Biotransformation of ß-Aminonitriles to ß-Aminoacids

Beta-amino acids, although less abundant in nature than their alpha-counterparts, are essential key components of many physiologically active natural products (peptides, depsipeptides and antibiotics). They exhibit strong pharmacological effects also in their free form and therefore have become very important targets in medicinal chemistry and related fields. The chemical synthesis of carboxylic acids and amides from the corresponding nitrile moiety requires the application of strong chemicals, such as acids or bases. The same reaction can be carried out in a much more compatible fashion when employing nitrile hydrolyzing enzymes or microorganisms. In this case the transformations can be carried out in aqueous phase and compared with the chemical transformation much fewer side products are produced. Another crucial advantage of such an enzyme-catalyzed preparation is the fact that it proceeds with stereoselectivity. The use of this biocatalytic method for the preparation of beta-amino acids/amides from their corresponding amino nitriles is new. The establishment of a feasible biotransformation protocol for a selection of substrate nitriles and the development of the analytical tool to determine the enantioselectivity of the products by chiral chromatographic methods is thought to contribute to the understanding of the rather complex nitrile transforming enzyme system as far as the enantioselectivity of the involved enzymatic steps is concerned. Subsequently, the application of this transformation reaction to the synthesis of cispentacin, an antifungal antibiotic, as well as to the preparation of central key intermediates for the important class of carbapenems (beta- lactam antibiotics) is envisaged.

The transformation of nitriles into carboxylic acids and amides is an important and useful chemical reaction and represents in most cases the best route to these compounds. However, the chemical hydrolysis requires very strong acidic or basic catalysis and is in most cases accompanied by decomposition, low product yields and low selectivity of the desired hydrolysis product when other functionalities besides the nitrile moiety are present in the molecule. Thus, these methods are generally restricted to structurally simple nitriles. The use of biocatalysts (enzymes) and their mild reaction conditions - usually they are applied in aqueous solution around pH 7 at room temperature - provides an excellent alternative for the transformation of functionalized nitriles with high product selectivity. The aim of the project was the application of nitrile converting enzymes - nitrilases and nitrile hydratases/amidases (the latter by means of resting whole cells) to synthesize beta-amino acids and amides from the respective beta- amino nitriles in a combined chemical and enzymatic approach. Cis- and trans-diastereomeric carbocyclic amino nitriles were synthesized in racemic form and subjected to enantioselective biotransformation using different Rhodococcus strains. Highly selective transformations with respect to the nature of products (either amide or acid) and with respect to enantiomeric purity of the product in dependence on ring size and relative configuration of the 1, 2-substituents were achieved. Carbocyclic beta-amino acids can be used as constituents of certain natural products, peptidomimetics and as monomeric units for beta-peptides, molecules which exhibit unique folding properties. The respective beta amino amides are novel compounds, they have not been reported in the literature before.