The present invention relates to an improved method for the preparation of an intermediate for bambuterol, i.e. for the preparation of 2'-bromo-3,5-di[N,N-dimethyl- σarba oyloxy]acetophenone, having the formula

In US-A-4 451 663 a method of manufacturing the inter- mediate 2'-bromo-3,5-di[N,N-dimethylcarbamoγloxy]- acetophenone of the formula (1) is disclosed, called route A2 therein, according to which 3,5-dihydroxyacetophenone, having .the formula

is reacted with N,N-dimethγlcarbamoγlchloride (DMCC) having the formula

CH ₃ J

N-C-Cl (3)

CH.

using pyridine as solvent and base for the uptake of hydrochloric acid formed during the reaction. The resulting compound, 3,5-di-[N,N-dimethylcarbamoyloxy]- acetophenone having the formula

is not isolated but further reacted with bromine in dioxan solvent resulting in the formation of 2'-bromo-3,5-di[N,N- dimethylcarbamoyloxy]acetophenone (1) .

There is a great desire to avoid working with pyridine as a solvent. One method was tested in order to avoid using pyridine as a solvent, according to which the reaction was performed in methylene chloride with potassium carbonate as a base with a catalytical amount of pyridine present. The drawbacks with this process was a long reaction time for N,N-dimethγlcarbamoγlchloride (DMCC) , > 24 hours, and a long quenching time for DMCC, > 24 hours, and separation problems in the work-up step (emulsion and small density difference). Methylene chloride is not a preferred solvent either, because of its toxicity.

The object of the invention is to provide an improved method for the preparation of the intermediate 2'-bromo- 3,5-di-[N,N-dimethylcarbamoyloxy]acetophenone (1) for the production of bambuterol avoiding the use of pyridine and methylene chloride as solvents and eliminating the separation problems in the work-up step.

This is attained according to the present invention by reacting 3,5-dihγdroxγacetophenone

with N,N-dimethylcarbamoγlchloride, (CH ₃ ) ₂ NCOCl, in a solvent, quenching with water, removing the solvent and dissolving of the resulting 3,5-di-[N,N-dimethγl- carba oyloxy]acetophenone

in ethyl acetate, to which is added dissolved hydrogen bromide and then bromine, whereupon the resulting inter¬ mediate (1) is collected in the form of crystals, wherein the reaction between said 3,5-dihydroxyaceto¬ phenone (2) and N,N-dimethγlcarbamoylchloride (3) is carried out in ethylacetate, isopropylacetate, butyl- acetate, ethylmethylketone or isobutylmethylketone as the solvent with a small amount of pyridine as a catalyst using a base comprising crystallized potassium carbonate.

According to a preferred embodiment of the invention potassium carbonate having a crystal water content of 11 - 13 % is used. This might be achieved by using a mixture of crystallized and calcinated potassium carbonate, resulting in a easily stirred two-phase system.

Calcinated potassium carbonate (completely dry) gives a slow and non-complete reaction, and the addition of water to the calcinated potassium carbonate results in a sticky mass which is difficult to stir. Further, free water should not be present in the reaction mixture because of the risk of hydrolysis of the reactants.

The bromination step which according to the invention first involves the addition of HBr, preferably dissolved in ethanol, and a second step charging with bromine, Br-, has the advantage that the bromination will proceed calmly. When bromination is performed directly with bromine, the reaction will start very slowly and at a certain point it will proceed vigorously and will be hard to control.

The invention will now be described more in detail.

In the first step of the process the base, potassium carbonate, will take care of hydrochloric acid formed.

According to the prior art process pyridine will act both as base and solvent. In the process according to the invention however, pyridine will only be present in catalytic amounts. Here the pyridine will initially take up the hydrochloric acid formed, which then will be transferred to the potassium carbonate, which is consumed.

Dry, that is calcinated, potassium carbonate, does not work very well as a base, and the reason is believed to be that it forms a separate phase which is non-miscible with the other phases. If calcinated potassium carbonate and water is used, the water will hydrolyze the N,N-dimethyl- carbamoylchloride (DMCC) .

According to the invention crystallized potassium carbonate is used as the base. The water will released slowly and the base will be slowly consumed by the

hydrochloric acid, which is first taken up and then transferred from the catalytic amount of pyridine present in the reaction mixture.

The crystallized potassium carbonate contains about 1.5 moles of H-O which corresponds to about 16 %. The optimum water content is 11 - 13 %, and this content is obtained using a mixture of calcinated potassium carbonate and crystallized potassium carbonate.

The synthetic route is described more in detail below.

Flowsheet diagram of synthetic route

Step 1

3,5-dihydroxyacetophenone

N,N-dimεthγlcarbamoyl- chloride

3 , 5-di- [N ,N-dime thy lcarba¬ moyloxy ] cetophenone ( not isolated)

Hydrogene bromide Bromine

Step 1

Synthesis of 3,5-di-[N,N-dimethylcarbamoyloxy]acetophenone 500 litre scale

Chemicals

3,5-di-[N,N-dimethylcarba¬ moyloxy]acetophenone Yield calc. 95 % 44.1 294.3 149.8

*) The use of the hydrate form of potassium carbonate is essential for a successful reaction.

Procedure

Into a 500 litre enameDled reactor, connected to a scrubber containing ammonia/ethanol, is charged:

24 kg 3,5-dihydroxyacetophenone 41 kg potassium carbonate x 1,5 H^O 9.4 kg potassium carbonate calcinated 96 kg (150 1) ethyl acetate 1.0 kg pyridine 50 kg dimethylcarbamoylchloride

This mixture is stirred and heated to 70 ± 2°C, and after stirring for 2 hours at this temperature 120 kg water is charged at 70°C and the mixture is stirred for 1,5 hours at 70 ± 2°C. After cooling to 20-30°C the reaction mixture is separated and the lower water phase is discarded.

To -the organic phase is ^' charged 60 kg water and pH is adjust-.ed to 2 - 3 with about 1 kg sulphuric acid.

The phases are separated and the lower water phase is discarded. Ethyl acetate and water are removed by evaporation in the reactor at a jacket temperature of 60°C and vacuum (< 100 mbar). The evaporation residue is dissolved in 40 kg (50 1) ethyl acetate and is used directly in the bromination step.

Step 2

Synthesis of 2'-bromo-3,5-di[N,N-dimethylcarbamoyl- oxy]acetophenone in laboratory scale

Chemicals

3,5-di-[N,N-dimethylcarba¬ moyloxy3acetophenone dissolved in ethyl acetate Ethanol Bromine Water Ethanol Hydrogen bromide compr. Out

2'-bromo-3,5-di[N,N-dimethγl- carbamoyloxy]acetophenone Yield calc, 75 % 20.7 373.2 0.055

Procedure

Into a 100 ml three necked flask is charged 21.8 g 3,5- di[N,N-dimethylcarbamoyloxy]acetophenone dissolved in ethylacetate and 3.2 g hydrogen bromide dissolved in 12.8 g (16 ml) ethanol.

At 12 ± 2°C is charged 12.6 g bromine in 30-60 minutes.

After stirring for further 10 minutes at 12 ± 2°C 36 ml (36 g) water is charged in about 15 minutes.

Thereafter the crystal slurry is cooled to - 14 ± 2°C and the crystals are collected on a filter and washed with 12 g (15 ml) cold (-14°C) ethanol. The crystals are dried at 40°C in vacuum.

The yield is about 20.7 g (75 %).