WO2000058266A1

WO2000058266A1 - Divergent method for the synthesis of diethylene triaminepenta aceticacid esters

Info

Publication number: WO2000058266A1
Application number: PCT/US2000/007460
Authority: WO
Inventors: Samuel I. Achilefu; Ananthachari Srinivasan
Original assignee: Mallinckrodt Inc.
Priority date: 1999-03-26
Filing date: 2000-03-21
Publication date: 2000-10-05
Also published as: AU4017600A

Abstract

The synthesis of compounds of formula (X) is disclosed. These compounds are useful in the preparation of nuclear pharmaceuticals such as those used for tumor imaging or tumor therapy.

Description

DIVERGENT METHOD FOR THE SYNTHESIS OF DIETHYLENE TRIAMINEPENTA ACETICACID ESTERS

BACKGROUND OF THE INVENTION

This invention relates to the synthesis of esters of diethylenetriaminepentaacetic acid (DTP A) and of intermediates useful in the synthesis of such esters. These esters have the general formula:

(I)

wherein R, and R₂ are H, and R₃, R₄, and R₅ are t-butyl (compound (II)) or a similar protecting group. Such compounds are useful in the preparation of nuclear pharmaceuticals where they serve as a metal chelate and a link between a peptide and a radionuclide. Activation of the free dicarboxylic acid rapidly forms intramolecular acid anhydride which then reacts with the amino group on a peptide to form the DTPA-peptide conjugate. Acid mediated cleavage of the esters gives free tetra-carboxylic acid which readily forms stable metal complexes with the radionuclide of choice.

Using conventional techniques, these compounds are prepared by the following sequence (t-Bu = t-butyl):

(III) (IV)

Compound (IN) is commercially available. Compound (III) was prepared by the method taught in Rapoport, J. Org. Chem. 1993, 58, 1151-1158 (incorporated herein by reference). This reaction yields two compounds in about a 1 :4 ratio. The major product is the penta ester (compound (N)), (i.e.: compound (I) in which all of R,-R₅ = t-butyl). This product is useless and must be disposed of. The minor product is:

(VI)

Compound (NI) is reacted with the compound (Bz = benzyl):

(Nil) This compound is not commercially available, but may be synthesized by literature method as described by Rapoport (see the reference for compound (III) above).

The reaction product of (NI) and (Nil) is:

(VIII)

This compound is subjected to catalytic hydrogenation at room temperature to yield

(II) (i.e.: compound (I) where R, and R₂ are H and R₃, R₄, and R₅ are t-butyl).

The synthesis described above suffers from several problems, one being that compounds (III) and (Nil) are not commercially available, and have to be synthesized in several steps involving expensive reagents and purification processes, and may not be amenable to large-scale production.

Accordingly it would be desirable to have an alternative synthesis of these compounds.

US 5,618,513 (Mallinckrodt: A. Srinivasan) teaches a general method for using DTPA compounds in the preparation of radiopharmaceuticals. This reference also teaches the preparation of such compounds as outlined above.

US 4,479,930 (Univ. of Massachusetts: D. Hnatowich) teaches the use of a dicyclic dianhydride compound to couple polypeptides. Although useful, this method results in diaddition products which are not medically useful.

S. Ram and L. Spicer, Rapid Debenzylation of N-Benzylamino Derivatives to Amino-Derivatives Using Ammonium Formate as Catalytic Hydrogen Transfer Agent, Tetrahedron Letters, Vol. 28, No. 5, pp 515-516, 1987, teaches the deprotection of various N-benzyl compounds using ammonium formate as the hydrogen source.

S. Ram and L. Spicer, Debenzylation of N-Benzylamino Derivatives by Catalytic Transfer Hydrogenation with Ammonium Formate, Synthetic Communication, 17(4), 415-418 (1987), is similar to the first Ram and Spicer reference. C. Grote, D. Kim, and H. Rapoport, Stereocontrolled Synthesis of DTPA Analogues Branched in the Ethylene Unit, J. Org. Chem., 1995, 60, 6987-6997, teaches a synthesis similar to that outlined above, with the addition of stereo control of the reaction.

M. Brechbiel and O. Gansow, Backbone-Substituted DTPA Ligands for ⁹⁰Y Radioimmunotherapy, Bioconjugate Chem. 1991, 2, 187-194, teaches the synthesis of new bifunctional DTPA ligands.

SUMMARY OF THE INVENTION

Briefly, the invention comprises a method of synthesis of compounds of the formula

(X)

DETAILED DESCRIPTION OF THE INVENTION

In this specification and claims, numerical values and ranges are not critical unless otherwise stated. That is, the numerical values and ranges may be read as if they were prefaced with the word "about" or "substantially".

