of cysteine. Interestingly, no previous synthesis of this compound (or a protected derivative) has been reported in the literature. Herein, we report a concise synthesis of protected boronocysteine, and its use in the synthesis of N-acyl derivatives including dipeptide analogues.

Matteson has previously noted that rearrangement of a thioether-functionalised alkylboronate using dichloromethyllithium was unsuccessful, probably due to loss of a sulfur-stabilised carbanion from the ‘ate’ complex.[12] We therefore envisaged that boronocysteine 1 could readily be constructed by Cu-catalysed borylation of a suitably protected sulfinimine 2 (Scheme [2]).[5] The required imine 2 should be readily available from commercially available bromoacetaldehyde diethyl acetal.

Scheme 2 Proposed synthetic strategy for preparing boronocysteine

Bromoacetaldehyde dimethyl acetal 3a was converted into the corresponding sulfide 3b by reaction with p-methoxybenzyl thiol (PMBSH) and sodium hydroxide (Scheme [3]).[13] After deprotection of the acetal, the aldehyde 4 [14] was then converted into sulfinimine 5 through condensation with the sulfinimide.

Scheme 3 Synthesis of imine 5. Reagents and conditions: a. NaOH, PMBSH, EtOH, 52%; b. HCl, acetone, 99%. c. ^t BuSONH₂, CuSO₄, CH₂Cl₂, 80%.[16]

With the required sulfinimide in hand, the Cu-catalysed borylation reaction was investigated. Pleasingly, by using CuCl in the presence of KO ^t Bu and rac-BINAP as a ligand,[5a] the desired boronate 6 was obtained in 60% isolated yield as a single diastereoisomer (Scheme [4]). As reported previously, the sulfinimide could be removed using HCl to give the corresponding α-aminoboronate as the hydrochloride salt 7.[5a] This compound required careful handling as it readily underwent protodeboronation to give the corresponding 2-aminoethylthio ether 8.[15] For example, deboronated compound 8 was obtained when 6 was exposed to HCl for 24 h instead of the 3 h reaction time required to produce 7.

Scheme 4 Copper-catalysed borylation of imine 5 and sulfinamide deprotection. Reagents and conditions: a. CuCl, (±)-BINAP, KO ^t Bu, B₂pin₂, THF, 60%;[17] b. HCl, dioxane, MeOH, 82%.[18]

We were, however, able to prepare boronocysteine amides derived from 7 through reaction with appropriate acylating reagents (Scheme [5]). Thus, reaction of 7 with acid chlorides provided the acetamide 9a and chloroacetamide 9b. Dipeptide analogues 10 were obtained through reaction with either an in situ generated mixed anhydride 10a, or a pre-formed acyl fluoride 10b. These reactions demonstrate that boronocysteine can readily be converted into amide derivatives through reaction with a range of different active acylating agents.

Scheme 5 Synthesis of boronocysteine amides. Reagents and conditions: a. MeCOCl, pyridine, MeCN, 22% (9a); b. ClCH₂COCl, N-methylmorpholine, CH₂Cl₂, 85% (9b); c. Boc-Gly-OH, N-methylmorpholine, ⁱ BuOCOCl, CH₂Cl₂, then 7, 89% (10a);[19] d. FMoc-Gly-F, ⁱ Pr₂NEt, CH₂Cl₂, 60% (10b).

In summary, we have described the first synthesis of protected boronocysteine, and demonstrated its application in the formation of amide derivatives.

Acknowledgment

We would like to thank the Department of Chemistry, University College London for providing a studentship to support this work.

