# Patent

## Title
High purity phthalein derivatives and method for preparing same

## Abstract

The invention concerns high purity phthalcin derivatives enabling their use for medical applications or in the field of biotechnology, as well as their preparation method whereby a phthatic anhydride derivative is condensed with a naphthol or phenol derivative in an organic acid ester and the crystals of the resulting condensate are converted by action of a strong acid or one of its precursors in anhydrous medium.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is the National Stage of International Application No. PCT/FR2003/003205, filed Oct. 28, 2003, which claims the benefit of France Patent Application Serial No. 02/13,528, filed on Oct. 29, 2002. The contents of both applications are hereby incorporated by reference in their entireties.
FIELD OF THE INVENTION
The present invention relates to high purity phthaleins and to the method for preparing them. It relates more particularly to high purity fluorescein.
In the present description, the term “high purity phthalein” is intended to mean a phthalein containing at most 1% by weight, preferably at most 0.5% by weight, of impurities.
BACKGROUND OF THE INVENTION
Phthaleins are molecules having the following xanthene unit:




These products are useful as dyes in various industries, in particular the textile industry, the paper industry, printing, reprography, the food industry, the cosmetics industry and the pharmaceutical industry. Phthaleins are, currently, the subject of a considerable resurgence of interest in the health field, for their diagnostic use, in particular in the context of medical imaging and in the field of the biotechnology for labeling biological molecules (nucleic acids, proteins, lipoproteins, membrane lipids) and following the intracellular or extracellular biochemical activity of biological molecules.
For example, fluorescein is a phthalein commonly used in ophthalmology for performing retinal angiography by fluorescence. The diagnostic advantage of fluorescein angiography is currently increased by the appearance of new medicinal products for treating vascular pathologies of the retina and of the choroid and the availability of a new generation of retinographs that make it possible to perform digital fluorescence imaging that has higher performance levels and gives greater resolution than the former systems of acquisition on photographic emulsions.
In parallel, the quality and safety requirements of the new international pharmaceutical standards (ICH: International Commission of Harmonization, ICH Q Topic Q3A 1999) have considerably increased. The same is true with regard to the use of phthaleins in biotechnologies that require reagents of increasingly high quality. In order to satisfy the current requirements, the level of purity of the phthaleins used in the pharmaceutical field or that of biotechnology must necessarily be very high. By way of example, in the publication “Effective differences in the formulation of intravenous fluorescein and related side effects”, Am. J. Ophthalmol. 78, 2: 217-221, 1974, L. Yannuzzi showed a correlation between the purity of phthaleins, in particular that of fluorescein, and the tolerance of these substances when they are administered to humans by injection.
SUMMARY OF THE INVENTION
The high purity phthaleins according to the invention have the general formula (I):




in which R1, R2, R3, R4 and R5, which are identical to or different from one another, are chosen from the group comprising the following radicals or groups: hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, preferably hexoses or pentoses, oligosides and polyosides, polypeptides, proteins and phospholipids,

R1 and R5 are not hydrogen when R1 is a group —CH2—CH2—COOH, R2 is a hydroxyl group and R4 is a group —COOH,

these phthaleins containing no more than 1% by weight, preferably no more than 0.5% by weight, and even more preferably not more than 0.2% by weight, of residual impurities.

