Method for preparing .SUP.18.F-BPA and intermediate

Abstract

A method for preparing .sup.18F-BPA and an intermediate, by which high-purity .sup.18F-BPA is obtained. The method simplifies the synthesis steps after .sup.18F labeling, and is easy to operate and efficient.

Claims

1. A method for preparing .sup.18F-BPA with the structure: ##STR00032## comprising reacting intermediate I-2 with a .sup.18F ion to obtain a .sup.18F-substituted compound ##STR00033## and deprotecting the .sup.18F-substituted compound to obtain the .sup.18F-BPA ##STR00034## wherein R.sub.10 and R.sub.20 are OH, or taken together with a boron atom to which they are attached, represent a substituent that is hydrolyzable to —B(OH).sub.2 group; R.sub.3 or R.sub.4 independently represents hydrogen, an amino-protecting group, or an imino group combined with an amino group for protecting the amino group; and R.sub.5 represents hydrogen or a carboxyl-protecting group.

2. The method according to claim 1, wherein the reaction of the intermediate I-2 with the .sup.18F ion further comprises using a copper catalyst.

3. The method according to claim 1, wherein the reaction of the intermediate I-2 with the .sup.18F ion is carried out at a temperature of 20-150° C.

4. The method according to claim 1, wherein the reaction of the intermediate I-2 with the .sup.18F ion is carried out in a solvent comprising water, methanol, DMF, DMA, DMSO, acetonitrile, n-butanol, ethanol, dichloromethane, or any mixed solvents thereof.

5. The method according to claim 1, further comprising: using intermediate I-1 to react with boric acid or boric ester to obtain the intermediate I-2 ##STR00035## where both of X.sub.1 and X.sub.2 are halogens.

6. The method according to claim 5, wherein X.sub.1 or X.sub.2 independently represents Cl, Br, or I, X.sub.1 and X.sub.2 are the same.

7. The method according to claim 5, further comprising: chirally resolving the intermediate I-1, and then reacting with boric acid or boric ester respectively.

8. The method according to claim 1, wherein the intermediate I-2 is selected from ##STR00036## and ##STR00037##

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the .sup.1H-NMR spectrum of N,N-bis(tert-butoxycarbonyl)-2,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-phenylalanine tert-butyl ester prepared in Example 6;

(2) FIG. 2 is the Radio-HPLC chromatogram of .sup.18F-BPA according to the present disclosure; and

(3) FIG. 3 is the Radio-HPLC chromatogram of .sup.18F-BPA according to the present disclosure mixed with the standard .sup.19F-BPA.

(4) The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

(5) After extensive and in-depth research, the inventor prepared an intermediate for preparing .sup.18F-BPA. The intermediate includes a bis(pinacolato)diboron-substituted phenylalanine intermediate. .sup.18F-BPA may be obtained by substituting .sup.18F for a boric acid pinacol ester group at the ortho position of the alanine group. The method shortens the steps of preparing .sup.18F-BPA after .sup.18F labeling, which is simple and efficient, has high yield and high product purity, and improves the radiochemical yield of the synthesis. The present disclosure is made on such basis.

Terminology

(6) Unless otherwise defined, all scientific and technological terms herein have the same meanings as commonly understood by those skilled in the art to which the claims belong. Unless otherwise specified, all patents, patent applications, and publications cited in this specification are incorporated herein by reference in their entirety.

(7) It should be understood that the above brief description and the following detailed description are exemplary and only used for explanation, and are not intended to limit the subject of the present disclosure. In this disclosure, unless otherwise specified, the plural forms are included when the singular form is used. It should be noted that, unless otherwise clearly specified in this specification, the singular form used in this specification and claims includes the plural referents. It is also noted that, unless otherwise specified, the use of “or”, “alternatively” means “and/or”. In addition, the terms “comprise”, “include”, and other grammatical forms such as “comprising”, “including” and “having” are not limiting.