• References and Notes

• 1a Yang W. Gao X. Wang B. Med. Res. Rev. 2003; 23: 346
• 1b Dembitsky VM. Al Aziz Quntar A. Srebnik M. Mini-Rev. Med. Chem. 2004; 4: 1001
• 1c Dembitsky VM. Srebnik M. Tetrahedron 2003; 59: 579
• 1d Matteson DS. Med. Res. Rev. 2008; 28: 233
• 1e Andrés P. Ballano G. Calaza MI. Cativela C. Chem. Soc. Rev. 2016; 45: 2291
• 2 Matteson DS. Sadhu KM. Organometallics 1984; 3: 614
• 3 Matteson DS. Cheng T.-C. J. Organomet. Chem. 1966; 6: 100
• 4a Tsai DJ. S. Jesthi PK. Matteson DS. Organometallics 1983; 2: 1543
• 4b Matteson DS. J. Organomet. Chem. 1999; 581: 51
• 5a Beenen MA. An C. Ellman JA. J. Am. Chem. Soc. 2008; 130: 6910
• 5b Buesking W. Bacauanu V. Cai I. Ellman JA. J. Org. Chem. 2014; 79: 3671
• 5c Solé C. Gulyás H. Fernández E. Chem. Commun. 2012; 48: 3769
• 6 Jagannathan S. Forsyth TP. Kettner CA. J. Org. Chem. 2001; 66: 6375
• 7 He Z. Zajdlik A. St Denis JD. Assem N. Yudin AK. J. Am. Chem. Soc. 2012; 134: 9926
• 8 Batsanov AS. Grosjean C. Schütz T. Whiting A. J. Org. Chem. 2007; 72: 6276

