Abstract:
Provided is a compound of the following formula (I) for preparing 4-( 10 B)borono-L-phenylalnine: 
     
       
                 
         
             
             
         
       
     
     wherein R group represents a protecting group and is selected from the group consisting of: tert-butoxycarbonyl (Boc) group, trityl (Trt) group, 3,5-dimethoxyphenylisopropoxycarbonyl(Ddz) group, 2-(4-Biphenyl)isopropoxycarbonyl (Bpoc) group, and 2-nitrophenylsulfenyl (Nps) group, and the compound has a  10 B purity higher than or equal to 98% and an enantiomeric excess higher than or equal to 99%.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Pursuant to 35 U.S.C. §119(e), this application claims the benefit of U.S. Provisional Patent Application No. 61/657,069, filed Jun. 08, 2012. The content of the prior application is incorporated herein by its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a compound for preparing 4-( 10 B)borono-L-phenylalanine ( L - 10 BPA), particularly to a compound for preparing  L - 10 BPA with high isotopic purity and high optical purity. 
         [0004]    2. Description of the Prior Arts 
         [0005]    4-Borono-L-phenylalanine ( L -BPA) is an important boronated compound known to be useful for treatment of cancer through boron neutron capture therapy (BNCT). Furthermore,  10 B contained in  L -BPA is known as the critical factor accumulated in tumor cells and subsequently irradiated with thermal neutron. Thus  10 B renders  L -BPA a treatment of cancer through boron neutron capture therapy (BNCT). However, natural boron exists as 19.9% of  10 B isotope and 80.1% of  11 B isotope. Therefore, many researchers have been developing synthetic processes and compounds suitable for producing  10 B-enriched  L -BPA. 
         [0006]    Nevertheless, most conventional synthetic processes and compounds are not applicable for producing 4-( 10 B)borono-L-phenylalanine ( L - 10 BPA) but only applicable for producing 4-borono-L-phenylalanine ( L -BPA). For example, U.S. Pat. No.6,031,127 discloses a process and an intermediate for preparing 4-borono-L-phenylalanine ( L -BPA), wherein the intermediate is of formula (A): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0007]    According to the disclosure, the boron source for producing the intermediate is bis(pinacolato)diboron and 4,4′,6,6′-tetraphenyl-2,2′-bi(1,3,2,-dioxaborinane), which is not commercial available and is hardly prepared from reacting diboron with propane diols. Therefore, such disclosed preparation is difficult and time-consuming. 
         [0008]    To overcome the shortcomings, the present invention provides a compound for preparing 4-( 10 B)borono-L-phenylalanine to mitigate or obviate the aforementioned problems. 
       SUMMARY OF THE INVENTION 
       [0009]    Given that the aforesaid drawbacks of the prior art such as difficulties of preparing  L - 10 BPA, needs for hydrogenation to provide 4-borono-L-phenylalanine, and thus being inapplicable for producing  10 B-enriched  L -BPA, one objective of the present invention is to provide a compound for preparing 4-( 10 B)borono-L-phenylalanine in a facile method and without tedious hydrogenation. Accordingly, the compound of the present invention is applicable for preparing 4-( 10 B)borono-L-phenylalanine and the obtained 4-( 10 B)borono-L-phenylalanine has high isotopic purity and high optical purity. 
         [0010]    To achieve the aforementioned objective, the compound for preparing 4-( 10 B)borono-L-phenylalanine in accordance with the present invention is of formula (I): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0011]    wherein R group represents a protecting group and is selected from the group consisting of: tert-butoxycarbonyl (Boc) group, trityl (Trt) group, 3,5-dimethoxyphenylisopropoxycarbonyl(Ddz) group, 2-(4-Biphenyl)isopropoxycarbonyl (Bpoc) group, and 2-nitrophenylsulfenyl (Nps) group, and the compound has a  10 B purity higher than or equal to 98%. 
         [0012]    The present invention provides a novel compound for preparing 4-( 10 B)borono-L-phenylalanine, particularly to a compound that is easy to be prepared such as from commercially available starting materials, and the compound of the present invention is with high isotopic purity and high optical purity. Furthermore, the compound in accordance with the present invention provides 4-( 10 B)borono-L-phenylalanine by undergoing simple deprotection step such as an acid deprotection step, and the obtained 4-( 10 B)borono-L-phenylalanine is with high isotopic purity and high optical purity. Accordingly, the compound is applicable for preparing 4-( 10 B)borono-L-phenylalanine. 
         [0013]    Preferably, the compound has an enantiomeric excess higher than or equal to 99%. 
         [0014]    Preferably, the R group is tert-butoxycarbonyl (t-Boc) group. 
         [0015]    Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The present invention provides solutions to solve the problems of the conventional processes for preparing 4-( 10 B)borono-L-phenylalanine. A compound for preparing 4-( 10 B)borono-L-phenylalanine from (S)-N-Boc-4-iodophenylalanine is provided as a preferred embodiment for illustrating but not limiting the scope of the present invention. 
         [0017]    For a better understanding about the technical features of the present invention and its effect, and for implements in accordance with the disclosures of the specification, preferred embodiment, details and figures are further shown as follows. 
       Embodiment 1  
     Preparation of (S)-N-Boc-4-( 10 B) boronophenylalanine of formula (a) from (S)-N-Boc-4-iodophenylalanine of formula (b) 
       [0018]    
       
