Process for preparing erythromycin A oxime or a salt therof.

The present invention relates to a process for preparing erythromycin A oxime or a salt thereof useful as an intermediate for the synthesis of macrolide antibiotics.

Recently, various chemical modifications of natural macrolide antibiotics have been made to develop synthetic macrolide antibiotics having medicinal features of even greater excellence. Erythromycin A oxime and the salts thereof are important as intermediates of these synthetic macrolide antibiotics.

There are known processes for preparing erythromycin A oxime by reacting erythromycin A with hydroxylamine hydrochloride using various bases, for example, barium carbonate (GB-A-1100504), pyridine (EP-A-109253), sodium carbonate (JP-A-62-081399), imidazole (US-A-4672109) and sodium acetate (JP-A-62-087599).

Also known is a process in which erythromycin A is reacted with hydroxylamine in dry methanol at room temperature to obtain erythromycin A oxime in a moderate yield (Tetrahedron Letters, p. 157, 1970).

Of the above processes, the processes using hydroxylamine hydrochloride and the base have a problem in that hydroxylamine hydrochloride is expensive. In addition, the use of an inorganic base as the base results in formation of a large amount of the inorganic salt, and therefore, troublesome procedures are required for isolation and purification of erythromycin A oxime as the end compound from the reaction mixture.

On the other hand, the process using free hydroxylamine, which as far as we are aware is the only previously reported process using free hydroxylamine, provides only less than 20% yield of erythromycin A oxime even after reaction for 35 days, so that it is not practical (See the Reference Example described below).

As a result of various investigations to solve the drawbacks of the prior art processes, we have found that the above problem may be solved by reacting erythromycin A with inexpensive hydroxylamine in place of hydroxylamine hydrochloride using an acid in place of the base under certain conditions, as described below.

The present invention seeks to provide a process on an industrial scale for preparing erythromycin A oxime or a salt thereof in good yield and good purity by simple procedures.

According to the present invention, erythromycin A oxime or a salt thereof is prepared by:    admixing erythromycin A, hydroxylamine and an acid to form a reaction mixture, the acid being added in an amount sufficient to adjust the reaction mixture to a pH of 5.5 to 7.5; and    reacting the erythromycin A with the hydroxylamine in the reaction mixture to form erythromycin A oxime or a salt thereof.

Hydroxylamine used in the process of the present invention may be an aqueous hydroxylamine solution or dry hydroxylamine.

Examples of the acid are organic acids such as formic acid, acetic acid and propionic acid, and inorganic acids such as hydrochloric acid. Accordingly, the salts of the present invention are those with the organic and inorganic acids, which can be prepared by reaction of the corresponding acid.

The reaction solvents are preferably alcohols, and more preferably methanol is used. The reaction temperature may be chosen from a range of room temperature to the reflux temperature of the solvent. The conclusion of the reaction can be recognized by observing the disappearance of erythromycin A from the reaction material by means of thin layer chromatography or high performance liquid chromatography.

When reacting erythromycin A with hydroxylamine, the addition of the acid increases the rate of formation of erythromycin A oxime. However, when more than a certain amount of the acid is added, an hemiacetal which is formed by the reaction of the 9-carbonyl group with the 6-hydroxyl group of erythromycin A is dehydrated to give the corresponding enol-ether form. In order to obtain erythromycin A oxime in good yield, it is necessary to prevent the formation of the enol-ether form. For the purpose, each amount of hydroxylamine, the acid and the reaction solvent may be as follows. The amount of hydroxylamine is more than 1.5 equivalents, and preferably from 3 to 6 molar equivalents relative to erythromycin A, while more than 6 molar equivalents can give comparable yield to the above. The acid is added in an amount to adjust the pH of the reaction solution from 5.5 to 7.5, and preferably from 6.0 to 7.0. The amount of reaction solution is preferably from 0.7 to 6 ml, and more preferably from 0.8 to 4 ml relative to 1 g of erythromycin A.

The reaction of erythromycin A with hydroxylamine using an acid under the above conditions precipitates the salt of erythromycin A oxime in the reaction mixture. After completion of the reaction, crystals of the salt of erythromycin A oxime can be collected in good purity merely by filtration.

To the salt is added an aqueous ammonia or an aqueous alkali solution (e.g., sodium hydroxide, potassium hydroxide, sodium carbonate or sodium hydrogen carbonate), the mixture is extracted with an organic solvent such as dichloromethane or ethyl acetate, and the extract is concentrated to give erythromycin A oxime easily.

Alternatively, the salt of erythromycin A oxime in the reaction mixture without isolation is mixed with an aqueous ammonia or an aqueous alkali solution as described above, and the mixture is extracted with an organic solvent such as dichloromethane or ethyl acetate, and the extract is concentrated to give crude crystals, which are then recrystallized to give erythromycin A oxime.

According to the present invention, erythromycin A oxime can be obtained in good yield by using hydroxylamine and an acid by simple procedures. Accordingly, erythromycin A oxime can be prepared inexpensively.

Furthermore, large amounts of the inorganic base and the inorganic salt do not exist in the reaction system of the present invention so that the end product can be easily isolated and purified and the amount of the reaction solvent can be reduced without any hindrance against stirring of the reaction system. Accordingly, a large amount of the end product can be prepared for the scale of the equipment. In addition, since the concentration of the reaction solution rises, the side formation of the enol-ether form which proceeds in an intramolecular reaction can be reduced to give the end product in good purity and good yield.

The present invention will be illustrated in more detail by the following Examples. The erythromycin A used in the Examples and Reference Example is of 90% purity.

