Process for preparing 2-unsubstituted imidazoles

Details Process for preparing 2-unsubstituted imidazoles Process for preparing 2-unsubstituted imidazoles

1987-09-25

1990-01-02

Lee, Mary C.

Organic compounds -- part of the class 532-570 series

Organic compounds

Heterocyclic carbon compounds containing a hetero ring...

548346, C07D23358

Patent

active

048914312

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a process for preparing imidazoles. More particularly, the present invention relates to a process for preparing 2-unsubstituted imidazoles in high yields by reacting .alpha.-hydroxycarbonyl compounds with formamide under specified conditions.


BACKGROUND TECHNOLOGY

Imidazoles are very useful compounds not only as starting materials for producing medicines and agricultural chemicals, but also as curing agents or curing accelerators for epoxy resins, industrial sterilizers, rust inhibitors and antistatic agents. Especially, 2unsubstituted imidazoles are useful compounds for producing antitumor agents for the digestive organs or curing agents for epoxy resins.
Such 2-unsubstituted imidazoles have heretofore been prepared by, for example, reacting a 1,2-dicarbonyl compound with an aldehyde and ammonia. However, this process has the disadvantage in that the material 1,2-dicarbonyl compound cannot be produced on a mass production scale. A process for preparing 2-unsubstituted imidazoles by reacting an .alpha.-hydroxyketone with ammoniacal cupric acetate in the presence of an aldehyde has also been known. However, this process has had a problem in that the yield of the obtained imidazoles is low, the steps are complicated and a large amount of expensive cupric acetate is consumed. Further, it has been known that when an imidazoline obtained by reacting a 1,2-diamine with a cyanogen compound is dehydrogenated in the presence of a nickel formate catalyst, an imidazole is obtained. However, this process also has a problem in that it cannot give 2-unsubstituted compounds.
In order to solve these problems, a process for preparing imidazoles by mixing an .alpha.-hydroxycarbonyl compound (hereinafter may be referred to as HC) and a formaldehyde amide (hereinafter may be referred to as FA) at an FA/HC molar ratio of 10-20 and reacting them at a temperature of 180.degree. C.-200.degree. C. was proposed (see Chem. Ber., 86, page 88, 1953).
According to this process, when the FA/HC molar ratio is made 10-20, imidazoles are obtained in a yield of 50-90%, but when the FA/HC molar ratio is made 2 which is the stoichiometric ratio, the yield of imidazoles decreases to as low as 10-20%. Because of this, the reaction has to be carried out at an FA/HC molar ratio of 10-20. As a result, after completion of the reaction, a large amount of unreacted FA must be recovered by distillation. Because FA is unstable and tends to be decomposed, as much as 30-40% of FA is lost during distillation. Because of this, in order to bring the FA/HC molar ratio close to 2 which is the stoichiometric ratio, a method in which HC is gradually added to FA while keeping the pH of the reaction liquid at 4-10 was also proposed (see Japanese Laid-Open Patent Publication No. 30574/1986 laid open on Feb. 12, 1986).
However, there is still a problem in that when the FA/HC molar ratio is made 2 or less, the yield of imidazoles became about 40%, and the imidazoles cannot be obtained in high yields.
The present inventors conducted strenuous studies with a view to obtaining 2-unsubstituted imidazoles in high yields from FA and HC, and as a result found that when the reaction is carried out under specified conditions, even if the FA/HC molar ratio is not more than 2, it is possible to obtain 2-unsubstituted imidazoles in high yields. This finding has led to completion of the present invention.
An object of the present invention is to solve such problems of the prior art as mentioned above and provide a process capable of preparing 2-unsubstituted imidazoles in high yields from FA and HC even if the FA/HC molar ratio is not more than 2.
Another object of the present invention is to provide a method of removing by-product formic acid in the preparation of imidazoles from an .alpha.-hydroxycarbonyl compound and formamide, in which the corrosion of a separating device such as a distillation column is inhibited by efectively removing the by-product formic acid from the reaction mixture.
Further objects and advantages of

REFERENCES:
Bredereck et al., Chem. Ber., vol. 86, p. 88, 1953.

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