A phthalein that is particularly advantageous, in particular for ophthalmic applications, is fluorescein having such a purity.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
It is known practice to prepare the phthaleins of formula (I) by the condensation of a phthalic anhydride derivative with a phenol derivative having a free ortho-position with respect to a hydroxyl group.
This condensation is carried out by heating at the melting temperature of a mixture of the phthalic anhydride and of the phenol derivative, in the desired proportions.
This condensation can optionally be carried out in a dilution solvent. It can also be carried out in the presence of a catalyst.
In the absence of solvent, during the heating, the reaction medium rapidly thickens and has a tendency to harden. Within the reaction medium, areas where the temperature is too high and areas where the temperature is not high enough then form.
In the areas where the temperature is too high, the reactants and/or the reaction product are degraded and, in the areas where the temperature is not high enough, the reaction is not complete. The product obtained is of mediocre quality since it contains by-products that are very difficult to remove.
In order to improve this known method, the addition of an inert solvent or the use of a catalyst has been considered.
Thus, U.S. Pat. No. 1,931,049 describes the addition to the reaction medium of an inert solvent consisting of a benzene-based or aliphatic hydrocarbon, more particularly ortho-dichlorobenzene. However, the condensation reaction is then incomplete, and generate intermediate products that are subsequently difficult to remove. Therefore the method described in U.S. Pat. No. 1,931,049 does not make it possible to obtain a high purity phthalein according to the definition of the present application. Moreover, higher aliphatic hydrocarbons are not miscible with the reaction medium; consequently, they introduce no improvement in terms of thermal transfer and impair the elimination of the water formed, thereby slowing down the reaction, which takes place exclusively in a hydrophobic medium.
Regarding the catalysts that are used to improve the reaction yield, these are concentrated sulfuric acid, anhydrous zinc chloride and tin chloride.
The German patent DE 360691, describes the use of an aromatic sulfonic acid as a catalyst, in particular benzenesulfonic acid, alone or combined with one of the abovementioned three catalysts.
It so happens, however, that the addition of these catalysts leads to a reaction product that sets and hardens with entrapment of impurities, which can no longer be eliminated from the desired product.
In order to eliminate both the generated by-products and the impurities, isolation and purification methods have been developed, but none have made it possible to substantially improve the quality of phthaleins.
A conventional method consists in basifying the phthalein in an aqueous medium so as to dissolve it, and then in acidifying it so as to allow it to precipitate. These two steps are repeated successively in an attempt to eliminate the impurities. However, this method does not bring about a noticeable improvement in the purity of the product since, while the impurities dissolve with the phthalein during the basifying step, they precipitate again with the phthalein during the acidification step. In addition, this purification method has the drawback of producing a considerable amount of salts which are difficult and expensive to subsequently eliminate.
U.S. Pat. No. 1,965,842 describes the purification of phthaleins, derived from hydroxybenzene, by direct extraction of the by-products with dichlorobenzene alone or in a mixture with other solvents. This direct extraction of the crude product with the solvent in question does not, however, make it possible to obtain extensive elimination of impurities, which remain partly trapped in the phthalein crystals.
None of the purification methods described in the prior art makes it possible to reach a level of purity that is sufficient to allow the pharmaceutical use of phthaleins.
SUMMARY OF THE INVENTION
Given the advantage of these molecules for medical diagnosis, the production of high purity phthaleins is a real medical need and would satisfy a high demand, in particular for pharmaceutical applications in ophthalmology, for diagnosis, in particular in medical imaging, or in the field of biotechnological applications (for example, as dye to label molecules).
The inventors have, to their credit, at the end of extensive research, found that it is possible to obtain high purity products by the condensation of a phthalic anhydride with a phenol or naphthol derivative in a specific solvent which consists of an organic acid ester.
They have also found that the use of organic acid esters as solvents makes it possible to carry out this condensation with an excellent yield, greater than 75%.
Specifically, these solvents have the particularity

    
    
        firstly, of leading to specific crystallization of the phthalein that derives from the condensation and of excluding, from the generated crystals, all the impurities that remain dissolved in the reaction medium, and
        secondly, of allowing a complete condensation that is predominant, to detrimental and unwanted side reactions, which results in the total consumption of the reactants and minimizes the formation of by-products.
    
    


The method in accordance with the invention thus makes it possible to obtain high purity phthaleins with a yield that is satisfactory from an industrial point of view.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a radiocrystallography spectrum of crude fluorescein.
FIG. 2 is a radiocrystallography spectrum of the fluorescein after conversion with acid (yellow).
FIG. 3 is a radiocrystallography spectrum of the crude 4′,5′-dimethyifluorescein.
FIG. 4 is a radiocrystallography of the 4′,5′-dimethyifluorescein after conversion with acid.
FIG. 5 is a radiocrystallography spectrum of the crude 4′,5′-dihydroxyfluorescein.
FIG. 6 is a radiocrystallography spectrum of the 4′,5′, dihydroxyfluorescein after conversion with acid.



DETAILED DESCRIPTION OF THE INVENTION
More particularly, the invention is related to a method for preparing phthaleins, from which the residual impurities have been removed, having general formula (I):




in which R1, R2, R3, R4 and R5, which are identical to or different from one another, are chosen from the group comprising the following radicals or groups: hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, preferably hexoses or pentoses, oligosides and polyosides, polypeptides, proteins and phospholipids,

R1 and R5 are not representing hydrogen when R1 is a group —CH2—CH2—COOH, R2 is a hydroxyl group and R4 is a group —COOH,

by condensation of a phthalic anhydride derivative of formula (II) with a phenol or naphthol compound of formula (III)





in which R1, R2, R3, R4 and R5 have the same meanings as above,

in a solvent consisting of an organic acid ester.

Advantageously, the starting compound (III), which is condensed with the phthalic anhydride (II), is chosen from the group comprising in particular resorcinol, orcinol, naphthol, pyrogallol, alkylaminophenol and arylaminophenol.
When resorcinol is used as starting product, the method in accordance with the invention makes it possible to prepare fluorescein.
Advantageously, the solvent used in the method in accordance with the invention is an organic acid ester of formula (IV)

R6—COOR7  (IV)

in which R6 is chosen from the group comprising the following radicals or groups: hydrogen, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, and alkyloxy having from 1 to 30 carbon atoms, R7 representing one of the following groups: alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, or alkyloxy having from 1 to 30 carbon atoms.