Example 1

To 6.40 kg of erythromycin A was added 6.00 ℓ of methanol, and the mixture was stirred at 60°C for an hour. To the suspension was added a mixture of 2.88 kg of 50% aqueous hydroxylamine solution and 1.18 kg of 90% formic acid, and the mixture (pH 6.26) was stirred at 58°C for 4.5 hours and then at 40°C overnight. After further stirring at 5°C for 3 hours, the reaction mixture was filtered using a centrifuge to give crystals, which were then dried at 60°C with a forced air dryer for 12 hours to give 5.75 kg of erythromycin A oxime formate, m.p. 156 - 158°C.

To 5.75 kg of the salt was added 30 ℓ of 10% aqueous ammonia, the mixture was extracted with 67 ℓ of dichloromethane, and the extract was dried over 1.70 kg of anhydrous magnesium sulfate. The dichloromethane was evaporated to give crystals, which were then dried at 60°C with a forced air dryer for 12 hours to give 5.30 kg of erythromycin A oxime, m.p. 156 - 159°C.

Example 2

To a suspension of 14.68 g of erythromycin A in a 16.1 ml of methanol was added 6 ml of 50% aqueous hydroxylamine at 56°C, and the mixture was stirred for 30 minutes. Then, the mixture was adjusted to pH 6.30 by adding 4.35 ml of acetic acid at 60°C over a 20-minute period, and stirred at 60°C for 5 hours and then at 3°C for 2 hours. The crystals which formed were collected by filtration, washes with 5 ml of cold methanol and dried to give 13.47 g of erythromycin A oxime acetate, m.p. 152 - 154°C.

The salt obtained above was treated in a similar manner to that of Example 1 to give crystals, which were then recrystallized from dichloromethane-hexane to give 10.79 g of erythromycin A oxime.

Example 3

To a suspension of 14.68 g of erythromycin A in 15 ml of methanol was added at 57°C a mixture of 4.14 ml of 50% aqueous hydroxylamine solution and 2.31 ml of 12N hydrochloric acid, and the mixture (pH 6.40) was stirred at 60°C for 8 hours and then at 3°C for 4 hours. The crystals which formed were collected by filtration, washed with 10 ml of cold methanol and dried to give 9.98 g of erythromycin A oxime hydrochloride, m.p. 189 - 192°C.

The salt obtained was treated in a similar manner to that of Example 1 by using 2N aqueous sodium hydroxide solution and dichloromethane to give crude crystals, which were then recrystallized from dichloromethane-hexane to give 9.30 g of erythromycin A oxime.

Example 4

To a solution of 15.8 g of erythromycin A in 79 ml of dry methanol was added 3.3 g of hydroxylamine, and the solution was adjusted to pH 6.40 (27°C) by adding 6.7 ml of acetic acid at room temperature. The reaction solution was allowed to stand at room temperature for 6 days, the methanol was evaporated, and the mixture was cooled on ice for 3 hours. The crystals which formed were collected by filtration and dried to give 9.58 g of erythromycin A oxime acetate.

The crystals were treated in a similar manner to that of Example 1 to give 9.27 g of erythromycin A oxime.

Reference Example [Preparation without acid]

To a solution of 15.8 g of erythromycin A in 79 ml of dry methanol was added 3.3 g of hydroxylamine, and the mixture was allowed to stand at room temperature. The reaction rate was calculated by determining the formed erythromycin A oxime and the remaining erythromycin A with lapse of time by means of high performance liquid chromatography.

The conditions of high performance liquid chromatography and the results are as follows.

〈The conditions of high performance liquid chromatography〉

<dl id="dl0001"><dt>Column:</dt><dd>TSK gel, ODS-120A (Toyo Soda) 4.6 mm ID x 25 cm</dd><dt>Solvent:</dt><dd>methanol/water (70/30, 0.04% monoethanolamine)</dd><dt>Flow rate:</dt><dd>1.0 ml/min.</dd><dt>Temperature:</dt><dd>65°C</dd><dt>Detection:</dt><dd>UV (220 nm)</dd></dl>

〈Results〉

<tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="3" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center">Reaction time</entry><entry namest="col2" nameend="col3" align="center">Peak area (%)</entry></row><row><entry namest="col1" nameend="col1" /><entry namest="col2" nameend="col2" align="left">(a)</entry><entry namest="col3" nameend="col3" align="left">(b)</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">2 days</entry><entry namest="col2" nameend="col2" align="char" char=".">92.8</entry><entry namest="col3" nameend="col3" align="char" char=".">2.9</entry></row><row><entry namest="col1" nameend="col1" align="right">7 days</entry><entry namest="col2" nameend="col2" align="char" char=".">89.1</entry><entry namest="col3" nameend="col3" align="char" char=".">6.7</entry></row><row><entry namest="col1" nameend="col1" align="right">21 days</entry><entry namest="col2" nameend="col2" align="char" char=".">71.6</entry><entry namest="col3" nameend="col3" align="char" char=".">14.3</entry></row><row><entry namest="col1" nameend="col1" align="right">35 days</entry><entry namest="col2" nameend="col2" align="char" char=".">46.4</entry><entry namest="col3" nameend="col3" align="char" char=".">29.8</entry></row><row rowsep="1"><entry namest="col1" nameend="col3" align="justify">(note) (a): erythromycin A (b): erythromycin A oxime</entry></row></tbody></tgroup></table></tables>

The molar extinction coefficient of (b) under the conditions was about 5 times as large as that of (a).

It is apparant from the results of the above test that when the process is carried out without using an acid, the yield of erythromycin A oxime obtained by the reaction even for 35 days does not reach 20%.