Degussa Aktiengesellschaft

Michael Karrenbauer

Axel Kleemann

Karlheinz Drauz

Andreas Bommarius

Wolfgang Leuchtenberger

Theodor Lussling

Gunter Knaup

Fritz Schafer

Rudi E Moerck

Rudi Moerck

Kurt Klostermann

The disclosure relates to a process for obtaining optically active L-.alpha.-aminocarboxylic acids from the corresponding racemic D,L,.alpha.-aminocarboxylic acids. The following steps are involved: (a) the D,L,.alpha.-aminocarboxylic acids are acetylated; (b) the N-acetyl-L-.alpha.-aminocarboxylic acid present in the mixture of N-acetyl-D,L,.alpha.-aminocarboxylic acids thus obtained is broken down enzymatically into the L-.alpha.-aminocarboxylic acid; (c) the L-.alpha.-aminocarboxylic acid is separated from the mixture, a quantity of a solution of N-acetyl-D(L)-.alpha.-aminocarboxylic acids and a quantity of acetate equivalent to the L-.alpha.-aminocarboxylic acid being retained; and (d) the N-acetyl-D(L)-.alpha.-aminocarboxylic acid is racemized and recycled for enzymatic breakdown. Known extraction processes involving steps (a) to (d) have the disadvantage of producing large quantities of salt. Specifically, the processes are far removed from the ideal equation, according to which one hundred percent of the acetic anhydride and the D,L,.alpha.-aminocarboxylic acids are converted to L-.alpha.-aminocarboxylic acids and acetic acid. Adjusting the retained solution from step (c) in such a way as to obtain a solution consisting essentially of N-acetyl-D-(L)-.alpha.-aminocarboxylic acid salt and free acetic acid in equilibrium with acetate and free N-acetyl-D(L)-.alpha.-aminocarboxylic acid and from which acetic acid is extracted by distillation makes it surprisingly easy to feed the solution formed as 'mother liquor' following separation of the L-.alpha.-aminocarboxylic acid in the circuit and to achieve a materials balance as close as possible to the ideal.

Process for obtaining active L-.alpha.-amino carboxylic acids from

With known methods of racemizing N-acetyl-D(L)-.alpha.-amino carboxylic acids in the non-aqueous state by heating them to temperatures above room temperatures, significant quantities of by-products are formed, especially acetyl dipeptides. By converting at least a proportion of the N-acetyl-D(L)-.alpha.-amino carboxylic acids to corresponding N-acetyl-D(L)-.alpha.-amino carboxylic acid salts before or during the heating, it is possible to increase the sojourn time of the educt which is to be racemized at higher temperatures without any evident increase in the quantity of by-product formed (in particular acetylated dipeptides). Also disclosed is the production of optically active amino acids by enzymatic splitting of racemic compounds.

Process for racemization of N-acetyl-(D)L-.alpha.-amino carboxylic acids

L-amino acids present in cleavage solutions obtained in the acylase-catalyzed cleavage of N-acetyl-D,L-amino acids are isolated by use of a strongly acidic ion exchanger in H+ form. First, the mother liquor from a previous treatment cycle is fed to the ion exchanger, and then the cleavage solution and finally the wash water and the discharge from the ion exchanger which is pH-controlled is analyzed into an 'effluent water fraction', an 'N-acetyl-D-amino acid fraction' and an 'L-amino acid fraction'. The last fraction is adjusted by addition of fresh cleavage solution to a pH of between 4.0 and 6.0, and from the mixture which is thus obtained the L-amino acid is isolated by crystallization. The mother liquor which is obtained is recycled back into a subsequent treatment cycle.

Process for the isolation of L-amino acids

The reaction mixture obtained in the enzymatic reaction of fumaric acid to form L-malic acid contains the L-malic acid and unreacted fumaric acid as their salts. To recover pure solutions of L-malic acid the reaction mixture is treated at 50.degree. to 150.degree. C. with cation exchangers. The eluate after concentration to 30 to 80 weight % content of L-malic acid is filtered. The pure L-malic acid is recovered by evaporating the solutions, in a given case after treatment with activated carbon.

Process for the recovery of malic acid

N-acetyl-D(L)-.alpha.-aminocarboxylic acids are thermally racemized by melting, adding a small amount of acetic anhydride to the melt and heating to a temperature between the melting temperature and about 210.degree. C. and subsequently quenching the melt with an aqueous alkali metal hydroxide or ammonia solution. The residence time needed for complete racemization depends on the heating temperature of the melt in the manner that the higher the heating temperature the shorter the residence time.

Process for the racemization of N-acetyl-D,L-.alpha.-aminocarboxylic

D,L-Homocystine is obtained by treating an aqueous solution of the disodium salt of D,L-homocysteine having a concentration between 0.4 and 1.6 moles/l and an initial pH between 7.0 and 8.0 under vigorous stirring with at least the equivalent amount of an aqueous solution of hydrogen peroxide and after the end of the oxidation adjusting the pH to about 5.3. The hydrogen peroxide solution can be employed with a concentration between 20 and 60 weight percent.

Process for obtaining D,L-homocystine (I)

D,L-Homocystine is obtained by introducing molecular oxygen into an aqueous solution of the disodium salt of D,L-homocysteine having a concentration between 0.8 and 1.2 moles/l and an initial pH between 7.0 and 8.0 in the presence of a catalytic amount of iron(III) ions until there is no longer an increase in pH and subsequently adjusting the pH to about 5.3 with stirring.

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Rodenbach, Germany

Michael Karrenbauer