The invention includes a method for synthesizing compounds of the formula:

(X) wherein Rg and R₈ are linking moieties each having 1 to 10, desirably 1 to 6, preferably 1 to 4, and most preferably 2 carbon atoms; and each R₄ is an alkyl group having 1 to 15, desirably 2 to 10, more desirably 2 to 8, preferably 3 to 6, and more preferably 4 carbon atoms, and most preferably being t-butyl.

The synthesis begins with the following compound:

(XIV)

This compound can be made by the conventional methods mentioned above in the

Background, or can be made by the following sequence: H₂N R₆ NH ^ X^{' ^}COOR₄

(XI) (XII) wherein R₄, R , are as defined above; R₉ is a removable protecting group, desirably benzyl, trifluoroacetyl, methoxy benzyl, chlorotrityl, or nitrobenzyl, preferably benzyl, trifluoroacetyl, or methoxy benzyl, and most preferably benzyl; and X is a group that will react with the amines of compound (XI), desirably a halogen or a mesylate or a triflate, more desirably a halogen, preferably CI or Br, and most preferably Br. This reaction produces:

(XIII)

When subjected to selective removal of the R₉ group (for instance, by hydrogenation), the compound becomes:

(XIV)

Compound (XIV) is known from the conventional synthesis described in the Background of the Invention, above. However, in the reaction sequence just shown, it is produced as essentially the sole product, rather than as the minor product.

The instant invention uses compound (XIV) as a raw material and reacts it with a compound of the formula D R₈ NE (XXI)

Wherein R₈ is as defined above; D is a group that will react with the secondary amine of compound (XIV), desirably a halogen, triflate, mesylate, or tosylate, preferably a halogen, and most preferably Br; and E is a removable protecting group that can be removed to leave its associated nitrogen as a primary nitrogen, desirably dibenzyl, trifluoroacetyl, phthalimide, benzyloxycarbonyl, di-p- methoxybenzene, or di-p-nitrobenzene, preferably dibenzyl or trifluoroacetyl, and most preferably dibenzyl.

The reaction of compounds (XIV) and (XXI) yields:

(XXII)

Selective removal of the protecting group E (for instance, by hydrogenation) yields:

(XXIII) This compound is reacted with a compound of the formula:

Di COOE-,

(XXIV) wherein D, is a group that will react with the primary amine of compound (XXIII), desirably a halogen, triflate, mesylate, or tosylate, preferably a halogen, and most preferably Br; and E, is hydrogen or a removable protecting group, desirably benzyl, allyl, p-methoxybenzyl, or p-nitrobenzyl, and most preferably benzyl.

This reaction yields the compound:

(XXV)

In the case where E, is hydrogen, then compound (XXV) is the same as compound (X). In the case there E, is other than hydrogen, compound (XXV) can be subjected to hydrogenation to replace both E, moieties with hydrogens, to yield compound (X).

This new procedure has several advantages over the prior art. Some of these include the elimination of the need to synthesize and purify several intermediate products because all of the starting materials for this new procedure are commercially available. It further reduces the time and cost of production of the desired non-symmetrical DTPA derivatives. This new procedure is also flexible, allowing for the selective removal of one benzyl group (compound (XXI), where E is dibenzyl) and the incorporation of other functional reactive radicals or by selective substitution of the active amide hydrogen when E is trifluoroacetyl in compound (XXI). This method is also amenable to large-scale production and it is applicable to the production of a variety of DTPA derivatives. EXAMPLE 1

Synthesis of N-benzyl-N,N',N'-tris(t-butyloxycarbonylmethyl) ethylenediamine (Compound (XIII) where R₄ = t-butyl, R₆ = ethyl, and R₉ = benzyl). N-Benzylethylenediamine (Compound (XI) where R_ = ethyl and R₉ = benzyl) (5g, 33.28 mmol) and potassium bicarbonate (19.3g, 139.7 mmol) were added to 200 ml of anhydrous acetonitrile and stirred vigorously under argon, t- Butyl bromoacetate (Compound (XII) where X = Br and R₄ = t-butyl) was diluted in 30 ml of anhydrous acetonitrile and the solution was added drop wise to the reaction mixture over 90 minutes. The progress of the reaction was monitored by thin-layer chromatography and was essentially complete in about 4 hours but was stirred at room temperature for about 12 hours in order to assure complete alkylation of the amine. The insoluble residue was filtered and washed with acetonitrile. The filtrate was evaporated to give 20g of a yellow liquid. Hexane (100 ml) was added to the crude mixture and stirred vigorously until white precipitate formed. The precipitate was filtered and the filtrate was evaporated to give a yellow liquid. The pure compound was obtained by washing the crude product over dry flash chromatographic column and the desired compound was eluted with 10% diethyl ether in hexane (12.5 g, 80% yield).

