PROCESS FOR SYNTHESIZING PHOSPHODIESTERS

Abstract: An improved process for the production of phosphodiester compounds having formula (1). These compounds are particularly useful as contrast agents for diagnostic imaging. The process avoids the need for multiple isolation and purification steps otherwise reduced due to the formation of multiple intermediates.

Description: PROCESS FOR SYNTHESIZING PHOSPHODIESTERS Field of the Invention The present invention relates to an improved process for the production of phosphodiester compounds. In particular, the invention relates to an improved process for preparing phosphodiester compounds which are useful as contrast agents for diagnostic imaging, and more particularly, for preparing diethylenetriamine- pentaacetic acid ("DTP A") compounds comprising phosphodiesters. Background of the Invention Many important biological substances, including phospholipids, oligonucleotides, deoxynucleosides, nucleotides and nucleosides, exist as symmetrical and unsymmetrical phosphodiesters. The usefulness of such phosphodiester compounds in medical applications is well known. See, e.g., Desseaux et al., "Synthesis of Phosphodiester and Triester Derivatives of AZT with Tethered N-Methyl Piperazine and N,N,N'trimethylethylenediamine," Bioorg. & Med. Chem. Letters, vol. 3, no. 8, pp. 1547-50 (1993); PCT publication no. WO 96/27379. Recently, PCT publication no. WO 96/23526, incorporated herein by reference, describes phosphodiester compounds which are useful as contrast agents for diagnostic imaging A number of methods of making phosphodiester compounds, based on P(III) chemistry, are known. In general, phosphorylation plays an important role in the synthesis uf phosphodiester compounds. But the known phosphodiester synthetic methods all suffer from a number of problems including how phosphorylation is accomplished. One method for making phosphodiesters involves the use of phosphoramidite chemistry. See, e.g., Bannwarth et al., "A Simple and Effective Chemical Phosphorylation Procedure for Biomolecules," Helvetica Chimica Acta. vol. 70, pp. 175-186 (1987); Bannwarth et al., "Bis(allyloxy)(diisopropylamino) phosphine as a New Phosphinylation Reagant of the Phosphorylation of Hydroxy Functions," Tetrahedron Letters, vol. 30, no. 32, pp. 4219-22 (1989); Moore et al., "Conceptual Basis of the Selective Activation of E is ' ■ '.ialkylamino) methoxyphosphines by Weak Acids and Its Application toward the Preparation of Deoxynucleoside Phosphoramidites in Situ," J.Oηa Chem.. vol. 50, pp. 2019-2025 (1985); Hebert et al., "A New Reagant for the Removal of the 4-Methoxybenzyl Ether: Application to the Synthesis of Unusual Macrocyclic and Bolaform Phosphatidycholines," J.Org.Chem.. vol. 57, pp. 1777-83 (1992); Desseaux et al., "Synthesis of Phosphodiester and Triester Derivatives of AZT with Tethered N-Methyl Piperazine and N,N,N'trimethylethylenediamine," Bioorg. & Med. Chem. Letters, vol. 3, no. 8, pp. 1547-50 (1993); Pirrung et al., "Inverse Phosphotriester DNA Synthesis Using Photochemically-Removable Dimethoxybenzoin Phosphate Protecting Groups," J.Org.Chem.. vol. 61, pp. 2129-36 (1996). Such phosphoramidite methods, however, suffer from the fact that the phosphoramidites are typically unstable compounds (both chemically and kinetically) and upon purification by distillation may ignite or cause an explosion. Further, phosphoramidite methods are generally not suitable for manufacturing phosphodiester compounds on a commercial basis. This is so because the phosphoramidite starting materials are very expensive and are not readily available, and because methods using phosphoramidites tend to involve additional process steps (e.g., additional step of cleaving protecting groups after phosphorylation) as well as multiple isolation and/or purification steps of the intermediates. Methods involving the use of phosphodichloπdates as the phosphorylating agent suffer from similar problems See, e g , Martin et al , "General Method