Particularly advantageously, the abovementioned solvent is chosen from the group comprising methyl, ethyl, propyl or butyl benzoates, methyl, ethyl, propyl or butyl heptanoates, methyl, ethyl, propyl or butyl octanoates, methyl, ethyl, propyl or butyl laurates, methyl, ethyl, propyl or butyl myristates and methyl, ethyl, propyl or butyl palmitates, and mixtures thereof.
The solvent is chosen according to its boiling point, so as to make it possible to carry out the condensation reaction at a temperature of between 150° C. and 250° C.
The condensation reaction can be carried out at atmospheric pressure or under a pressure that is adjusted according to the difference that exists between the temperature corresponding to the boiling point of the solvent and the temperature necessary to carry out the reaction, in particular under a pressure greater than atmospheric pressure when the boiling point of the compound (IV) is lower than the reaction temperature.
The condensation reaction can be carried out in the presence of a catalyst chosen from the group comprising in particular Lewis acids, such as ZnCl2 or AlCl3, Brönsted acids such as H2SO4, polyphosphoric acid, or salts thereof.
Advantageously, the catalyst used is an alkali metal salt of hydrogen sulfate. Particularly advantageously, the catalyst used is potassium hydrogen sulfate (KHSO4) or sodium hydrogen sulfate. The use of hydrogen sulfate as catalyst makes it possible to obtain an excellent yield for the condensation reaction and has the advantage, unlike other catalysts, of making it possible to obtain complete condensation of the reactants, of being able to be completely eliminated from the phthalein obtained, and of not inducing the formation of tars in the reaction medium.
At the end of the condensation reaction, a crude product is obtained, the organic purity of which is already much higher than that of the products obtained with the methods of the prior art, since it is equal to or greater than 95%.
However, this purity is not yet sufficient to allow pharmaceutical use, in particular by injection. Furthermore, as shown above, the purification methods described in the prior art do not result in a process that makes it possible to notably improve this purity.
At the end of extensive research, surprisingly and unexpectedly, the inventors have found a method that considerably increases the purity of crude phthaleins resulting from the condensation reaction, by treating them with a strong acid or one of its precursors in an anhydrous organic medium.
According to an advantageous embodiment, the method in accordance with the invention consequently comprises, after the condensation reaction, a step consisting in acidifying the product resulting from the condensation, in an anhydrous organic medium, by addition of a strong acid or one of its precursors, chosen from the group comprising in particular sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P2O5), and mixtures thereof. This acidification is carried out until the phthalein crystals resulting from the condensation are converted to phthalein crystals having a different structure.
The effect of this acidification step is to convert the crude phthalein that is insoluble and dispersed in the organic medium into a form that has a different crystalline structure and is slightly soluble in this same medium. This conversion is very rapid, and takes place via an intermediate solubilization phase during which the impurities are released and completely eliminated from the phthalein crystals. This new purification method is very advantageous since it requires very little solvent and yields to a very high purity in a very short time.
According to the method of the present invention, this purification step is carried out by dispersing the crude phthalein resulting from the condensation, in an anhydrous solvent, preferably in an alcohol, a ketone, an ether or a halogenated solvent, that is either used alone or as a mixture, even more preferably in absolute ethanol or acetone, alone or as a mixture.
The dispersion of crude phthalein thus obtained is acidified by the addition of a strong acid or one of its precursors, chosen from the group comprising in particular sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P2O5), and mixtures thereof.
According to a particularly advantageous embodiment, the acidification is carried out either by sparging gaseous hydrochloric acid into the phthalein dispersion, or by adding hydrochloric acid dissolved beforehand in an organic solvent.
According to another advantageous embodiment of the method in accordance with the invention, the product obtained after acidification is washed with a washing solution chosen from the group comprising water, polar solvents such as alcohols or ketones, or slightly polar solvents such as ethers and halogenated solvents, used pure or as a mixture.
At the end of the purification step, the phthaleins thus treated have a purity of greater than 98%, preferably greater than 99%, more preferably greater than 99.5%, and even more preferably of 99.8%.
The phthaleins thus obtained have the suitable quality to prepare other phthaleins of formula (I) by chemical modification of the groups R1, R2, R3, R4 and R5 according to the conventional methods of the art, in particular to obtain phthaleins that can be used for labeling biological molecules (nucleic acids, proteins, lipoproteins, membrane lipids) and in the field of biotechnological applications (for example, for the labeling of molecules and of their intracellular or extracellular biochemical activity).
The invention is particularly suitable for preparing very high purity fluorescein, i.e. such that its content of each of the by-products of the reaction is less than or equal to 0.2%, and preferably less than or equal to 0.1%, the sum of the contents of each of these by-products being less than or equal to 0.5%.
The method for preparing a fluorescein having the abovementioned purity comprises the following successive steps:

    
    
        condensing phthalic anhydride with resorcinol, in a solvent which is an ester of an aliphatic or aromatic organic acid, preferably methyl benzoate, in the presence of a catalyst chosen from the group of Brönsted acids,
        suspending the red-colored crystals obtained in the preceding step in an anhydrous solvent chosen from the group comprising alcohols such as absolute ethanol, ketones such as acetone, ethers, halogenated solvents, or mixtures thereof,
        acidifying the suspension obtained, by the addition of a strong acid or one of its precursors, chosen from the group comprising sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P2O5), or mixtures thereof, until yellow-colored crystals are obtained,
        washing the crystals thus obtained with a washing solution chosen from the group comprising water, alcohols, ketones, ethers and halogenated solvents, pure or as a mixture, until red-colored crystals are obtained.
    
    


An X-ray diffraction analysis of the red-colored fluorescein crystals is represented in FIG. 1; an X-ray diffraction analysis of the yellow-colored fluorescein crystals is represented in FIG. 2; these X-ray-diffraction analysis being obtained on the following equipment: Philips 1729 generator, Philips 1050 goniometer, Cu Kα radiation, Gonio acquisition software and Rayon processing software, under the following operating conditions:
















Voltage
40 mV



Intensity
40 mA



Number of points
4000



Number of passages
10



Acquisition period
250 ms



Start angle (°θ)
3.000



End angle (°θ)
23.000



Standard
silicon










According to an advantageous embodiment of the method for preparing fluorescein, the catalyst used for the condensation reaction is the hydrogen sulfate of an alkali metal, preferably potassium hydrogen sulfate or sodium hydrogen sulfate.
According to another advantageous embodiment of the method for preparing fluorescein, the acidification is carried out by sparging gaseous hydrochloric acid into the fluorescein suspension or by the action, on this fluorescein, of hydrochloric acid in solution in an anhydrous organic solvent, preferably chosen from the group comprising alcohols, ketones, ethers and halogenated solvents, used alone or as a mixture, even more preferably isopropanol, absolute ethanol or acetone, pure or as a mixture.
Advantageously, the washing step is carried out with a mixture of water, of ethanol and of acetone.
By means of this method in accordance with the invention, it is possible to prepare fluorescein having a purity of greater than or equal to 99.7%, which provides unquestionable advantages, in particular for pharmaceutical uses in diagnosis, especially in medical imaging or else in the field of biotechnological applications.
The inventors have demonstrated a novel crystallographic form of fluorescein being yellow-colored crystals. This novel crystallographic form is identified by the X-ray diffraction analysis given in FIG. 2, which was determined under the conditions mentioned above.
They have also demonstrated a novel crystallographic form of the following compounds: 4′,5′-dihydroxyfluorescein and 4′,5′-dimethylfluorescein.
Thus, the invention relates to the yellow-colored fluorescein crystals for which the X-ray diffraction analysis is given in FIG. 2.
The invention also relates to the yellow-colored 4′,5′-dihydroxyfluorescein crystals for which the X-ray diffraction analysis is given in FIG. 4.
It also relates to the dark red-colored 4′,5′-dimethylfluorescein crystals for which the X-ray diffraction analysis is given in FIG. 6.
All these spectra were determined with the equipment and under the conditions mentioned above.
The invention will be described in more detail with the aid of the following examples which are not limiting but relate to advantageous embodiments.
EXAMPLES
Example 1
Preparation of High Purity Fluorescein
Synthesis of Fluorescein
A mixture comprising 55 g of resorcinol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of methyl benzoate is brought to 200° C. for 6 hours. After cooling, the red crystals of crude fluorescein are washed with acetone and dried.
Mass obtained=51.8 g (78%).
These crystals are analyzed by X-ray crystallography under the following operating conditions:
Equipment
Philips 1729 generator
Philips 1050 goniometer
Cu Kα radiation
Gonio acquisition software
Rayon processing software










Conditions


















Voltage
40 mV



Intensity
40 mA



Number of points
4000



Number of passages
10



Acquisition period
250 ms



Start angle (°θ)
3.000



End angle (°θ)
23.000



Standard
silicon










The radiocrystallogram of FIG. 1 is obtained, for which the peaks are identified below:
