EXAMPLE 2

Synthesis of N,N',N'-tris(t-butyloxycarbonylmethyl)ethyienediamine

(Compound (VI) or Compound (XIV) where R₄ = t-butyl and Rg = ethyl). N- benzyl-N,N'N'-tris(t-butyloxycarbonylmethyl) ethylenediamine (from Example 2) (6 g, 12 mmol) was added to a heterogeneous mixture of 10% palladium on carbon (6 g, 1 weight equivalent) in 100 ml of methanol. Anhydrous ammonium formate (3.8 g, 60.26 mmol) was added to the reaction mixture in one bulk. The mixture was stirred at room temperature for 2 hours. The mixture was filtered over celite and the residue was washed with chloroform. The filtrate was evaporated until white precipitates began to form. The residue was triturated in chloroform and the insoluble formate was filtered. Evaporation of the filtrate gave a pale yellow liquid (4.6 g, 96% yield) which was identified as the pure compound by NMR analysis.

EXAMPLE 3 Synthesis of N",N"-bis(benzyl)-N,N',N'-tris(t-butyloxycarbonylmethyl) diethylenetriamine (Compound (XXII) where, R₄ = t-butyl, R₆ R₈ = ethyl, E = dibenzyl). A mixture of N,N'N'-tris(t-butyloxycarbonylmethyl) ethylenediamine (4.4 g, 9.82 mmol) and N(2-bromoethyl)dibenzylamine (3.9 g, 12.82 mmol), diisopropylethylamine (3.8 g, 29.45 mmol) and 100 ml of acetonitrile was refluxed under argon for 24 hours. After the reaction was complete, the solvent was evaporated and the residue was partitioned between dichloromethane (100 ml) and distilled water (100 ml). The organic layer was washed with 100 ml of water and 100 ml of brine. It was dried over magnesium sulfate and the crude product was purified by dry flash chromatography.

EXAMPLE 4

Synthesis of N,N',N'-tris(t-butyloxycarbonylmethyl)diethylenetriamine (Compound (XXIII) where R₄ = t-butyl and ^, R₈ = ethyl). N",N"-bis(benzyl)- N,N',N'-tris(t-butyloxycarbonylmethyl)diethylenetriamine (from Example 3) (8 g, 12.78 mmol) was added to a heterogeneous mixture of 10% palladium on activated carbon (8 g, 1 weight equivalent) in 100 ml of methanol. Anhydrous ammonium formate (3.8 g, 60.26 mmol) was added to the reaction mixture in one bulk. The mixture was stirred at room temperature for 8 hours. The mixture was filtered over celite and the residue was washed with chloroform. The filtrate was evaporated until white precipitates began to form. The residue was triturated in chloroform and the insoluble formate was filtered. The crude product obtained after evaporation of the filtrate is used in the next reaction without further purification.

EXAMPLE 5 Preparation of N",N"-bis(benzyloxycarbonylmethyl)-N,N',N'-tris(t- butyloxycarbonylmethyl) diethylenetriamine (Compound (XXIII) where R₄ = t-butyl, Rg, R₈ = ethyl and E, = benzyl). N,N',N'-tris(t-butyloxycarbonylmethyl) diethylenetriamine (from example 4) (5 g, 11.22 mmol), and potassium bicarbonate (3.9 g, 28.05 mmol) were added to 100 ml of anhydrous acetonitrile and stirred vigorously under argon. Benzyl bromoacetate (5.7 g, 24.88 mmol) was diluted in 20 ml of anhydrous acetonitrile and the solution was added dropwise to the reaction mixture over 15 minutes. The progress of the reaction was monitored by thin-layer chromatography and was essentially complete in about 3 hours but was stirred at room temperature for about 12 hours in order to assure complete alkylation of the amine. The insoluble residue was filtered and washed with acetonitrile. The filtrate was evaporated to give an orange oil which was purified by dry flash chromatography, eluting the product with 30% diethyl ether in hexane.