for the Synthesis of Phosphohpid Deπvatives of 1,2-O-Dιacyl-sn-glycerols," J Org.Chem . vol 59, pp 4805-20 (1 94), Martin et al , "A General Protocol for the Preparation of Phospholipids via Phosphate Coupling," Tetrahedron Letters, vol 29, no 30, pp 3631-34 (1988), Lammers et al , "Synthesis of Phospholipids via Phosphotπester Intermediates," J.Roya Netherlands Chem Soc'y. 98/4, pp 243-250 (Apπl 1979), Martin et al , "Synthesis and Kinetic Evaluation of Inhibitors of the Phosphatidyhnositol-Specific Phosphohpase C from Bacillus cereus " J.Org.Chem.. vol 61, pp 8016-23 (1996) Another method used for making phosphodiester compounds involves the use of PC1 3 to generate hydrogen-phosphonate intermediates See, e g , Lmdh et al , "A General Method for the Synthesis of Glycerophospholipids and Their Analogues via H-Phosphonate Intermediates." J Org.Chem.. vol 54, pp 1338-42 (1989), Garcia et al , "Synthesis of New Ether Glycerophospholipids Structurally Related to Modulator," Tetrahedron, vol 47, no 48, pp 10023-34 (1991), Gaπgapati et al , "Synthesis of Short Chain Phosphatidy nositols," Tetrahedron Letters, vol 34, no 5, pp 769-72 (1993) This method, however, requires the use of a coupling reagent which can either be purchased or independently synthesized, and thus renders such methods expensive or more complex In addition, multiple isolation and puπfication steps of the intermediates are required, often with laboπous drying conditions for the H-phosphonate intermediate Consequently, there remains a need for a safe, efficient and inexpensive process for the production, in high yields, of phosphodiester compounds with the potential of having a wide vaπety of substituents which does not require either the use of a protecting group or a coupling agent In particular, there remains a need for a process which could be performed in one reaction vessel and does not require multiple isolation and purification steps because of the formation of multiple intermediates Sum arv of the Invention The present invention relates to a safer, more efficient and less expensive process for prepaπng phosphodiester compounds, and more particularly, phosphodiesters having the formula In accordance with the present invention, the process compnses the steps of (a) coupling PC1 3 with an alcohol to obtain a substituted dichlorophosphine, (b) coupling of said dichlorophosphine with an amine base to obtain a bιs(amιno)phosphιno, (c) coupling of said bιs(ammo)phosphιno with a second alcohol, which can be the same or different from that alcohol used in step (a), to obtain a disubstituted (amιno)phosphιno, (d) and reacting said (amιno)phosphιno with water and an oxidant to obtain the desired phosphodiester compound The process according to this invention avoids the use of unstable phosphorylating agents as well as the need for using a protecting group or a coupling agent Thus, the present method avoids unnecessary process steps such as deprotection and coupling reagent syntheses In a preferred embodiment of this invention, the phosphodiester synthetic process takes place in one reaction vessel, avoidmg the need for multiple isolation and/or puπfication steps Detailed Description of the Invention In order that the invention herein described may be more fully understood, the following detailed description is set forth. The present invention provides an improved process for preparing phosphodiester compounds of general formula: 0 R— O— P — O— R 1 OH where R and R can be the same or different and are selected from the group consisting of linear, branched, or cyclic aliphatic, aryl, heterocyclic, peptidic, peptoid, deoxyribo- or ribo-nucleotidic or nucleosidic, or cyclic or acyclic organic chelating agent groups, all of which may optionally be substituted with one or more nitrogen, oxygen, sulfur, halogen, aliphatic, amide, ester, sulfonamide, aryl, acyl, sulfonate, phosphate, hydroxyl, or organometallic substituents. In a preferred aspect of the invention, all synthetic steps