Process Development of Enantioselective Imine Reductase-Catalyzed Syntheses of Pharmaceutically Relevant Pyrrolidines

Enantioselective Synthesis of Secondary Amines by Combining Oxidative Rearrangement and Biocatalysis in a One-Pot Process

This contribution describes the development of chemoenzymatic one-pot processes, which combine an oxidative rearrangement and a biotransformation catalyzed by an imine reductase (IRED), for the synthesis of highly enantiomerically enriched secondary amines, such as an aryl-substituted pyrrolidine and a benzazepine. The benefits of this chemoenzymatic one-pot approach include high overall conversions (up to >99%), high enantiomeric excesses (up to >99% ee), and a straightforward synthetic approach toward secondary amines without the need to isolate the formed intermediate. For the initial chemical reaction, namely, the oxidative rearrangement, PhI(OAc)2 in methanol is used as a non-natural reagent, whereas the enzymatic step requires only stoichiometric amounts of d-glucose along with catalytic amounts of IRED, glucose dehydrogenase (GDH), and the cofactor NADPH. This methodology, demonstrating the compatibility of a "classic" organic synthesis using a non-natural, highly reactive reagent and a subsequent biocatalytic step, can be applied for different amines as substrates, thus making this concept a versatile tool in synthetic organic chemistry in general and for enantioselective synthesis of heterocyclic secondary amines in particular.

Asymmetric Reduction of Cyclic Imines by Imine Reductase Enzymes in Non-Conventional Solvents

The first enantioselective reduction of 2-substituted cyclic imines to the corresponding amines (pyrrolidines, piperidines, and azepines) by imine reductases (IREDs) in non-conventional solvents is reported. The best results were obtained in a glycerol/phosphate buffer 1 : 1 mixture, in which heterocyclic amines were produced with full conversions (>99 %), moderate to good yields (22-84 %) and excellent S-enantioselectivities (up to >99 % ee). Remarkably, the process can be performed at a 100 mM substrate loading, which, for the model compound, means a concentration of 14.5 g L-1 . A fed-batch protocol was also developed for a convenient scale-up transformation, and one millimole of substrate 1 a was readily converted into 120 mg of enantiopure amine (S)-2 a with a remarkable 80 % overall yield. This aspect strongly contributes to making the process potentially attractive for large-scale applications in terms of economic and environmental sustainability for a good number of substrates used to produce enantiopure cyclic amines of high pharmaceutical interest.

Enantio-Complementary Synthesis of 2-Substituted Pyrrolidines and Piperidines via Transaminase-Triggered Cyclizations

Chiral N-heterocycles are a common motif in many active pharmaceutical ingredients; however, their synthesis often relies on the use of heavy metals. In recent years, several biocatalytic approaches have emerged to reach enantiopurity. Here, we describe the asymmetric synthesis of 2-substituted pyrrolidines and piperidines, starting from commercially available ω-chloroketones by using transaminases, which has not yet been comprehensively studied. Analytical yields of up to 90% and enantiomeric excesses of up to >99.5% for each enantiomer were achieved, which has not previously been shown for bulky substituents. This biocatalytic approach was applied to synthesize (R)-2-(p-chlorophenyl)pyrrolidine on a 300 mg scale, affording 84% isolated yield, with >99.5% ee.