The invention relates to a new method for the preparation of 3,4-dihydroxy-5- n ^' rtro θnzaldθhydθ, which is an intermediate in the synthesis of several pharmaceutically important catechol compounds.

GB-A-2200109 and EP 237929 describe a method for the preparation of 3,4- dihydroxy-5-nitrobenzaldehydθ, in which method 4-hydroxy-3-methoxy-5- nitrobenzaldehyde is refluxed in concentrated hydrobromic acid. This method has many disadvantages which make it industrially inapplicable. For example, hydrobromic acid causes serious corrosion problems and results in the formation of the by-product 2-bromo-3,4-dihydroxy-5-nitrobenzaldehyde and dark coloured decomposition products which both interfere with the purification of the desired 3,4-dihydroxy-5-nitrobenzaldehyde. The emission of the toxic gaseous by-product methyl bromide is also a serious problem.

It has now been surprisingly found that the above disadvantages may be avoided if the dealkylation reaction is performed by using a strong nucleophilic reagent. Preferably, the nucleophile is thiolate anion of an aromatic mercapto compound such as thiophenol, 2-, 3- or 4-aminothiophenol, 2-, 3- or 4-thiocresol or 1- or 2- thionaphthol. The thiolate anion is preferably created with the aid of a strong organic or inorganic base such as an alkali metal hydroxide, hydride or amide. Especially preferable are the lithium bases.

It is advisable to carry out the reaction in an inert atmosphere, thus preventing the formation of disulphide impurities. The reaction is preferably performed in an aprotic polar solvent, such as 1-methyl-2-pyrrolidinone, N,N-dimethylformamide or N,N-dimethylacetamide under reduced or normal pressure at elevated temperature in the range of about 80 to 160, most preferably about 130 °C.

The reaction is very suitable for industrial production because the reagents are readily available, cheap, noπcorrosive and easily handled. The solvent used can be easily recirculated and there is no emission of toxic gases.

Example 1

20 g of 4-hydroxy-3-methoxy-5-nitrobenzaldehyde, 5.4 g of lithium hydroxide, 12 ml of thiophenol and 40 ml of NMP (1-methyl-2-pyrrolidinone ) were mixed for two hours at 130 °C under nitrogen atmosphere. The mixture was cooled to 90 °C and 125 ml of water, 40 ml of heptane and 30 ml of strong hydrochiorid acid was added. The mixture was stirred overnight at room temperature, kept for two hours at 0 °C, filtered, washed with 20 ml of cold water and dried. Yield 16.52 g (88.9 %), m.p. 135-137 ^© C.

Example 2

15 g of 4-hydroxy-3-methoxy-5-nitrobenzaldehyde, 4.1 g of lithium hydroxide, 9 ml of thiophenol and 25 ml of NMP were mixed for two hours at 130 °C under nitrogen atmosphere. The mixture was cooled to 100 °C and 50 ml of glacial acetic acid and 30 ml of concentrated hydrochloric acid were added. The mixture was stirred overnight at room temperature, kept for two hours at 0 °C, filtered, washed with 20 ml of cold water and dried. Yield 11.38 g (81.7 %), m.p. 135-137 ©

Example 3

15 g of 4-hydroxy-3-methoxy-5-nitrobenzaldehyde, 3.7 g of lithium hydroxide, 13 g of 2-mercaptobenzothiazole, 40 ml of NMP and 30 ml of toluene were refluxed with water separation for 20 hours under nitrogen atmosphere. The mixture was cooled to 80 °C and 150 ml of water and 20 ml of toluene were added. After stirring for half an hour the phases were separated and the toluene phase discarded. To the water phase 45 ml of concentrated hydrochloric acid was added. The mixture was stirred overnight at room temperature, kept for two hours at 0 °C, filtered, washed with 20 ml of cold water and dried. Yield 12.64 g (90.7 %), m.p. 135-137 ^O C.

■Example 4

150 g of 4-hydroxy-3-methoxy-5-nitrobenzaldehyde, 280 ml of NMP, 39 g of lithium hydroxide and 90 ml of thiophenol were kept at 130 °C for three hours under reduced pressure and the distillate was collected. The mixture was cooled to 100 °C, the pressure normalized and 1000 ml of hot water and 250 ml of concentrated hydrochloric acid were added. The mixture was stirred overnight at room temperature, kept for two hours at 0 °C, filtered, washed with 200 ml of cold water and dried. Yield 135 g (96.9 %), m.p. 135-137 °C.