Organic compounds -- part of the class 532-570 series – Organic compounds – Cyclopentanohydrophenanthrene ring system containing
Reexamination Certificate
2001-08-23
2002-11-12
Qazi, Sabiha (Department: 1616)
Organic compounds -- part of the class 532-570 series
Organic compounds
Cyclopentanohydrophenanthrene ring system containing
C552S521000
Reexamination Certificate
active
06479681
ABSTRACT:
The present invention relates to a novel process for the preparation of bile acid derivatives in which an amino group is present at the 3&bgr; position.
The most important bile acids are represented in the following scheme 1:
More particularly, the invention relates to a process for the preparation of compounds of formula (I) from compounds of formula (II):
in which
R
1
is H or OH;
R
2
is H, &agr;-OH, or &bgr;-OH; and
R
3
is a straight or branched C
1
-C
4
alkyl group, or a benzyl group.
The compounds of general formula (I) are important intermediates for the preparation of compounds useful for a number of pharmaceutical applications. They are, for example, used for the preparation of inhibitors of bile acids intestinal absorption (see EP-A-0489423) or as carriers for active compounds in the enter-hepatic circulation (see EP-A-0417725).
Last, but not least, is the use thereof for the preparation of contrast agents for medical diagnosis using Magnetic Resonance, such as those described in WO-A-95/32741, resulting from the conjugation of a bile acid with a chelating agent, which are capable of chelating the ions of paramagnetic bi- and trivalent metals, in particular the gadolinium ion, or in the publication: Anelli P. L. et al., Acta Radiologica, 38, 125, 1997.
EP-A-614,908 discloses the preparation of the derivatives of general formula (I), comprising the following steps:
a) formation of phthalimido derivatives of general formula (III) by reacting compounds of general formula (II) with phthalimide;
b) treatment of compounds of general formula (III) with hydrazine hydrate or phenylhydrazine;
c) subsequent treatment of the reaction products from step
b) with mineral acids to form the addition salts, and
d) liberation from the salt to give the compounds of general formula (I).
As already widely discussed in EP-A-614,908, the formation of the phthalimido derivative is carried out by means of the well known Mitsunobu reaction (see Synthesis, 1, 1981; Org. React. Vol. 42, 335 (1992)), namely in the presence of a suitable phosphine and DEAD (diethylazodicarboxylate) or DIAD (diisopropylazodicarboxylate), in an organic solvent such as dioxane or tetrahydrofuran, at a temperature which ranges from 20 to 50° C. The Mitsunobu reaction yields the final products with inversion of configuration.
The process illustrated makes use of hydrazine hydrate or phenylhydrazine, particularly dangerous products due to their ascertained cancerogenicity.
It has now surprisingly been found that the reduction of the phthalimido group in the compounds of formula (III) can be advantageously carried out with sodium borohydride.
It is therefore the object of the present invention a process for the preparation of the compounds of general formula (I) comprising the reduction reaction of the compounds of general formula (III) in the presence of sodium borohydride, according to the following Scheme 2:
in which
R
1
is H or OH;
R
2
is H, &agr;-OH, or &bgr;-OH; and R
3
is a straight or branched C
1
-C
4
alkyl group, or a benzyl group.
A further object of the present invention is the process for the preparation of the compounds of general formula (I) comprising the reduction of the compounds of general formula (III) in the presence of sodium borohydride, through formation of the novel compounds of general formula (IV), and subsequent deprotection by treating said compounds with acids, according to the following Scheme 3:
in which R
1
, R
2
and R
3
have the meanings defined above.
Compounds of general formula (IV) are novel, useful intermediates in the process according to Scheme 3 and their recovery will be described in the Experimental Section.
Particularly preferred is the process for the preparation of compounds of formula (Ia), according to Scheme 3, starting from compounds of formula (IIIa), which are derivatives of cholic or deoxycholic acid,
in which
R
3
has the meanings defined above and
R
4
can be a hydrogen atom or a hydroxy group.
