Process for preparing racemic phenethylamines

Details Process for preparing racemic phenethylamines Process for preparing racemic phenethylamines

1999-01-19

2000-04-04

Richter, Johann

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

Organic compounds

Sulfonate esters

558 61, 558418, 564381, C07C30504, C07C31312, C07C25550, C07C21103

Patent

active

060463511

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION

The present invention relates to a novel process for the preparation of known phenethylamines by racemization of cally active phenethylamines.


BACKGROUND OF THE INVENTION

Several processes for the preparation of optically active phenethylamines are already known (cf. EP-A 0 341 475). In these processes, the racemic starting materials are resolved into the individual enantiomers by adding auxiliary reagents. In order to make these known processes for racemate resolution economical, the enantiomers which are in each case undesired must be racemized again and returned to the cycle.
In this connection, several methods have already been described which are suitable for racemizing enantiomers of phenethylamines which are not required. For example, optically active phenethylamines can be converted into the corresponding racemates by treatment with alkoxides in the presence of dimethyl sulphoxide (cf. EP-A 0 489 682). The process does, however, have the disadvantage that the alkoxides which are used as auxiliary reagents are relatively costly.
In addition, it has already been disclosed that racemic phenethylamines can be prepared by reacting optically active phenethylamines with acetophenone derivatives, then racemizing the resulting optically active Schiff bases using potassium tert-butoxide, and cleaving the resulting racemic Schiff bases using acids (cf. JP-A 07-188 120 and Derwent Abstract No. 95-290 356/38). A disadvantage of this process, however, is that the yields of the desired racemates are not always satisfactory. It is also unfavourable that the racemization requires potassium tert-butoxide, which is expensive.


DETAILED DESCRIPTION OF THE INVENTION

It has now been found that racemic phenethylamines of the formula ##STR5## in which R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 independently of one another are hydrogen, halogen, cyano, nitro, alkyl, alkoxy, alkylthio, alkylsulphinyl, alkylsulphonyl, dialkylamino, halogenoalkyl, halogenoalkoxy, halogenoalkylthio, halogenoalkylsulphinyl or halogenoalkylsulphonyl, ##STR6## in which R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are as defined above, ##STR7## in which R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are as defined above, used each being identically substituted in the phenyl moiety, catalyst, the formula ##STR8## in which R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are as defined above, 50% by weight, optionally under a protective-gas atmosphere, and ##STR9## in which R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are as defined above,
To indicate optically active compounds, the chiral centre in the above formula and also in the text below is in each case marked by (*).
It is extremely surprising that racemic phenethylamines of the formula (I) can be prepared by the process according to the invention in higher yields than by the most similar previously described process in which optically active Schiff bases are racemized using potassium tert-butoxide. Moreover, on the basis of the previously disclosed prior art, it must be assumed that optically active phenethylamines of the formula (I*) are considerably decomposed when treated with hydrous metal hydroxides. Contrary to expectations, however, this is not the case.
The process according to the invention is distinguished by a number of advantages. For example, it permits the preparation, in very high yield, of racemic phenethylamines from undesired enantiomers which are inevitably produced during racemate resolutions. It is also favourable that both the (R)- and also (S)-enantiomers can be readily racemized to the same extent and that the substituents on the phenyl ring can be varied widely. It is also advantageous that the racemization is carried out using low-cost metal hydroxide. Finally, the carrying out of the reactions and isolation of the desired substances do not present any problems of any sort.
If (S)-1-(4-chloro-phenyl)-ethylamine and 4-chloracetophenone are used as starting materials, aqueous potassium hydroxide as racemization reagent, and

REFERENCES:
patent: 4988734 (1991-01-01), Kraatz et al.
patent: 5183939 (1993-02-01), Jansen et al.
patent: 5668140 (1997-09-01), Schaper et al.
patent: 5728876 (1998-03-01), Balkenhohl et al.
JP Abstracts of Japan, vol. 95, No. 010 & JP 07188120 Jul. 25, 1995.
JP Abstracts of Japan, vol. 96, No. 005 & JP 08027073 Jan. 30, 1996.

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