For other approaches, see:
• 9a Zheng B. Deloux L. Pereira S. Skrzypczak-Jankun E. Cheesman BV. Sabat M. Morris S. Appl. Organomet. Chem. 1996; 10: 267
• 9b Touchet S. Carreaux F. Molander GA. Carboni B. Bouillon A. Adv. Synth. Catal. 2011; 353: 3391
• 9c Kawamorita S. Miyazaki T. Iwai T. Ohimiya H. Sawamura M. J. Am. Chem. Soc. 2012; 134: 12924
• 9d Hu N. Zhao G. Zhang Y. Liu X. Li G. Tang W. J. Am. Chem. Soc. 2015; 137: 6746
• 9e Li C. Wang J. Barton LM. Yu S. Tian M. Peters DS. Kumar M. Yu AW. Johnson KA. Chatterjee AK. Yan M. Baran PS. Science 2017; 357: in press; DOI: 10.1126/science.aam7355
• 10a Kettner C. Mersinger L. Knabb R. J. Biol. Chem. 1990; 265: 18289
• 10b Lebarbier C. Carreaux F. Carboni B. Boucher JL. Bioorg. Med. Chem. Lett. 1998; 8: 2573
• 10c Watanabe T. Momose I. Abe M. Abe H. Sawa R. Umezawa Y. Ikeda D. Takahashi Y. Akamatsu Y. Bioorg. Med. Chem. Lett. 2009; 19: 2343
• 10d Lebarbier C. Carreaux F. Carboni B. Synthesis 1996; 1371
• 10e Matteson DS. Michnick TJ. Willett RD. Patterson CD. Organometallics 1989; 8: 726
• 11a Cowper B. Shariff L. Chen W. Gibson SM. Di W.-L. Macmillan D. Org. Biomol. Chem. 2015; 13: 7469
• 11b Macmillan D. Synlett 2017; 28: 1517
• 12 Matteson DS. J. Organomet. Chem. 1999; 581: 51
• 13 Lantos I. Razgaitis C. Sutton BM. J. Heterocycl. Chem. 1982; 19: 1375
• 14 Yoneda K. Ota A. Kawashima Y. Chem. Pharm. Bull. 1993; 41: 876
• 15 Ghosh S. Tochtrop GP. Tetrahedron Lett. 2009; 50: 1723
• 16 (E)-N-{2-[(4-Methoxybenzyl)thio]ethylidene}-2-methylpropane-2-sulfinamide (5) Copper(II) sulfate (1.27 g, 7.94 mmol) and aldehyde 4 (779 mg, 3.97 mmol, 1.1 equiv) were added to a solution of (±)-tert-butyl sulfinamide (438 mg, 3.61 mmol) in anhydrous CH₂Cl₂ (7.2 mL). The reaction was stirred at r.t. for 18 h, before filtering through Celite. The solvents were removed in vacuo and the residue obtained was purified by column chromatography to give an orange oil (865 mg, 2.89 mmol, 80%). ¹H NMR (600 MHz, CDCl₃): δ = 7.98 (1 H, t, J = 5.6 Hz, NCH), 7.23 (2 H, d, J = 6.5 Hz, ArH), 6.85 (2 H, d, J = 6.5 Hz, ArH), 3.79 (3 H, s, OCH₃), 3.66 (2 H, s, ArCH₂), 3.35 (1 H, dd, J = 14.3, 6.0 Hz, 1 × SCH ₂CH), 3.31 (1 H, dd, J = 14.3, 5.3, 1 × SCH ₂CH), 1.22 (9 H, s, ^t Bu). ¹³C NMR (150 MHz, CDCl₃): δ = 164.2, 158.9, 130.3, 129.2, 114.1, 57.0, 55.4, 35.02, 34.3, 22.5. LRMS (CI): m/z (%) = 420 (100), 300 (37) [M + H⁺], 240 (30), 195 (32) [M – SO ^t Bu]⁺), 121 (88) [PMB⁺]. HRMS: m/z calcd for C₁₄H₂₂NO₂S₂: 300.10865; found: 300.10877. IR (film): ν_max = 2958 (C–H), 1609 (C=C), 1510 (C=N), 1458 (C=C), 1083 (S=O) cm^-1.
• 17 N-{2-[(4-Methoxybenzyl)thio]-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethyl}-2-methylpropane-2-sulfinamide (6) Using flame-dried glassware under an argon atmosphere, CuCl (38.4 mg, 0.388 mmol), (±)-BINAP (111.1 mg, 0.1784 mmol), and B₂pin₂(1.331 g, 5.242 mmol) were dissolved in anhydrous THF (4 mL). KO ^t Bu (1 M in THF, 1.4 mL, 1.4 mmol) was added whilst stirring at r.t. After 10 min, the reaction was cooled to –20 °C, and aldehyde 5 (1.0338 g, 3.4522 mmol) was added followed by MeOH (300 μL, 7.41 mmol) and the reaction stirred overnight. The solvent was removed in vacuo and the resultant oil purified by flash column chromatography using EtOAc in CH₂Cl₂ (20 → 35%) to give 6 as an orange oil (878 mg, 2.056 mmol, 60%); R_f = 0.08 (EtOAc/CH₂Cl₂ = 1:4). ¹H NMR (600 