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

Abstract

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.

Figure 1

(A) Biocatalytic strategies for the synthesis of chiral cyclic amines. (B) Literature examples of the transamination of carbonyl substrates bearing a terminal leaving group, followed by cyclization. (C) Proposed synthesis of 2-substituted chiral pyrrolidines and piperidines starting from commercially available ω-chloroketones.

Figure 2

GC yields (determined using a calibration curve of the products) and ees for the TA-catalyzed synthesis of (A) 2-methylpyrrolidine 3a and (B) 2-methylpiperidine 3b. Enzymes labeled with an asterisk produce the (S)-enantiomer, while those without produce the (R)-enantiomer. Conditions: TA (10 mg/mL), 1a-b (50 mM), PLP (1 mM), IPA (0.5), DMSO (5% v/v), KP_i-buffer (100 mM), pH 8, 30 °C, 700 rpm, final volume 0.5 mL. Reaction time: 22 h (1a; except PjSTA-R6-8: 24 h) or 24 h followed by the addition of NaOH (100 μL, 10 M) and further incubation for 3h (1b). Data are the average of duplicates.

Figure 3

Docked quinonoid intermediate of 5-chloropentan-2-one 1a. (A) HEwT wt, (B) HEwT W56G. Docking was carried out with the dock_run.mcr macro in YASARA 20.12.24; the figure was generated using Open Source PyMOL 2.5.0.

Scheme 1. GC Yields (Determined Using a Calibration Curve of the Products) and ees (in Parentheses) for the Best-Performing Variants in the TA-Catalyzed Synthesis of 3a and 3b

Conditions: TA (10 mg/mL), 1a-b (50 mM), PLP (1 mM), IPA (1 M or 0.5 M (TsRTA and ATA-117)), DMSO (5% v/v), KP_i-buffer (100 mM), pH 8, 30 °C, 700 rpm, final volume 0.5 mL Reaction time: 22 h (1a) or 24 h followed by the addition of NaOH (100 μL, 10 M) and further incubation for 3h (1b). Data are the average of duplicates.

Figure 4

Structures of bioactive molecules containing 2-arylpyrrolidines.

Figure 5

Relative HPLC areas of TA-catalyzed reactions with 1f as substrate, using varying temperatures (30, 37, and 45 °C). Conditions: ATA-117-Rd6 (10 mg/mL), 1f (50 mM), PLP (1 mM), IPA (1 M), DMSO (20% v/v), KP_i-buffer (100 mM), pH 8, 700 rpm, final volume 0.5 mL. Data are the average of duplicates; error bars represent standard errors.

Scheme 2. HPLC Yields (Determined Using a Calibration Curve of the Products) and ees (in Parentheses) for the Best-Performing Variants in the Synthesis of 3c-m

Conditions: TA (10 mg/mL), 1c-g (50 mM), PLP (1 mM), IPA (1 M), DMSO (20% v/v), KP_i-buffer (100 mM), pH 8, 37 °C, 700 rpm, final volume 0.5 mL Reaction time: 48 h. For 3l: subsequent addition of NaOH (50 μL, 10 M) and further incubation for 1 h. Data are the average of duplicates. N.d.: not detected.

Scheme 3. Preparative-Scale Synthesis of (R)-3f from 1f, Catalyzed by ATA-117-Rd6

Conditions: ATA-117-Rd6 (250 mg), 1f (300 mg), PLP (1 mM), IPA (1 M), DMSO (20% v/v), KP_i-buffer (100 mM), pH 8, 40 °C, vigorous stirring (magnetic stirrer), final volume 25 mL. Reaction time: 72 h.