(8) When a substituent is described by a conventional chemical formula written from left to right, the substituent also includes chemically equivalent substituents obtained when the structural formula is written from right to left. For example, —CH.sub.2O— is equivalent to —OCH.sub.2—.

(9) The section titles used herein are only for the purpose of organizing the article, and should not be construed as a limitation on the subject. All documents or parts of the documents cited in this disclosure, including but not limited to patents, patent applications, articles, books, operating manuals, and papers, are incorporated herein by reference in their entirety.

(10) In addition to the foregoing, when used in the specification and claims of this disclosure, unless otherwise specified, the following terms have the following meanings.

(11) In this disclosure, the term “halogen” refers to F, Cl, Br, and I. “F” refers to fluorine, including radioactive and non-radioactive fluorine, such as .sup.18F and .sup.19F, preferably, .sup.18F. “B” refers to boron, including radioactive and non-radioactive boron, preferably, .sup.10B. “N” refers to nitrogen. “Nitro” refers to —NO.sub.2 group. “Amino” refers to —NH.sub.2 group. “Boric acid group” refers to —B(OH).sub.2 group.

(12) In this disclosure, the term “alkyl”, as a group or a part of other groups (for example, in a halogen-substituted alkyl group, etc.), refers to a straight or branched hydrocarbon chain group consisting only of carbon and hydrogen atoms, without unsaturated bonds, having, for example, 1 to 10 carbon atoms, and connected to the rest of the molecule by a single bond. Examples of the alkyl group include, but are not limited to methyl, ethyl, n-propyl, isopropyl, n-butyl, and the like. The term “C1-10 alkyl group” refers to the alkyl group having 1-10 carbon atoms, such as methyl, ethyl, propyl, isopropyl, pentyl, and the like.

(13) Preferably, the inert solvent is selected from the group consisting of toluene, benzene, water, methanol, ethanol, isopropanol, ethylene glycol, N-methylpyrrolidone, dimethyl sulfoxide, tetrahydrofuran, dichloromethane, chloroform, 1,2-dichloroethane, acetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, dioxane, or a combination thereof. More preferably, the inert solvent is a mixed solvent of benzene and water.

(14) The term “substituent that is hydrolyzable to a boric acid group” represents a substituent that may generate a boric acid group (—B(OH).sub.2) after hydrolysis, for example, a boric ester group. The substituent includes, but is not limited to the following substituents:

(15) ##STR00020##
and preferably, a boric acid pinacol ester group.

(16) In the present disclosure, R.sub.3 or R.sub.4 as an amino-protecting group may be independently protecting groups, including but not limited to, an alkoxycarbonyl protecting group, an acyl protecting group, and an alkyl protecting group, or R.sub.3 or R.sub.4 may also combine with N to form imino (C═N) for protecting the amino group, for example,

(17) ##STR00021##
The above two forms of protecting groups should fall within the scope of the “amino-protecting group” in the present disclosure. Among those protecting groups, the alkoxycarbonyl protecting group includes, but is not limited to: benzyloxycarbonyl (Cbz), tert-butyloxycarbonyl (Boc), fluorenylmethoxycarbonyl (Fmoc), allyloxycarbonyl (Alloc), trimethylsilyl ethoxycarbonyl (Teoc), and methoxycarbonyl or ethoxycarbonyl. The acyl protecting group includes, but is not limited to: phthaloyl (Pht), tosyl (Tos), trifluoroacetyl (Tfa), o-(p-) nitrobenzenesulfonyl (Ns), pivaloyl, and benzoyl. The alkyl protecting group includes, but is not limited to: trityl (Trt), 2,4-dimethoxybenzyl (Dmb), p-methoxybenzyl (PMB), and benzyl (Bn).

(18) The term “carboxyl-protecting group” refers to a protecting group that may combine with carboxyl to form ester group, amide, or hydrazide, including but not limited to alkyl, phenyl, and alkyl-substituted amino. “Alkyl” is preferably a straight or branched and substituted or unsubstituted alkyl with a substituent having 1-20 carbon atoms, such as methyl, ethyl, isopropyl, tert-butyl, benzhydryl, benzyl, p-nitrobenzylp, p-methoxybenzyl, 4-pyridylbenzyl, trichloroethyl, methylthioethyl, p-toluenesulfonylethyl, p-nitrophenylthioethyl, and the like.