Theta

%
Number



(degrees)
Distance (Å)
Intensities
of counts





















5.3600
8.2457
18.09
647



5.9500
7.4306
31.10
1112



6.7000
6.6020
36.30
1298



8.3750
5.2883
45.95
1643



9.1050
4.8675
10.79
386



9.4300
4.7012
14.09
504



9.9950
4.4379
8.03
287



10.9150
4.0678
9.73
348



11.6200
3.8241
16.41
587



11.7300
3.7888
5.40
193



12.4200
3.5813
2.82
101



13.2150
3.3694
100.00
3576



13.5050
3.2983
7.47
267



13.8850
3.2097
79.03
2826



14.9650
2.9828
5.62
201



15.3100
2.9172
3.91
140



15.7600
2.8359
11.69
418



16.0000
2.7945
8.67
310



16.9100
2.6481
4.28
153



17.4750
2.5650
3.97
142



17.8550
2.5122
3.80
136



18.2300
2.4622
2.52
90



18.5150
2.4256
2.96
106



18.8250
2.3871
5.03
180



19.0800
2.3563
3.27
117



20.4300
2.2066
3.05
109



20.6800
2.1811
4.53
162



20.9300
2.1562
3.08
110



21.1800
2.1319
2.38
85



22.9400
1.9762
4.53
162










Wavelength: 1.54051 Å
Purification of Crude Fluorescein
50 g of crude fluorescein obtained in the previous step are stirred into 1000 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of the red crystals to yellow crystals is obtained.
The yellow crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 2 is obtained, for which the peaks are identified below:
















Theta
Distances
%
Number of



(degrees)
(Å)
Intensities
counts





















3.3500
13.1813
8.55
56



4.0050
11.0283
47.18
309



4.8550
9.1010
12.98
85



5.2650
8.3940
9.01
59



5.4500
8.1099
11.45
75



5.7900
7.6352
52.98
347



6.3000
7.0193
16.95
111



6.7700
6.5340
17.86
117



7.4900
5.9090
17.56
115



8.0300
5.5140
69.77
457



8.4250
5.2572
33.13
217



8.7800
5.0462
8.40
55



9.1550
4.8411
15.88
104



9.3400
4.7461
42.44
278



9.6550
4.5926
34.66
227



9.8800
4.4891
24.43
160



10.1950
4.3518
57.71
378



10.5800
4.1951
58.02
380



10.7350
4.1352
38.17
250



11.1650
3.9779
28.85
189



11.4800
3.8701
30.23
198



11.7200
3.7919
59.85
392



12.1100
3.6716
26.41
173



12.3950
3.5884
58.47
383



12.6400
3.5200
100.00
655



12.9450
3.4384
30.84
202



13.1550
3.3845
56.64
371



13.8350
3.2211
14.66
96



14.0550
3.1717
24.12
158



14.3550
3.1068
36.34
238



14.6700
3.0415
42.90
281



15.1300
2.9511
42.90
281



15.7500
2.8377
18.47
121



15.9400
2.8047
18.17
119



16.2500
2.7526
15.42
101



16.7550
2.6719
17.71
116



16.8500
2.6573
19.39
127



17.2250
2.6011
18.17
119



17.7300
2.5293
15.73
103



18.4200
2.4377
10.23
67



18.5750
2.4180
10.23
67



19.2650
2.3345
15.88
104



19.5800
2.2984
10.84
71



19.8000
2.2739
11.91
78



20.1150
2.2397
13.28
87



20.6500
2.1841
14.81
97



21.1500
2.1348
11.30
74



21.2750
2.1228
10.08
66



21.8100
2.0732
7.94
52



22.2500
2.0342
9.16
60



22.6250
2.0022
9.62
63



22.9100
1.9786
9.01
59










Wavelength: 1.54051 Å
These crystals are filtered off and then washed with an ethanol/acetone/water (40/40/20) mixture. The washing turns the yellow fluorescein crystals to red fluorescein crystals, which have a higher purity.
Purity by HPLC: 99.8%
Example 2
Preparation of high purity 4′,5′-dimethylfluorescein
Synthesis of 4′,5′-dimethylfluorescein
A mixture comprising 62 g of 2-methylresorcinol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of ethyl laurate is brought to 200° C. for 3 hours. After cooling, the crude product is filtered and washed with acetone and then dried. The product obtained is a dark orange powder.
Mass obtained=49.7 g (69%)
These crystals are analyzed by X-ray crystallography in the same way as in example 1. The X-ray diffraction analysis of FIG. 3 is obtained, for which the peaks are identified below.
