                 
         
             
             
         
       
     
         [0019]    Set up a 3-L, three-necked flask equipped with a mechanical stirrer, a thermometer, and a nitrogen inlet adaptor capped with a rubber septum. Charge the flask with 2-methyltetrahydrofuran (750 mL), followed by (S)-N-Boc-4-iodophenylalanine (50.0 g, 100% pure, 128 mmol), stirred to form a solution, and added tributyl  10 B borate (99% of  10 B purity, 106 mL, 90.1 g, 393 mmol) to form a mixed solution. The mixed solution was cooled to a temperature ranging from −76° C. to −85° C., and n-butyllithium (1.6 M in hexanes, 375 ml, 600 mmol) was added dropwise to the mixed solution over 3 h to form a reaction mixture. After the addition, the reaction mixture was stirred for an additional 0.5 h at −80° C. HPLC analysis of a quenched sample of the reaction mixture showed the starting material (S)-N-Boc-4-iodophenylalanine was less than 0.5%. The reaction mixture was quenched slowly with 900 mL of cold water over 15 to 20 min, then allowed to warm to a temperature ranging from 5° C. to 10° C. The resulted mixture was filtered to remove insoluble solid, and 100 mL of water was adopted for transfer and rinse. The obtained filtrate was transferred to a separatory funnel to separate the layers, and the basic lower aqueous layer was separated to obtain a first aqueous layer. The first aqueous layer was extracted with isobutyl alcohol and then separated from the isobutyl alcohol to obtain a second aqueous layer. 
         [0020]    The pH of the second aqueous layer was adjusted to 3 to 4 by using 37% hydrochloric acid at a temperature ranging from 20° C. to 25° C., the product (S)-N-Boc-4-( 10 B) boronophenylalanine started to precipitate during this period. The second aqueous layer mixture was stirred for 30 minutes, then the pH of the second aqueous layer mixture was further adjusted to 3.0 and then the second aqueous layer mixture was stirred for another 2 hours. The second aqueous layer mixture was filtered to obtain solid (S)-N-Boc-4-( 10 B) boronophenylalanine, which was then washed twice with water and dried in a vacuum oven at 50° C. for a minimum of 4 hours to an LOD of less than 0.5% to afford 25.8 g of (S)-N-Boc-4-( 10 B) boronophenylalanine as white solid, which was 99.6% pure determined by HPLC. The yield was 65.1%. 
         [0021]    The melting point, specific rotation,  1 H NMR data,  13 C NMR data, IR data and MS data of the obtained (S)-N-Boc-4-( 10 B) boronophenylalanine are as follows. 
         [0022]    Melting point: 150° C. (decomp.); 
         [0023]    [α] 25   D :+14° (c=0.5, MeOH); 
         [0024]      1 H NMR:(500 MHz, DMSO-d 6 ):δ 8.0 (s, 2H), 7.7 (d, J=7.7 Hz, 2H), 7.2 (d, J=7.6 Hz, 2H), 7.0 (d, J=8.4 Hz, 2H), 4.1 (m, 1H), 3.0 (dd, J=13.8, 4.5 Hz, 1H), 2.8 (dd, J=13.7, 10.3 Hz, 1H), 1.3 (s, 9H); 