A further object of the present invention is the process for the preparation of compounds of formula (Ib) according to Scheme 4, starting from compounds of formula (IIIb), which are deoxycholic acid derivatives,
in which R
3
has the meanings defined above.
Particularly preferred is the process for the preparation of compound (Ic), according to Scheme 5, starting from compound (IIIc), a deoxycholic acid derivative,
The compounds of formula (IIIc) and (IVc), (3&bgr;,5&bgr;,12&agr;)-3-[1,3-dihydro-1,3-dioxo-2H-isoindol-2-yl)]-12-hydroxycholan-24-oic acid methyl ester and (3&bgr;,5&bgr;,12&agr;)-12-hydroxy-3-[[(2-hydroxymethyl)benzoyl]amino]cholan-24-oic acid methyl ester, respectively, are novel and they are useful intermediates for the preparation of compound of formula (Ic), a deoxycholic acid derivative.
The compounds of formula (III) are synthesized according to the general procedure already described above and exemplified in EP-A-614,908.
Preferred reaction conditions comprise:
1) selection of a reaction temperature from 15 to 25° C., thus decreasing the amount of diisopropylazodicarboxylate to a small excess to the stoichiometric;
2) crystallization of phthalimido derivatives (III) from MeOH instead of 2-PrOH, thereby reducing the volume of the crystallization solvent by at least four times.
The reduction of phthalimido derivatives with NaBH
4
has never before been described in the field of the compounds of the present invention, but references exist in the literature, (T. W. Greene; P. G. M. Wuts “Protective groups in Organic Synthesis”; 3
rd
Ed John Wiley and sons, New York, 1999), reporting the use of a large excess of this reducing agent (5-10 mol) to deprotect a phthalimido group.
The teaching contained in the most important paper [Osby, Tetrahedron Letters, Vol. 25, 2093 (1984)] i.e. the use of 2-PrOH/H
2
O=6/1 as a reduction solvent proved to be definetly unsuitable for the process of the present invention. In fact, due to the poor solubility of compounds of formula (III), it was necessary to operate at temperatures around 40° C. and with a 2.5% maximum concentration which is of course industrially unacceptable.
On the other hand, Osby already evidenced that the proposed conditions were not particularly suitable to deprotect phthalimido groups from derivatives containing an ester function. Osby himself in fact observed that the deprotection of this group in said derivatives was accompanied by reduction and hydrolysis of the ester group with consequent decrease in reaction yields.
The Applicant found that when reducing the compounds of formula (III) under the Osby conditions, compounds of formula (I) were recovered in around 65% yields, but contaminated by remarkable amounts of by-products, mainly the product from the reduction of the ester group at 24- to alcohol. This phenomenon is still significant even when markedly reducing the NaBH
4
excess. In order to better understand the genesis of the by-products, the reaction carried out under the osby conditions was worked up at the end of the reduction with NaBH
4
and before the acid hydrolysis and the side-products recovered by silica chromatography had the formulae (V)-(VIII). These by-products are of course the precursors of the impurities evidenced in compounds of formula (I) and account for the low reaction yields.
In the Experimental section the recovery and the characterization of the various intermediates, in particular in the case of deoxycholic acid, will be reported.
Compounds of formula (IV) are substantially inseparable from compounds (V) by crystallization from different solvents. Not even the conversion of compounds (IV) into the corresponding acids of formula (VI) allows the elimination of the by-products of formula (V).
It has surprisingly been found that using dipolar aprotic solvents such as DMA (dimethylacetamide), DMF (dimethylformamide), DMSO (dimethylsulfoxide), NMP (N-methyl-pyrrolidone), in place of an alcoholic solvent, such as isopropanol described by Osby, these problems could be overcome. Particularly preferred are dimethyl
Anelli Pier Lucio
Brocchetta Marino
Gallotti Chiara
Visigalli Massimo
Bracco Imaging S.p.A.
Nixon & Vanderhye P.C.
Qazi Sabiha
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