are performed in one reaction vessel, precluding the need for multiple isolation and/or purification steps. The present invention demonstrates an efficient and high-yielding process for producing phosphodiester compounds which does not rely on expensive or unstable starting materials and does not require the use of either protecting groups or coupling agents. Moreover, said process is efficient for the generation of phosphodiester linkages between a wide variety of substituents. Process Scheme In accordance with this invention, an alcohol ROH, where R has the same meaning as stated above, is reacted with PC1 3 , preferably at a molar ratio of 1 1, to form a dichlorophosphine reaction product (I) PCI3, solvent (I) ROH ► ROPCl 2 This reaction takes place in the presence of an ethereal or hydrocarbon solvent and is earned out at a temperature of from about -50°C to about 15°C, preferably from about -10°C to about -5°C, for a peπod of from about 30 minutes to about 3 hours, preferably from about 1 to about 1 5 hours The solvent may be any ethereal or hydrocarbon solvent and preferably, may be selected from the group consisting of heptanes, methyl-t-butyl ethers, tetrahydrofurans, diethyl ethers, and ethylene glycol dialkyl ethers More preferably, the solvent is tetrahydrofuran The dichlorophosphine (I) is then reacted with from about 5 to about 6 equivalents of an amine base to form a bιs(amιno)phosphιno reaction product (II) amino amine base, solvent ROPC „,I 2 1 ! ^ *- D R Λ O 1 P v (ID V amino This reaction also takes place in the presence of an ethereal or hydrocarbon solvent, as descπbed above, and is earned out at a temperature of from about -50 °C to about 15°C, preferably from about -10°C to about -5°C, for a peπod of from about 30 minutes to about 3 hours, preferably from about 15 to about 30 mmutes The base used to form reaction product (II) may be any amine b^se, preferably a base having a pKa value of from about 5 to about 11, and more preferably selected from the group consisting of lmidazole, 2,4-dιmethyhmιdazole, lH-tetrazole, dialkylamines (methyl, ethyl, butyl), pyndine, piperazme, pipendine, pyrrole, 1H-1, 2, 3-tπazole, and 1,2,4- tπazole In a more preferred embodiment, the base is lmidazole The bιs(amιno)phosphιno compound (II) is then reacted with from about 0 75 to about 1 0 equivalents of a second alcohol R OH, where R has the same meaning as stated above, to form an (ammo)phosphιno reaction product (III) amino solvent / (III) ROP(amιno) 2 + R ] OH *- ROP ORi This reaction takes place in the presence of an ethereal or hydrocarbon solvent and earned out at a temperature of from about -50 °C to about 15°C, preferably from about -10°C to about -5 °C, for a penod of from about 30 minutes to about 3 hours, preferably from about 1 0 to about 1 hours The solvent may be any ethereal or hydrocarbon solvent and preferably may be selected from the group consisting of heptanes, methyl-t-butyl ethers, dioxanes, tetrahydrofurans, 1,3-dιoxolanes, diglymes, diethyl ethers, dialkvl ethers, and ethylene glycol dialkyl ethers More preferably, the solvent is tetrahvdrofuran Finally, the (amιno)phosphmo compound (III) is reacted with about one equivalent of acidic water, preferably having a pH of about 2 5 to about 5, and about 1 or more equivalents of an oxidant to form the desired phosphodiester compound (IV) The oxidant may be any peroxide type oxidant and preferably selected from the group consisting of peπodates More preferably, the oxidant is sodium peπodate The above hydrolysis and oxidation is carried out in a solvent mixture at a temperature of from about -15°C to about 25 °C, preferably from about 0°C to about 2°C, for a period of from about 10 to about 24 hours, preferably from about 10 to about 15 hours. The solvent mixture comprises any combination of solvents selected from the group consisting of ethereal or hydrocarbon solvents. Preferably, the solvent mixture comprises tetrahydrofuran, heptane and toluene in the volume ratio