MHz, CDCl₃): δ = 7.24 (2 H, d, J = 8.6 Hz, ArH), 6.82 (2 H, d, J = 8.6 Hz, ArH), 3.78 (3 H, s, OCH₃), 3.71 (1 H, d, J = 5.6 Hz, NH), 3.69 (s, 2 H, ArCH₂S), 3.22 (1 H, m, CHB), 2.77 (1 H, dd, J = 13.4, 6.3 Hz, 1 × SCH ₂CH), 2.72 (1 H, dd, J = 13.4, 7.9 Hz, 1 × SCH ₂CH), 1.25 (s, 6 H, 2 × pinacol-CH₃), 1.23 (s, 9 H, ^t Bu), 1.20 (s, 6 H, 2 × pinacol-CH₃). ¹³C NMR (150 MHz, CDCl₃): δ = 158.7, 130.1, 130.0, 114.0, 84.3, 56.2, 55.3, 41.3 (br), 35.2, 34.6, 25.0 (2 C), 22.6. LRMS (CI): m/z (%) = 428 (41), [M + H]⁺, 371 (18) [M⁺ – ^t Bu)], 322 (38) [M⁺ – SO ^t Bu), 121 (100) [PMB⁺]. IR: ν_max = 2977 (C–H), 2930 (C–H), 1609 (Ar), 1511 (Ar), 1544 (Ar), 1369 (B–O), 1246, 1140 (B–C), 1033 (S=O). HRMS: m/z calcd for C₂₀H₃₄BNO₄S₂: 428.2095; found: 428.2095.
• 18 2-[(4-Methoxybenzyl)thio]-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethanamine hydrochloride (7) A solution of HCl in dioxane (4 M, 585 μL, 2.34 mmol) was added to 6 (99.7 mg, 0.233 mmol) dissolved in anhydrous MeOH (3 mL) to give a pale yellow solution. The reaction was stirred for 3 h. The solvent was removed in vacuo to give an orange residue (75.3 mg). The residue was washed with Et₂O, sonicated, and centrifuged to give 7 as a light brown solid (68 mg, 0.190 mmol, 82%). ¹H NMR (400 MHz, MeOD-d ₄): δ = 7.29 (2 H, d, J = 8.7 Hz, ArH), 6.89 (2 H, d, J = 8.7 Hz, ArH) 3.80 (3 H, s, OCH₃), 3.79 (2 H, s, ArCH₂), 3.01 (1 H, dd, J = 8.7, 4.7 Hz, CHB), 2.85 (1 H, dd, J = 14.3,4.8 Hz, 1 × CΗ ₂CH), 2.73 (1 H, dd, J = 14.3, 8.8 Hz, 1 × CH ₂CH), 1.33 (12 H, s, 4 × CH₃). ¹³C NMR (100 MHz, MeOD-d ₄): δ = 159.1, 129.9, 129.5, 113.7, 85.5, 74.4, 54.5, 35.1, 30.4, 23.8, 23.7. LRMS (CI): m/z (%) = 323 (100) [M + H]⁺, 198 (18) [M – Bpin]⁺. HRMS: m/z calcd for C₁₆H₂₇BNO₃S: 323.1836; found: 323.18359. IR (solid): ν_max = 2975 (C–H), 2958 (C–H), 2831 (C–H), 1607, 1583, 1411 cm^–1.
• 19 tert-Butyl [2-({2-[(4-Methoxybenzyl)thio]-1-(4,4,5,5-tetra-methyl-1,3,2-dioxaborolan-2-yl)ethyl}amino)-2-oxoethyl]carbamate (10a) Using flame-dried glassware under an argon atmosphere, Boc-Gly-OH (87.5 mg, 0.50 mmol) was dissolved in anhydrous CH₂Cl₂ (1.5 mL) and cooled to –20 °C. To this was added NMM (66 μL, 0.60 mmol) followed by IBCF (58 μL, 0.45 mmol), and the mixture stirred for 5 h at –20 °C. HCl salt 7 (23.4 mg, 65.1 μmol) was added, followed by NMM (7 μL, 65 μmol), and the reaction stirred overnight. The reaction mixture was concentrated in vacuo and the resultant oil purified by flash column chromatography using deactivated silica (35% water w/w) eluting with MeOH in EtOAc (0 → 10%) to give 10a as a pale yellow oil (28 mg, 58.0 μmol, 89%). ¹H NMR (600 MHz, CDCl₃): δ = 7.51 (1 H, br s, CHNH), 7.21 (2 H, d, J = 8.7 Hz, ArH), 6.82 (2 H, d, J = 8.7 Hz, ArH), 5.29 (1 H, br s, CH₂NH), 3.93 (2 H, d, J = 5.7, NHCH ₂), 3.78 (3 H, s, OCH₃), 3.65 (2 H, s, ArCH₂), 2.81 (1 H, br d, J = 11.5 Hz, CHB), 2.75 (1 H, dd, J = 14.1, 3.2 Hz, 1 × SCH ₂CH), 2.46 (1 H, dd, J = 14.1, 11.5 Hz, 1 × SCH ₂CH), 1.44 (9 H, s, Bu), 1.18 (6 H, s, 2 × pinacol-CH₃), 1.16 (6 H, s, 2 × pinacol-CH₃). ¹³C NMR (150 MHz, CDCl₃): δ = 174.9, 158.7, 130.3, 130.1, 114.1, 81.6, 55.4, 54.0, 41.4, 35.2, 33.6, 29.8, 28.4, 25.0, 24.9, 14.3. LRMS (CI): m/z (%) = 481 (100) [M + H]⁺. HRMS: m/z calcd for C₂₃H₃₇BN₂O₆S: 480.2574; found: 480.2575. IR (film): ν_max = 2970 (C–H), 2926 (C–H), 1697 (br, C=O), 1609 (C=O), 1511, 1456 cm^–1.