Theta
Distances
%
Number of



(degrees)
(Å)
Intensities
counts





















3.5700
12.3700
7.38
81



5.0290
8.7938
45.49
499



6.6650
6.6365
62.26
683



6.7400
6.5630
60.07
659



7.1300
6.2057
87.24
957



7.4550
5.9366
20.97
230



8.3700
5.2915
6.56
72



8.9300
4.9621
30.63
336



9.0950
4.8728
10.12
111



9.3300
4.7511
21.24
233



9.4550
4.6889
19.05
209



10.0350
4.4204
12.12
133



10.3250
4.2975
18.87
207



10.6000
4.1873
11.49
126



10.8300
4.0994
39.65
435



11.2900
3.9344
12.49
137



11.4500
3.8801
10.85
119



11.7550
3.7808
28.17
309



11.9050
3.7339
52.42
575



12.2550
3.6288
79.95
877



12.7050
3.5023
8.11
89



13.2400
3.3631
21.15
232



13.4200
3.3188
100.00
1097



13.7100
3.2499
10.85
119



14.5250
3.0712
8.84
97



14.8350
3.0084
23.25
255



15.0550
2.9654
17.59
193



15.1550
2.9463
15.13
166



15.3650
2.9070
25.80
283



15.7050
2.8456
20.51
225



16.1900
2.7625
14.13
155



16.7550
2.6719
9.02
99



17.5400
2.5558
7.47
82



17.8850
2.5081
8.75
96



18.1050
2.4786
6.75
74



18.4900
2.4288
19.51
214



18.7950
2.3907
6.11
67



18.8900
2.3791
6.84
75



19.6800
2.2872
15.50
170



20.2100
2.2296
7.38
81



20.3950
2.2103
9.66
106



20.6800
2.1811
6.84
75



20.9950
2.1498
5.38
59



21.4650
2.1049
7.11
78



21.6500
2.0878
8.84
97



21.9350
2.0620
9.21
101



22.1250
2.0451
9.02
99



22.5300
2.0102
5.47
60



22.7850
1.9889
5.93
65










Wavelength: 1.54051 Å
Purification of 4′,5′-dimethylfluorescein
40 g of crude 4′,5′-dimethylfluorescein are added to 800 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of dark orange-colored crystals to yellow-colored crystals is obtained.
The yellow crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 4 is obtained, for which the peaks are identified below.
















Theta
Distances
%
Number of



(degrees)
(Å)
Intensities
counts





















3.7050
11.9199
22.22
172



4.6150
9.5732
45.61
353



4.9300
8.9629
46.77
362



6.3300
6.9861
8.53
66



6.6150
6.6864
6.20
48



7.3000
6.0619
19.51
151



7.4500
5.9405
32.56
252



7.6500
5.7861
26.61
206



8.1250
5.4499
64.21
497



8.6300
5.1332
22.48
174



9.5950
4.6211
6.59
51



10.4450
4.2487
41.34
320



10.8500
4.0919
10.21
79



11.3550
3.9122
14.60
113



11.7000
3.7983
9.17
71



11.7950
3.7682
9.30
72



12.2250
3.6375
23.13
179



12.4500
3.5728
58.79
455



12.7650
3.4861
19.90
154



12.8950
3.4515
18.60
144



13.1000
3.3984
100.00
774



13.6350
3.2675
67.44
522



14.1400
3.1530
30.75
238



14.5250
3.0712
14.08
109



14.8400
3.0074
22.74
176



15.1750
2.9425
19.38
150



15.3500
2.9098
30.23
234



15.6950
2.8474
20.93
162



16.0950
2.7784
7.24
56



16.2850
2.7468
7.49
58



16.5950
2.6969
8.01
62



16.9100
2.6481
14.08
109



17.3200
2.5873
8.66
67



17.5400
2.5558
7.88
61



17.9150
2.5040
13.31
103



18.3550
2.4460
9.69
75



18.4800
2.4300
10.59
82



18.8600
2.3828
15.37
119



19.2350
2.3380
9.43
73



19.4850
2.3092
7.75
60



20.2100
2.2296
14.99
116



20.7100
2.1781
7.11
55



20.8350
2.1656
6.59
51



21.0850
2.1411
6.33
49



21.5250
2.0993
12.92
100



21.6500
2.0878
8.79
68



21.8400
2.0705
6.98
54



22.0900
2.0482
10.34
80



226550
1.9997
7.11
55










Wavelength: 1.54051 Å
After filtration and recrystallization from an acetone/water mixture or washing in ethanol/acetone/water, the yellow-colored crystals turn to dark orange-colored crystals.
Example 3
Preparation of 4′,5′-dihydroxyfluorescein
Synthesis of 4′,5′-dihydroxyfluorescein
A mixture comprising 63 g of pyrogallol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of ethyl myristate is brought to 200° C. for 3 hours. After cooling, the crude product is filtered and washed with acetone and then dried. The product obtained is a grayish-brown- or anthracite-colored powder.
Mass obtained=43.5 g (59.7%)
The crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 5 is obtained, the peaks of which are identified below.
