(19) In this disclosure, “optional” or “optionally” represents that the event or condition described later may or may not occur, and the description includes both the occurrence and non-occurrence of the event or condition. For example, “optionally substituted aryl group” represents that the aryl group is substituted or unsubstituted, and the description includes both the substituted aryl group and the unsubstituted aryl group.

(20) The present disclosure will include various stereoisomers and mixtures thereof. The “stereoisomers” refer to compounds consisting of the same atoms bonded by the same bonds, but having different three-dimensional structures. All tautomeric forms of the compounds of the present disclosure will also be included in the scope of the present disclosure. “Tautomer” refers to an isomer formed by transferring a proton from one atom of a molecule to another atom of the same molecule.

(21) The intermediate compounds of the present disclosure contain chiral carbon atoms, and therefore may generate enantiomers, diastereomers, and other stereoisomeric forms. Each chiral carbon atom may be defined as (R)- or (S)-based on stereochemistry. The present disclosure is intended to include all possible isomers, as well as their racemates and optically pure forms. For the preparation of the compounds of the present disclosure, racemates, diastereomers, or enantiomers may be selected as raw materials or intermediates. Optically active isomers may be prepared by using chiral synthons or chiral reagents, or resolved by using conventional techniques, such as crystallization and chiral chromatography. The “intermediate” of the present disclosure has both L-configuration phenylalanine structure and D-configuration phenylalanine structure, which are encompassed within the protection scope of the present disclosure.

(22) Preparation Method for .sup.18F-BPA

(23) The present disclosure describes a new method for preparing .sup.18F-BPA, for example, by using the methods shown in following examples, but is not limited to the exemplary methods. The amino- and carboxyl-protecting groups, and the boric ester group used in the following reaction scheme may be appropriately changed, and are not limited to the exemplary method.

(24) The method includes reacting the intermediate I-2 with a .sup.18F ion to obtain a .sup.18F-substituted compound

(25) ##STR00022##
deprotecting the .sup.18F-substituted compound, for example, by hydrolyzing the amino- and carboxyl-protecting group, and the boric ester group to obtain the .sup.18F-BPA

(26) ##STR00023##

(27) For each step, the reaction temperature may be appropriately selected depending on solvents, starting materials, and reagents, and the reaction time may also be appropriately selected depending on reaction temperatures, solvents, starting materials, and reagents. After the reaction of each step is completed, a target compound may be separated and purified from a reaction system by conventional methods, such as filtration, extraction, recrystallization, washing, silica gel column chromatography, and the like. Without affecting the next reactions, the target compound may also directly used in the next steps without separation and purification.

(28) In the step of synthesizing .sup.18F-BPA with intermediate I, the .sup.18F ion ([.sup.18F]F.sup.−) used may be obtained by bombarding H.sub.2.sup.18O with a proton beam through .sup.18O (p, n) reaction, or by using methods known in the art. The .sup.18F ion is captured on an ion exchange column, eluted with a K2.2.2/K.sub.2CO.sub.3 mixed solution, dried to remove water and then used for a labeling reaction. Intermediate 1-2 is reacted with the .sup.18F ion, and the resulting mixed solution is purified by a cation column, dried, and hydrolyzed to remove the amino- and carboxyl-protecting group, thereby obtaining the .sup.18F-BPA. The reagent used includes hydrogen fluoride and potassium fluoride. The solvent used includes water, methanol, DMF, DMA, DMSO, acetonitrile, n-butanol, ethanol, dichloromethane, or any mixed solvents thereof. The reaction temperature is preferably 20-150° C., and more preferably, 100-130° C. The reaction time is preferably 5-60 minutes, and more preferably, 10-30 minutes. The drying may azeotropically remove water by adding a dry organic solvent. The organic solvent that may be used includes, but is not limited to: acetonitrile, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMA), or any combinations thereof. In a preferred implementation, a copper catalyst may be added to the reaction of the intermediate I and the .sup.18F ion. The copper catalyst that may be used includes, but is not limited to: Cu(OTf).sub.2Py.sub.4, Cu(OTf).sub.2, or a combination thereof.

