Biocatalytic Route to Chiral Precursors of β-Substituted-γ-Amino Acids

S derivatives of γ-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the mammalian brain, have been extensively used for the treatment of several nervous system disorders including epilepsy, neuropathic pain, anxiety, and social phobia. Several drugs such as Gabapentin and Pregabalin have achieved rapid success in the past two decades. The use of an enzymatic kinetic resolution in the manufacturing of Pregabalin was critical in providing an economical, high-yield synthesis of the final drug. The chemistry has been further developed and implemented by Pfizer, and the environmental benefits of racemizing and recycling the wrong enantiomer have been demonstrated at multiple metric ton scale. This prompted research to explore the possibility of using a biocatalyst like the Thermomyces Lanuginosa lipase in the resolution of related compounds, namely, 3-substituted-3-cyano-2-(ethoxycarbonyl)propanoic acid ethyl esters 2 (Scheme 1). Herein, we report high enantioselectivity of three lipases toward a variety of aliphatic and aromatic substrates, producing the corresponding enantiopure 2-carbethoxy-3-substituted-3-cyanopropionate salts 3. A general decarboxylation procedure for compounds 3 is also reported, yielding the final enantiopure compounds 4. Alkaline hydrolysis and reduction of these compounds by knownmethods produces the corresponding β-substituted-γ-amino acids 6 without affecting the chiral center generated in the enzymatic resolution. The substrates 2 were synthesized by a two-step method: Knoevenagel condensation of the corresponding aldehyde with diethyl malonate followed by nucleophilic cyanation of the R-β unsaturated precursor 1. The initial Knoevenagel reaction was carried out in n-heptane with piperidine/acetic acid as the catalytic systemwith continuous removal of H2O, and proceeded smoothly with good to high yields (Table 1). The resulting crude products 1 are used after minimal workup and without purification in the cyanation reaction. The cyanation reaction uses KCN in ethanol, and also proceeds with moderate to high yields, and the crude compound 2 thus produced can be used in the enzymatic step without purification (Scheme 2). Conceptually, the generation of compound 3 from 2 involves the hydrolysis of one diastereotopic carboxyethyl group, a desymmetrization of the prochiral C-2 center. The desired outcome was to find an enzyme that could only perform such a reaction on the S enantiomer of racemic 2 (kinetic resolution), thus generating one (or two) diastereomers from a single enantiomer at the C-3 chiral center, leaving behind the R enantiomer of 2. Thus, the diastereoselectivity in the desymmetrization reaction per se was not as important as the enantioselectivity of the kinetic resolution, as the chirality at the C-2 center will be lost while converting 3 to the desired product 4.