of 10: 10.1 Use of the Process Products It has beeri found that the above process is particularly useful in the preparation of contrast agents for diagnostic imaging. Examples of phosphodiester contrast agents that may be prepared by this improved process include the compounds shown below, as well as others described in PCT publication no. WO 96/23526 MS-322 MS-323 MS-325 MS-326 MS-327 MS-32Θ In such cases, it is contemplated that m l ast one of the two alcohols (ROH, R OH) as defined herein further compnse a cyclic or acyclic organic chelating ligand, with any sensitive functional groups (e.g., carboxylates) on such a chelate protected with appropriate groups (e.g., t-butyl groups) Suitable chelating ligands are well known in the art For example, where the phosphodiester compound is to be used as a contrast agent for magnetic resonance imaging, preferred chelating ligands include Magnev±st Dotarem ga opentetate d-lmeglumlne gadoterate n---.glu-----.ne DTPA DOTA The removal of any protecting groups on the chelate as well as the complexation of the chelate with the desired metal can be performed after carrying out the phosphodiester synthetic process of this invention by methods well known in the art. See, e.g., Grote et al , "StereocontroUed Synthesis of DTPA Analogues Branged in the Ethylene Unit," J. Org. Chem.. 60:6987-97 (1995); Kang et al , "Synthesis, Characterization, and Crystal Structure of the Gadolinium (III) Chelate of (lR,4R,7R)-α,α',α" - Trimethyl- l,4,7,10-tetraazacyclododecane-l,4,7-triacetic Acid (DO3MA)," Inorg. Chem . 32.2912-18 (1993) and references cited therein It is also contemplated that for such phosphodiester contrast agents, the alcohol (ROH or R OH) may comprise a moiety designed to facilitate localization of the resultant agent to the tissue, cell, protein, receptor or area desired to be imaged. Examples of such moieties include lipophilic or amphiphilic substances, receptor ligands, antibodies, or antibody fragments, peptides, or other biomolecules that are known to concentrate in the specific biological component desired to be imaged In order that this invention may be better understood, the following example is set forth. This example is for purposes of illustration only and is not intended to limit the scope of this invention in any way. Example The preparation of [(4,4-diphenylcyclohexyl)phosphooxymethyl] diethylene triaminepenta-acetic acid is shown below in Scheme I: Scheme I In a single reaction \ essel that contained a solution of phosphorous tnchlonde (13 2 mL, 0 151 mol) in tetrahydrofuran (202 ml) was added a solution of 4,4-dιphenyl-cyclohexanol (I) (38 34 g, 0 152 mol) in tetrahydrofuran (243 ml) while stirπng and maintaining an internal temperature of -6 2°C to -5 3 °C for 1 5 hours The mixture was then stirred for an additional 34 minutes yielding a dichlorophosphine reaction product (2), having a 3 I P NMR chemical shift of 174 28 ppm To this solution, lmidazole (51 34 g, 0 753 mol) in tetrahydrofuran (243 ml) was added while stirπng and maintaining an internal temperature of -7 8°C to - 3 6°C for 37 minutes The resulting mixture was then stirred for an additional 20 minutes yielding a solution of a bts(aιr---.no)phθi-phιno reaction product (3) having a 31 P NMR chemical shift of 106 36 ppm To this mixture was added a solution consisting of 2-(R)- hydroxymethyldiethylenetπamme pentaacetic acid, penta-t-butyl ester (4) (160 0 g, 0 128 mol, puπty 56 32% by weight) in heptane (114 ml) while stirnng and maintaining an internal temperature of -6 8°C to -4 8°C for 1 hour and 6 minutes This mixture was then stiπed for an additional 23 minutes yielding a solution (5) having a 31 P NMR chemical shift of 123 8 ppm Finally, water (202 ml) was added over a penod of about 1 minute while maintaining an internal temperature of -6 5°C to 6 5°C The mixture was stirred for 5 minutes followed by the addition of heptane (620 ml), toluene (70 ml) and 5N aqueous hydrochloπc acid (202 ml) over 5 minutes while