(29) It should be noted that the intermediate I and .sup.18F-BPA in the present disclosure include all optical isomers thereof, including an isomer with an L-configuration phenylalanine structure and an isomer with a D-configuration phenylalanine structure. For example, .sup.18F-BPA includes .sup.18F-L-BPA having the L-configuration phenylalanine structure or .sup.18F-D-BPA having the D-configuration phenylalanine structure. In the preparation method of the present disclosure, intermediate I may undergo chiral resolution to obtain a single-configuration compound for use in the reaction, or directly used in the reaction without resolution, to obtain .sup.18F-BPA.

(30) The present disclosure is further illustrated below with reference to specific examples. It should be understood that the following description is only the most preferred implementation of the present disclosure, and should not be considered as a limitation on the protection scope of the present disclosure. On the basis of a full understanding of the present disclosure, the experimental methods without specific conditions in the following examples are usually in accordance with the conventional conditions or in accordance with the conditions recommended by the manufacturer. A person skilled in the art may make non-essential changes to the technical solutions of the present disclosure, and such changes should be included in the protection scope of the present disclosure. Unless otherwise specified, the percentage and the parts are the percentage by weight and the parts by weight respectively.

Example 1 Preparation of N-diphenylmethylene-2,4-dibromophenylalanine tert-butyl ester

(31) N-diphenylmethylene-glycine tert-butyl ester (40 g, 135.42 mmol, 1 eq), 1,4-dibromobenzyl bromide (44.53 g, 135.42 mmol, 1 eq), and tetra-n-butylammonium bromide (TBAB, 436.55 mg, 1.35 mmol, 0.01 eq) were dissolved in 300 mL of toluene, and 80 mL of a solution of potassium hydroxide (100.00 g, 1.78 mol, 13.16 eq) in water was added. The mixture was stirred at 25° C. for 12 hours until the reaction was complete. The reaction mixture was diluted with 100 mL of ethyl acetate, and then extracted with 400 mL (200 mL×2) of ethyl acetate. The organic phases were mixed, washed with 600 mL (300 mL×2) of saturated brine, dried with Na.sub.2SO.sub.4, and filtered to obtain a solid. The crude product was separated by column chromatography (petroleum ether/ethyl acetate=100: 1-20:1) to obtain 15 g of product.

(32) LCMS: MS (M+H.sup.+)=544.0

(33) .sup.1H NMR: 400 MHz, CDCl.sub.3

(34) δ 7.56-7.46 (m, 3H), 7.35-7.16 (m, 7H), 7.01 (d, J=8.2 Hz, 1H), 6.59 (br d, J=6.8 Hz, 2H), 4.23 (dd, J=4.2, 9.5 Hz, 1H), 3.31 (dd, J=4.0, 13.4 Hz, 1H), 3.11 (dd, J=9.6, 13.5 Hz, 1H), 1.41-1.28 (m, 9H).

(35) ##STR00024##

Example 2 Preparation of N-tert-butyloxycarbonyl-2,4-dibromophenylalanine tert-butyl ester

(36) N-diphenylmethylene-2,4-dibromophenylalanine tert-butyl ester (30 g, 55.22 mmol, 1 eq) was dissolved in THF (70 mL), and 20 mL aqueous citric acid (31.83 g, 165.66 mmol, 31.86 mL, 3 eq) was added, and stirred at 25° C. for 12 h. 50 mL of a solution of Na.sub.2CO.sub.3 (29.26 g, 276.10 mmol, 5 eq) in water and Boc.sub.2O (13.26 g, 60.74 mmol, 13.95 mL, 1.1 eq) were added and stirred for additional 4 h. After the reaction was complete, the reaction mixture was extracted with 400 mL (200 mL×2) of ethyl acetate. The organic phases were mixed, washed with 400 mL (200 mL×2) of saturated brine, dried with Na.sub.2SO.sub.4, and filtered to obtain a solid. The crude product was separated by column chromatography (petroleum ether/ethyl acetate=100: 1-10:1) to obtain 19.6 g of product (73.48% yield, 99.2% purity).