EXAMPLE 5

Preparation of N,N',N'-tris(t-butyIoxycarbonylmethyl)-N",N"-bis(acetic acid)diethylenetriamine ((Compound (X) where R₄ = t-butyl, R₆, R₈ = ethyl and Ej = H)). A mixture of 10% palladium on carbon (0.21 g) and a solution of N",N"-bis(benzyloxycarbonylmethyl)-N,N',N'-tris(t-butyloxycarbonylmethyl) diethylenetriamine (3.3 g, 4.45 mmol) in 50 ml of methanol was hydrogenolyzed at 40 psi for 2 hours. The mixture was filtered over celite and the residue was washed with methanol. The solvent was evaporated to give an off-white powder which was shown by mass spectral analysis, HPLC and NMR to be the pure compound (2.4 g, 96% yield).

EXAMPLE 7

Synthesis of DTPA-Octreotate derivative. The DTPA-Octreotate conjugate was prepared by solid phase synthesis using pre-loaded fluorenemethoxycarbonyl- threonine (Fmoc-Thr) Wang resin on 0.025 mmol scale. Commercially available automated peptide synthesizer from Applied Biosystems (Model 432A SYNERGY Peptide Synthesizer) was used. Cartridges containing Fmoc-protected amino acids were used in the solid phase synthesis. Cysteines were protected with acetamidomethyl group. Coupling reaction was carried out with 0.075 mmol of the protected amino acid and 2-(lH-benzotriazole-lyl)-l,l,3,3-tetramethyluronium hexafluorophosphate (HBTU)/N-hydroxybenzotriazole (HOBT). The amino acids and tri-t-butyl DTPA (compound II) cartridges were placed on the peptide synthesizer and the product was synthesized from the C-terminal to the N-terminal position. After the synthesis was completed, the product was cleaved from the solid support with a cleavage mixture containing trifluoroacetic acid (85%):water (5%):phenol (5%):thioanisole (5%) for 6 hours. Note that the t-butyl esters of tri-t- butyl DTPA were also cleaved to give the free tetra-carboxylic acid. The DTPA- peptide conjugate was precipitated with t-butyl methyl ether and lyophilized with water : acetonitrile (2/3) mixture. Mass spectral analysis indicated that only the mono peptide-DTPA conjugate was obtained in accordance with the following sequence: DTPA-D-Phe-Cys(Acm)-Tyr-D-Trp-Lys-The-Cys(Acm)-Thr.

Claims

What is claimed is:

1. A method of synthesizing a compound of the formula:

(X) wherein each R₄ is an alkyl group having 1 to 15 carbon atoms, and Rg and R₈ are linking moieties each having 1 to 10 carbon atoms, comprising

(a) reacting together a compound of the formula:

(XIV) wherein R₄ and Rg are as defined above, with a compound of the formula: D R₈ NE

(XXI) Wherein R₈ is as defined above; D is a group that will react with the secondary amine of compound (XIV), and E is a removable protecting group that can be removed to leave its associated nitrogen as a primary nitrogen, to produce a compound of the formula:

(XXII) (b) selectively removing E from compound (XXII) to produce:

(XXIII)

(c) reacting compound (XXIII) with a compound of the formula:

Di COOE-,

(XXIV) wherein D, is a group that will react with the primary amine of compound

(XXIII), and E, is hydrogen or a removable protecting group, to produce a compound of the formula:

(XXV) and;

(d) if E, is other than hydrogen, removing both E, groups to produce compound (X).

2. The method of claim 1 wherein each R₄ has 2 to 8 carbon atoms; Rg and R_g each have 1 to 6 carbon atoms, D is a halogen, triflate, mesylate, or a tosylate; E is dibenzyl, trifluoracetyl, phthalimide, benzyloxycarbonyl, di-p-methoxybenzene, or di-p-nitrobenzene; D, is a halogen, triflate, mesylate, or tosylate; and E, is hydrogen, benzyl, allyl, p-methoxybenzyl, or p-nitrobenzyl.

3. The method of claim 1 wherein each R₄ has 3 to 6 carbon atoms; R₆ and R₈ each have 1 to 4 carbon atoms, D is a halogen; E is dibenzyl, or trifluoracetyl; D, is a halogen; and E, is benzyl.

4. The method of claim 1 wherein each R₄ is t-butyl; R^ and R₈ each are ethyl; D is Br; E is dibenzyl,; D, is a Br; and E, is benzyl.

5. The method of claim 1 wherein E_ is an alkyl group having 2 to 8 carbon atoms and R₄ is benzyl, trifluoroacetyl, methoxy benzyl, chlorotrityl, or nitrobenzyl.

6. The method of claim 5 wherein E, is t-butyl and R₄ is benzyl.