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The present invention relates to a process for the preparation of

2-amino-1 ,3-propanediol and, more particularly, it relates to a process for the preparation of 2-amino-1 ,3-propanediol starting from 1 ,3-dihydroxyacetone oxime.

2-Amino-1 ,3-propanediol, a compound better known as serinol, is a useful intermediate for the preparation of non-ionic iodinated contrast media (British patent No. 1472050 - Savac A.G.), in parti¬ cular for the synthesis of Iopamidol (The Merck Index, XI Ed., page 799, No. 4943).

Several syntheses of serinol are known in the literature. The patents US 4221740 (Schering A.G.) and US 4448999 (Dyna it Nobel A.G.), for instance, describe a process for the preparation of seri¬ nol through the reduction of the sodium salt of 2-nitro-1 ,3-propane- diol.

U.S. patent No. 4978793 (W.R. Grace & Co.) describes a synthetic process comprising at first the preparation of ketals of 2-nitro- -1 ,3-propanediol starting from nitro ethane and formaldehyde and, subsequently, their reduction to the corresponding amino ketals which, in turn, can be hydrolyzed to serinol.

The patent application EP 0238961 (Merck Patent GmbH) describes a process for the preparation of serinol comprising the reduction of the corresponding keti ine which is obtained, in turn, by reacting 1 ,3-dihydroxyacetone with ammonia or with a primary amine. It is also known in the literature, for instance as reported by Piloty et al. [Berichte, 30_, (1897), 2057], that serinol can be pre¬ pared by reduction of 1 ,3-dihydroxyacetone oxime with sodium amal- gam, in the presence of aluminum sulphate.

Alternatively, as described by M. Federonko [Chem. Papers, 43(2), 335-341, (1989)], serinol can be prepared by electrochemical reduction of 1 ,3-dihydroxyacetone oxime.

Said processes are of difficult industrial application and, with particular reference to the electrochemical reduction, remarkable quantities of by-products, variable depending on the selected ope¬ rating conditions, were identified. Methods for the reduction of 1 ,3-dihydroxyacetone oxime by hydrogenation under heterogeneous catalysis, however, are not reported in the literature.

In this connection, we carried out hydrogenation reactions in the presence of conventional catalysts based on platinum or palladium. Independently from the selected operating conditions, the results were completely unsatisfactory affording, in the most favourable cases, the formation of serinol in traces.

We have instead surprisingly found that the catalytic hydrogenation reaction of 1 ,3-dihydroxyacetone oxime for the preparation of serinol can be easily carried out in the presence of a catalyst consisting of rhodium supported on alumina.

It is therefore the object of the present invention a process for the preparation of 2-amino-1 ,3-propanediol comprising the catalytic hydrogenation of 1 ,3-dihydroxyacetone oxime in a suitable solvent and in the presence of a catalyst consisting of rhodium supported on alumina.

The serinol thus prepared is a useful intermediate in the synthesis of Iopa idol, for instance, as described in the aforementioned British patent No. 1 72050. The process object of the present invention is of easy industrial application and 1 ,3-dihydroxyacetone oxime is a well-known compound,

easily obtainable, according to conventional methods.

A convenient method for the preparation of 1 ,3-dihydroxyacetone oxime is described, for instance, in the literature by L. Reio [J.

5

Chromatogr. , 68j (1972), 183-205] and comprises the reaction between

1 ,3-dihydroxyacetone and hydroxyla ine hydrochloride. The reagents and the reaction products used in the process object of the present invention are stable and allow to operate on an industrial scale without using unusual technologies. 0 The reaction parameters of the process object of the present invention are reported and described as follows.

The hydrogenation reaction is carried out into a reactor suitable to sustain relatively high pressures, for instance into an autoclave, at a pressure preferably comprised between 40 and 70 bars 5 <4-10 ^* *-7-10 ^* * Pa).

The pressure does not seem to be a critical parameter. At lower pressures, however, the kinetics of the reaction results to be rather slow, requiring thus particularly long times for the conversion of the oxime; higher pressures instead do not justify the 0 relatively poor increase in the rate of the reaction.

The reaction temperature is comprised between 20°C and 80°C.

Preferably, the operating temperature is comprised between 60°C and

70°C.

At temperatures lower than 20°C the reaction is rather slow while 5 temperatures higher than 80°C are useless.

The catalyst consists of rhodium supported on alumina at 5%, easily commercially available, and it is used in amounts comprised between 0.01 and 0.001 moles per mole of 1 ,3-dihydroxyacetone oxime. Larger amounts of catalyst are equally effective but useless. n The hydrogenation reaction is carried out in the presence of

suitable inert solvents.

With the term inert solvents we intend the solvents which do not undergo chemical reactions with the reagents or with the reaction products.

Suitable solvents are those commonly used in the catalytic hydrogenation reactions such as, for instance, lower C _η -C_, alcohols.

Methanol is preferably used.

In a preferred embodiment, the process object of the present inven- tion is carried out according to the following operating conditions. A suitable amount of catalyst is added to a suitable amount of dihy- droxyacetone oxime in a suitable solvent (for example methanol). The resultant system is put under hydrogen atmosphere, according to conventional techniques, at a pressure of about 70 bars (7 ^* l0 ^ώ Pa) and kept under stirring for a few hours (12-20) at the preselected temperature (for example 70°C).

Due to the following practical features such as, for instance, the easy accessibility of the starting material, the stability of the reagents and of the products, the simple work-up of the reaction mixture and the easy industrial application, the present invention makes available a very advantageous process for the preparation of 2-amino-1 ,3-propanediol.

With the aim to better illustrate the present invention, without however limiting it, the following example is now given. Example 1

Preparation of 2-amino-1.3-propanediol

1 ,3-Dihydroxyacetone oxime (5 g; 47.6 mmoles), methanol (30 ml) and rhodium on alumina at 5% (0.1 g; 0.048 mmoles) were loaded into an autoclave provided with mechanical stirring. After removing the surrounding air, hydrogen at a pressure of 70

bars (7- 10* Pa) was loaded.

The resultant system was then kept under stirring at 70°C for 16 hours.

At the end of the reaction, after emptying the reactor, the catalyst was filtered off on a celite bed and the solvent was evaporated at reduced pressure.

A crude product (4.5 g) was thus obtained.

From the crude, pure serinol was obtained by conversion into the corresponding hydrochloride according to the following method.

The reaction crude was treated with HCl 1N up to nearly pH 1 and heated at 40°C for 2 hours.

At the end, water was evaporated at reduced pressure and the resultant residue was collected with acetone (20 ml). The mixture was kept under vigorous stirring for 3 hours.

Then, the precipitate was separated by filtration and dried at 50°C and 25 mm/Hg.

Serinol hydrochloride (2.1 g) was thus isolated.

Alternatively, serinol can be purified by converting it into the corresponding oxalate, for instance, as described in the aforemen¬ tioned patent application EP 0238961.