Theta
Distances
%
Number



(degrees)
(Å)
Intensities
of counts





















3.6450
12.1158
73.08
980



5.9200
7.4681
8.13
109



6.7200
6.5824
13.35
179



7.3350
6.0331
10.29
138



8.5050
5.2081
11.78
158



9.2300
4.8021
15.59
209



9.7400
4.5529
18.20
244



10.3400
4.2914
21.40
287



10.9150
4.0678
6.71
90



11.2600
3.9447
8.80
118



12.2550
3.6288
18.64
250



12.5550
3.5434
12.75
171



13.2000
3.3731
11.86
159



13.7200
3.2476
100.00
1341



14.7150
3.0324
17.75
238



14.8600
3.0034
13.57
182



16.1600
2.7675
6.79
91



17.2550
2.5967
7.83
105



18.3550
2.4460
6.86
92










Wavelenth: 1.54051 Å
Purification of 4′,5′-dihydroxyfluorescein
40 g of crude 4′,5′-dihydroxyfluorescein are added to 800 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of grayish-brown- or anthracite-colored crystals to reddish-brown- or mahogany-colored crystals is obtained.
The crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of FIG. 6 is obtained, for which the peaks are identified below:
















Theta
Distances
%
Number of



(degrees)
(Å)
Intensities
counts





















3.2500
13.5865
90.53
526



3.5350
12.4923
18.76
109



3.7850
11.6683
11.70
68



3.9450
11.1958
13.25
77



4.1650
10.6053
15.15
88



4.5400
9.7310
10.33
60



4.6350
9.5319
10.15
59



6.3950
6.9154
9.29
54



6.5500
6.7525
11.53
67



6.8200
6.4863
27.71
161



7.1150
6.2187
10.50
61



7.4800
5.9168
21.51
125



7.8700
5.6253
12.91
75



8.2200
5.3874
27.88
162



8.7800
5.0462
41.48
241



9.2200
4.8073
10.15
59



9.5350
4.6499
11.70
68



9.7950
4.5276
42.17
245



10.1000
4.3923
17.21
100



10.5400
4.2108
16.70
97



10.8850
4.0789
16.52
96



11.6500
3.8144
25.13
146



11.8900
3.7385
13.60
79



12.0450
3.6911
13.08
76



12.3900
3.5898
16.52
96



12.5800
3.5365
14.80
86



12.8750
3.4568
27.37
159



12.9950
3.4254
25.82
150



13.2900
3.3507
67.64
393



13.5750
3.2816
27.02
157



13.7150
3.2488
27.02
157



14.1100
3.1596
100.00
581



14.6200
3.0516
15.66
91



14.7750
3.0203
18.24
106



15.0400
2.9683
25.13
146



15.3050
2.9181
24.78
144



15.6850
2.8491
16.35
95



15.9400
2.8047
11.02
64



16.2500
2.7526
10.67
62



16.5950
2.6969
12.22
71



17.1650
2.6099
9.81
57



17.5700
2.5516
8.78
51



17.7600
2.5252
9.81
57



18.0750
2.4826
11.36
66



18.5150
2.4256
8.43
49



18.7650
2.3944
13.08
76



19.0450
2.3605
11.36
66



20.1950
2.2312
50.09
291



20.6150
2.1877
9.12
53



21.0550
2.1440
12.56
73



21.4650
2.1049
9.29
54



21.6850
2.0846
9.98
58



22.0600
2.0509
9.81
57










Wavelength: 1.54051 Å
After filtration and washing in water, the reddish-brown- or mahogany-colored crystals turn to grayish-brown- or anthracite-colored crystals.

## Claims

1. A method for preparing phthaleins, wherein the residual impurities have been removed, having the general formula (I):



wherein R1, R2, R3, R4 and R5, which are identical to or different from one another, are selected from the group consisting of hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, oligosides and polyosides, polypeptides, proteins and phospholipids,
R3 and R5 are not hydrogen when R1 is a group —CH2-CH2—COOH, R2 is a hydroxyl group and R4 is a group —COCH,
wherein a phthalic anhydride derivative of formula (II) is condensed with a phenol or naphthol compound of formula (III)




in which R1, R2, R3, R4 and R5 have the same meanings as above, the condensation being carried out in a solvent consisting of an organic acid ester of formula IV


R6 —COOR7  (IV)


wherein R6 is selected from the group consisting of hydrogen, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, and alkyloxy having from 1 to 30 carbon atoms, R7 being selected from the group consisting of alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, or alkyloxy having from 1 to 30 carbon atoms.

2. The method as claimed in claim 1, wherein the compound of formula (Ill) is selected from the group consisting of resorcinol, orcinol, naphthol, pyrogallol, alkylaminophenol and arylaminophenol.

3. The method as claimed in claim 1, wherein the organic acid ester is selected from the group consisting of methyl benzoate, ethyl benzoate, propyl benzoate, butyl benzoate, methyl heptanoate, ethyl heptanoate, propyl heptanoate, butyl heptanoate, methyl octanoate, ethyl octanoate, propyl octanoate, butyl octanoate, methyl laurate, ethyl laurate, propyl laurate, butyl laurate, methyl myristate, ethyl myristate, propyl myristate, butyl myristate, methyl palmitate, ethyl palmitate, propyl palmitate, butyl palmitate, and mixtures thereof.