maintaining an internal temperature of 1 0°C to 12 1 °C Sodium peπodate (22 6 g, 0 106 mol) was then added over a peπod of 3 minutes while maintaining an internal temperature of 10 5 ° C The reaction mixture was warmed to room temperature over 35 mmutes and stirred an additional 2 5 hours yielding a solution (6) with a P NMR chemical shift of 4 27 ppm The layers were separated and the organic layer was washed with 10% aqueous sodium thiosulfate (2 x 809 mL) To the above organic layer was added tetraoctylammonium bromide (8 21 g, 0 015 mol) Concentrated hydrochloπc acid (11 51 M, 405 mL) was then added over a period of 22 minutes while maintaining an internal temperature of 22 8° C to 25 0°C This mixture was stirred for 16 0 hours yielding a compound (7) with a P NMR chemical shift of 7 78 ppm The layers were separated and the organic layer discarded To the above aqueous layer was added 8M aqueous sodium hydroxide (630 mL) until a pH of 6 56 was recorded The solution was concentrated under reduced pressure (50°C to 55°C, vacuum 85 mm Hg) until 400 mL of solvent was collected (approximately 1 hour) The solution was cooled to room temperature and amberlite XAD-4 resin (92 0 g) was added The suspension was stirred for 50 minutes at room temperature and filtered to give a light yellow aqueous solution (1 1 L) The above solution was loaded onto C- 18 reversed phase silica gel (27 lg, packed wet in methanol and then washed with 800 mL methanol, 800 mL methanol water, 1 1 and 800 mL water) and eluted with water The first 1 0 L of elutent collected was discarded and the next 1.3 L collected were retained To the retained solution was added 6N aqueous hydrochloric acid (60 mL to a pH =2 15) and 3N aqueous hydrochloric acid (30 mL to a pH=l 63) The slurry was stirred for 1 25 hours and filtered The solid was washed with pH 1 67 aqueous solution (500 mL) and ddririeedd ((4488--5500 °CC,, 44--66 mmmm HHgg) to a constant weight (18 0 hours) to obtain an off-white solid, compound of formula (65 5 g, Yield: 68.89% Purity: 99 45% by weight, 98.95% by area, 3.02% water and 97 81% chelatables)

Claims: We Claim: 1. A process for preparing phosphodiester compounds having the formula: where R and R' can be the same or different and are chosen from the group consisting of linear, branched, or cyclic aliphatic, aryl, heterocyclic, peptidic, peptoid, deoxyribo- or ribo-nucleotidic or nucleosidic, or cyclic or acyclic organic chelating agent groups, all optionally substituted with one or more nitrogen, oxygen, sulfur, halogen, aliphatic, amide, ester, sulfonamide, acyl, sulfonate, phosphate, hydroxyl, or organometallic substituents, comprising the steps of: (a) reacting an alcohol ROH with PC1 3 in the presence of a solvent to form a dichlorophosphine compound having the formula: (b) coupling of the dichlorophosphine compound formed in step (a) with an amine base in the presence of a solvent to form a bis(amino)phosphino compound having the formula: (c) coupling of the bis(amino)phosphino compound formed in step (b) with a second alcohol R'OH, in the presence of a solvent, where the second alcohol can be the same or different from that of step (a), to form an (amino)phosphino compound having the following formula: (d) and subjecting the (amino)phosphino compound formed in step (c) to hydrolysis and oxidation. 2. The process according to claim 1 wherein the phosphodiester compound is prepared in one reaction vessel. 3. The process according to either claims 1 or 2, in which, the alkoxydichlorophosphine compound formed in step (a) is reacted with from about 5 to about 6 equivalents of the amine base. 4. The process according to any one of claims 1-3, wherein the amine base has a pKa value of from about 5.0 to about 11.0. 