         [0025]      13 C NMR (125 MHz, DMSO-d 6 ) δ 173.63, 155.48, 139.96, 134.06, 131.94, 128.18, 78.13, 55.06, 36.53, 28.19; 
         [0026]    IR (KBr) ν max : 3331, 2979, 1717, 1689, 1537, 1399, 1372, 1365, 1285, 1165, 1045 cm −1 ; and 
         [0027]    HRMS (ESI): calculated for C 14 H 20   10 BNO 6  [M-H] −  307.1420, found 307.1333. 
       Preparation of 4-( 10 B)borono-L-phenylalanine (L-( 10 B) BPA) from (S)-N-Boc-4-( 10 B)boronophenylalanine 
       [0028]    A suspension of (S)-N-Boc-4-( 10 B) boronophenylalanine (20.5 g, 99.6% pure, 66.2 mmol) in a mixture of acetone (122 ml) and water (14 ml) was stirred at room temperature and added hydrochloric acid (37%, 13.9 14 ml) to form an acidic mixture, the acidic mixture was stirred at 55° C. for 1.5 to 2 hours. HPLC analysis of the acidic mixture showed the completion of the reaction. The temperature of the acidic mixture was cooled to room temperature, and the pH of the acidic mixture was adjusted to 1.5 by using sodium hydroxide aqueous solution, 4-( 10 B)borono-L-phenylalanine started to precipitate during this period, and the acidic mixture was stirred for 50 min. The pH of the acidic mixture was readjusted to 6.2 by using sodium hydroxide aqueous solution, and the mixture was stirred for a minimum of 25 minutes at room temperature. The acidic mixture was filtered to obtain solid 4-( 10 B)borono-L-phenylalanine. The solid 4-( 10 B)borono-L-phenylalanine was washed with 50% aqueous acetone, followed by an acetone rinse, dried in a vacuum oven at 80° C. for a minimum of 6 hours to constant weight to afford 13.3 g (95.2% yield) of 4-( 10 B)borono-L-phenylalanine with 99.9% pure as white crystals, and was analyzed by chiral HPLC, indicating the ratio of L to D isomers to be 100 to 0 (100% enantiometric excess). 
         [0029]    The melting point, specific rotation,  1 H NMR data,  13 C NMR data, IR data, ICP-MS data and HRMS data of the obtained L-(10B) BPA are as follows. 
         [0030]    Melting point: 275 to 280° C. (decomp.); 
         [0031]    [α] 25   D :−5.4° (c=0.5, 1M HCl); 
         [0032]      1 H NMR (500 MHz, D 2 O, CF 3 COOD):δ 7.2 (d, J=8.0 Hz, 2H), 6.8 (d, J=8.0 Hz, 2H), 3.9 (dd, J=7.8, 5.7 Hz, 1H), 2.8 (dd, J=14.6, 5.6 Hz, 1H), 2.7 (dd, J=14.6, 7.9 Hz, 1H); 
         [0033]      13 C NMR: (125 MHz, D 2 O, CF 3 COOD):δ171.80, 137.31, 135.16, 132.37, 129.65, 54.64, 36.32; 
         [0034]    IR(KBr) ν max : 3585, 3148, 3038, 2923, 1636, 1610, 1507, 1410, 1398, 1345, 1085, 716 cm −1 . 
         [0035]    ICP-MS measurements for  10 B content is higher than 99.4 (w/w %), wherein  10 B is compared to  11 B; and 
         [0036]    HRMS (ESI): calculated for C 9 H 13 10BNO 4 , [M+H] +  209.0974, found 209.0970. 
         [0037]    Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and features of the invention, the disclosure is illustrative only. Changes may be made in the details, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.