4. The method as claimed in claim 1, wherein the condensation reaction is carried out at between 150° C. and 250° C., optionally under pressure.

5. The method as claimed in claim 1, wherein the reaction is carried out in the presence of a catalyst selected from the group consisting of Lewis acids, such as ZnCI2 or AICI3, Brödnsted acids such as H2SO4 or polyphosphoric acid.

6. The method as claimed in claim 5, wherein the catalyst is an alkali metal hydrogen sulfate.

7. The method as claimed in claim 6, wherein the catalyst is potassium hydrogen sulfate (KHSO4) or sodium hydrogen sulfate (NaHSO4 ).

8. A method for acidifying the product resulting from the condensation of a phthalic anhydride derivative of formula (II) with a phenol or naphthol compound of formula (Ill), the formulae (II) and (III) being those of claim 1, wherein the reaction is carried out in an anhydrous organic medium, by the addition of a strong acid or one of its precursors, selected from the group consisting of sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P2O5), and mixtures thereof, the acidification being carried out until the phthalein crystals resulting from the condensation are converted to phthalein crystals having a different structure.

9. The method as claimed in claim 8, comprising a step consisting in washing the product obtained after acidification, said washing step being carried out with a washing solution selected from the group consisting of water, alcohols, ketones, ethers and halogenated solvents, pure or as a mixture, until the crystals are reconverted to the structure that they had before the acidification reaction.

10. A method for preparing a fluorescein having a purity such that its content of each of the by-products of the reaction is less than or equal to 0.2%, the sum of the contents of each of these by-products being less than or equal to 0.5%, said method comprising the following successive steps:
condensing phthalic anhydride with resorcinol, in a solvent consisting of an ester of an aliphatic or aromatic organic acid, in the presence of a catalyst selected from the group consisting of Lewis acids or Brönsted acids, said ester of an aliphatic or aromatic organic acid having the formula IV


R6—COOR7  (IV)


wherein R6 is selected from the group consisting of hydrogen, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, and alkyloxy having from 1 to 30 carbon atoms, R7 being selected from the group consisting of alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, or alkyloxy having from 1 to 30 carbon atoms,
suspending the red-colored crystals obtained in the preceding step in an anhydrous solvent selected from the group consisting of alcohols such as absolute ethanol, ketones such as acetone, ethers, halogenated solvents, or mixtures thereof,
acidifying the suspension thus obtained by the addition of a strong acid or one of its precursors, selected from the group consisting of sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid pyrophosphate (P2O5), and mixtures thereof, until the red-colored crystals are converted to yellow-colored crystals,
washing the crystals obtained with a washing solution selected from the group consisting of water, alcohols, ketones, ethers and halogenated solvents, pure or as a mixture, this washing being continued until the yellow-colored crystals are reconverted to red-colored crystals.

11. The method for preparing a fluorescein as claimed in claim 10, having a purity such that its content of each of the by-products of the reaction is less than or equal to 0.1%.

12. The method for preparing a fluorescein as claimed in claim 10, wherein the solvent used in the condensation reaction is the ethyl or methyl benzoate or ethyl or methyl palmitate.

13. The method for preparing a fluorescein as claimed in claim 10, wherein the catalyst used for the condensation reaction is an alkali metal hydrogen sulfate.

14. The method for preparing a fluorescein as claimed in claim 13, wherein the catalyst is potassium hydrogen sulfate or sodium hydrogen sulfate.

15. The method as claimed in claim 8, wherein the acidification is carried out by addition of hydrochloric acid into the phthalein suspension in an anhydrous organic solvent.

16. The method of claim 15, wherein the addition of hydrochloric acid is effected by sparging gaseous hydrochloric acid Into the phthalein suspension.

17. The method of claim 15, wherein the anhydrous organic solvent is an alcohol, ketone, ether, halogenated solvent, or a mixture thereof.

18. The method of claim 17, wherein the anhydrous organic solvent is isopropanol, absolute ethanol, acetone, or a mixture thereof.

19. The method of claim 10, wherein the acidification Is carried out by addition of hydrochloric acid Into the fluorescein suspension In an anhydrous organic solvent.

20. The method of claim 19, wherein the addition of hydrochloric acid is effected by sparging gaseous hydrochloric acid into the fluorescein suspension.

21. The method of claim 19, wherein the hydrous organic solvent is an alcohol, ketone, ether, halogenated solvent, or a mixture thereof.

22. The method of claim 21, wherein the anhydrous organic solvent is isopropanol, absolute ethanol, acetone, or a mixture thereof.