5 The process according to claim 4, wherein the base is selected from the group consisting of imidazole, 2,4-dimethylimidazole, lH-tetrazole, dialkylamines (methyl, ethyl, butyl), pyridine, piperazine, piperidine, pyrrole, IH, 1, 2, 3-triazole, and 1,2,4-triazole 6 The process according to claim 5, wherein the base is imidazole 7 The process according to either claim 1 or claim 2, in which about one equivalent of ROH is reacted with about one equivalent of PC1 3 8 The process according to either claims 1 or 2, wherein the solvent used in steps (a), (b) and (c) may be the same or different and is be selected from the group consisting of ethereal and hydrocarbon solvents. 9 The process according to claim 8, wherein the solvent is selected from the group consisting of heptanes, methyl -t-butyl ethers, dioxanes, tetrahydrofurans, 1,3-dioxolane, diglymes, diethyl ethers, dialkyl ethers, and ethylene glycol dialkyl ethers 10 The process according to claim 9, wherein the solvent is tetrahydrofuran. 11 The process according to either claims 1 or 2, wherein the alkoxy( amino) phosphino compound formed in step (b) is coupled with about 1 equivalent of R'OH. 12 The process according to either claims 1 or 2, wherein the hydrolysis and oxidation of the dialkoxy (amino) phosphino compound formed in step (c) is performed with water and an oxidant in a aul»ent at a temperature range of about -15°C to about 25 °C for a peπod of 10 to 24 hours 13 The process according to claim 12, wherein the oxidant compnses sodium peπodate 14 The process according to claim 12, wherein the solvent compnses a mixture of tetrahydrofuran, heptane and toluene 15 A process for prepaπng phosphodiester compounds compnsing the steps of (a) reacting a 4,4-dιphenylcyclohexanol compound with PC1 3 to obtain 4,4- diphenylcyclohexyloxy dichlorophosphine having the formula (b) coupling the 4,4-dιphenylcyclohexyloxy-dιchlorophosphme formed in step (a) with an amine base to obtain 4,4-diphenylcyclohexyloxydιamιnophosphιne having the formula (c) coupling of the 4,4-diphenylcyclohexyloxy-dιamιnophosphme formed in step (b) with hydroxymethyl-DTPA penta tert-butyl ester to obtam 4,4- diphenylcyclohexyloxy (hydroxymethyl-DTPA -oxy, penta tert-butyl ester)amino- phosphino having the formula: (d) hydrolysis and oxidation of the 4,4-diphenylcyclohexyloxy (hydroxymethyl-DTPA oxy, penta tert butyl ester)aminophosphino formed in step (c) with dilute HC1 and an oxidant to form [(4,4- diphenylcyclohexyl)phosphonooxymethyl]diethylene triamine, penta t-butyl ester having the formula: 16. A process for preparing [(4,4- diphenylcyclohexyl)phosphooxymethyl] diethylene triaminepenta-acetic acid comprising the steps of: (a) phosphorylating 1 0 equivalents of 4,4-diphenylcyclohexanoι with about one equivalent of phosphorous trichloride to obtain 4,4-diphenylcyclohexyloxy dichlorophosphine having the formula' (b) coupling the 4,4-diphenylcyclohexyloxy-dichlorophosphine formed in step (a) with from about 5 to about 6 equivalents of imidazole to obtain 4,4- diphenylcyclohexyloxydiimidophosphine having the formula: (c) coupling of the 4,4-diphenylcyclohexyloxy-diimidophosphine formed in step (b) with from about 0 75 to about 1 0 equivalents of hydroxymethyl-DTPA penta tert-butyl ester to obtain 4,4-diphenylcyclohexyloxy (hydroxymethyl-DTPA -oxy, penta tert-butyl ester)imido-phosphino having the formula. (d) hydrolysis and oxidation of the 4,4-diphenylcyclohexyloxy (hydroxymethyl-DTPA oxy, penta tert butyl ester)imidophosphino formed in step (c) with dilute HC1 and from about 0.5 to about 2 0 equivalents to sodium periodate to form [(4,4-diphenylcyclohexyl)phosphonooxymethyl]diethylene triamine, penta t-butyl ester having the formula. 17 The process according to claim 16, further comprising the step of hydrolysis of the [(4,4-diphenylcyclohexyl) phosphonooxymethyl] diethylenetriamine, penta t-butyl ester formed in step (d) in HO to form [(4,4- diphenylcyclohexyl) phosphonooxymethyljdiethylene triaminepentaacetic acid having the formula

Inventors: AMEDIO JOHN C, BERNARD PAUL J, FOUNTAIN MARK, AMEDIO, JOHN, C., BERNARD, PAUL, J., FOUNTAIN, MARK
Applicants: EPIX MEDICAL INC, EPIX MEDICAL, INC.
Application year: 1998
Main classification: C07F9
IPC codes: C07F9