(37) LCMS: MS (M−155.sup.+)=323.9

(38) .sup.1H NMR: 400 MHz, CDCl.sub.3

(39) δ7.72 (d, J=1.5 Hz, 1H, 7.37 (dd, J=1.8, 8.1 Hz, 1H), 7.13 (br d, J=8.1 Hz, 1H), 5.06 (br d, J=8.3 Hz, 1H), 4.59-4.38 (m, 1H), 3.23 (dd, J=5.9, 13.9 Hz, 1H), 3.00 (br dd, J=8.6, 13.8 Hz, 1H), 1.40 (br d, J=16.6 Hz, 17H).

(40) ##STR00025##

Example 3 Chiral Resolution of N-tert-butyloxycarbonyl-2,4-dibromophenylalanine tert-butyl ester

(41) 19.6 g of N-tert-butyloxycarbonyl-2,4-dibromophenylalanine tert-butyl ester was separated by SFC (column: DAICEL CHIRALPAK AY (250 mm*50 mm, 10 μm); mobile phase: [0.1% NH.sub.3H.sub.2O MEOH]) to obtain 9.1 g of product (46.84% yield, 97.8% purity) and 9.2 g of product (48.18% yield, 99.5% purity).

(42) ##STR00026##

Example 4 Preparation of N-tert-butoxycarbonyl-2,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-phenylalanine tert-butyl ester

(43) N-tert-butyloxycarbonyl-2,4-dibromophenylalanine tert-butyl ester (7.50 g, 15.6 mmol, 1.00 eq), bis(pinacolato)diboron (19.8 g, 78.2 mmol, 5.00 eq), KOAc (6.14 g, 62.6 mmol, 4.00 eq), Pd(dppf)Cl.sub.2 (1.15 g, 1.57 mmol, 0.10 eq), and dioxane (75.0 mL) were stirred under nitrogen protection at 90° C. for 1 h. After the reaction was complete, the resulting reaction mixture was filtered, and 200 mL of water was added. The reaction mixture was extracted with 300 mL (100 mL×3) of ethyl acetate. The organic phases were mixed, washed with 200 mL (100 mL×2) of saturated brine, dried with Na.sub.2SO.sub.4, and filtered to obtain a solid. The crude product was purified by HPLC to obtain 6.5 g of product (96.3% purity).

(44) LCMS: MS (M+H.sup.+−156)=418.0

(45) .sup.1H NMR: 400 MHz, CDCl.sub.3

(46) 8.24 (s, 1H), 7.84 (d, J=7.6, 1H), 7.30 (d, J=7.6, 1H), 5.90 (d, J=8.4, 1H), 4.24-4.19 (m, 1H), 3.24-3.19 (m, 2H), 1.47 (s, 9H), 1.39 (s, 12H), 1.34 (s, 12H), 1.32 (s, 9H).

(47) ##STR00027##

Example 5 Preparation of N-tert-butoxycarbonyl-2,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-phenylalanine tert-butyl ester

(48) O-tert-butyloxycarbonyl-2,4-dibromophenylalanine tert-butyl ester (7.00 g, 14.6 mmol, 1.00 eq), bis(pinacolato)diboron (18.5 g, 73.0 mmol, 5.00 eq), KOAc (5.73 g, 58.4 mmol, 4.00 eq), Pd(dppf)Cl.sub.2 (5.73 g, 58.4 mmol, 4.00 eq), and dioxane (70.0 mL) were stirred under nitrogen protection at 90° C. for 3 h. After the reaction was complete, the resulting reaction mixture was filtered, and 200 mL of water was added. The reaction mixture was extracted with 300 mL (100 mL×3) of ethyl acetate. The organic phases were mixed, washed with 200 mL (100 mL×2) of saturated brine, dried with Na.sub.2SO.sub.4, and filtered to obtain a solid. The crude product was purified by HPLC to obtain 5.37 g of product (97.35% purity).

(49) LCMS: MS (M-100-55+H.sup.+): 418.3

(50) .sup.1HNMR: 400 MHz, CDCl.sub.3

(51) δ 1.31-1.35 (m, 21H), 1.39 (s, 12H), 1.47 (s, 9H), 3.16-3.29 (m, 2H), 4.19-4.26 (m, 1H), 5.89 (br d, J=8.40 Hz, 1H), 7.30 (d, J=8.00 Hz, 1H), 7.84 (dd, J=7.60, 1.47 Hz, 1H), 8.24 (s, 1H)

(52) ##STR00028##

Example 6 Preparation of N,N-bis(tert-butoxycarbonyl)-2,4-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-phenylalanine tert-butyl ester

(53) N-tert-butyloxycarbonyl-2,4-dibromophenylalanine tert-butyl ester (7.00 g, 14.6 mmol, 1.00 eq), bis(pinacolato)diboron (18.5 g, 73.0 mmol, 5.00 eq), KOAc (5.73 g, 58.4 mmol, 4.00 eq), Pd(dppf)Cl.sub.2 (5.73 g, 58.4 mmol, 4.00 eq), and dioxane (70.0 mL) were stirred under nitrogen protection at 90° C. for 3 h. After the reaction was complete, the resulting reaction mixture was filtered, and 200 mL of water was added. The reaction mixture was extracted with 300 mL (100 mL×3) of ethyl acetate. The organic phases were mixed, washed with 200 mL (100 mL×2) of saturated brine, dried with Na.sub.2SO.sub.4, and filtered to obtain a solid. The crude product was purified by HPLC to obtain 5.37 g of product (97.35% purity). .sup.1H NMR spectrum was shown in FIG. 1.

(54) ##STR00029##

Example 7 Preparation of .SUP.18.F-BPA

(55) H.sub.2.sup.18O was irradiated with accelerated protons to obtain .sup.18F ions through .sup.18O (p, n) reaction. The ions were captured on an ion exchange column, eluted with a K2.2.2/K.sub.2CO.sub.3 mixed solution, dried to remove water and then used for a labeling reaction. The compound obtained in Example 5 was reacted with the .sup.18F ions in DMA solution in the presence of Cu(OTf).sub.2Py.sub.4 for 15 min. After the reaction was complete, the mixed solution was purified by a cation column and dried. The reaction was performed in HCl solution at 110° C. for 10 min, and NaOH solution was added for neutralization. The amino- and carboxyl-protecting groups were removed by hydrolyzation to obtain the crude product. The crude product was purified by HPLC to obtain .sup.18F-BPA with a purity higher than 99%.

(56) Radio-HPLC chromatogram was shown in FIG. 2. The product was mixed with a small amount of standard .sup.18F-BPA and co-injected into the HPLC column. Under the same conditions, a peak appeared at the retention time (retention time 11 min±1 min), suggesting that the labeled product was .sup.18F-BPA. See FIG. 3.

(57) ##STR00030##

(58) By using the same preparation method above, .sup.18F-BPA was prepared from the compound obtained in Example 6, and .sup.18F-BPA with a purity higher than 99% may also be obtained.

(59) ##STR00031##

(60) All documents mentioned in the present disclosure are incorporated by reference, as if each document is individually incorporated by reference. In addition, it should be understood that, after reading the above teaching of the present disclosure, those skilled in the art can make various changes or modifications to the present disclosure, and these equivalent forms also fall within the scope defined by the appended claims of this disclosure.