Enantioselective Synthesis of Thiomorpholines through Biocatalytic Reduction of 3,6-Dihydro-2H-1,4-thiazines Using Imine Reductases

Abstract

In this work, an enantioselective biocatalytic synthesis of chiral thiomorpholines using imine reductases (IREDs) is described. As substrates, four prochiral and one chiral 3,6-dihydro-2H-1,4-thiazines were synthesized in a modified Asinger reaction and subsequently reduced using imine reductases as a biocatalyst, NADPH as a cofactor, and a glucose dehydrogenase (GDH)-glucose cofactor regeneration system. As a result, chiral thiomorpholines with a stereogenic center created in 3-position were obtained under mild process conditions with high conversions and excellent enantioselectivities of up to 99%. Furthermore, as a proof of concept, a sequential one-pot process combining both individual reaction steps was achieved.

Application of Imine Reductase in Bioactive Chiral Amine Synthesis

“…Chiral thiomorphology, with a high stereoscopic center, was obtained under mild process conditions, with high conversion and excellent enantioselectivity up to 99% ee. 88,89 Tetrahydroisoquinolines (THIQs) and tetrahydro-β-carbolines (THβCs) are important structural motifs found in natural products and clinical drugs, which have attracted the interest of synthetic chemists due to their wide-ranging biological activities. In 2022, Ma's group identified a set of active IRED capable of converting imine precursors into the corresponding enantiomerically complementary fused-ring THIQs and THβCs.…”

Biotransformations with Imine Reductases: Design of a Practical Process Avoiding an Extractive Work‐Up by Entrapment of Water and Enzymes in an Immobilized Phase

“…At first, we needed to develop an IRED-catalyzed reduction, for which no water-soluble co-substrates (which would result in a water-soluble co-product and consequently accumulate in the superabsorber) is used. So far, however, to the best of our knowledge in nearly all examples of IRED-based biotransformations with only a very few exceptions (see below), [3][4][5][6][7][8][9][10][11][12][13][14] in situ regeneration of NADP + is achieved by the use of a watersoluble co-substrate. The typical cofactor regeneration method is based on oxidation of D-glucose using a glucose dehydrogenase (GDH), thus forming gluconic acid as by-product.…”

“…On the other hand, water represents the favored solvent for enzyme catalysis since in general all enzymes can be applied in water as the "solvent of nature". In the following we will report a process solution addressing these "hurdles" for the enzyme class of imine reductase, which became very popular in the last years as catalyst for enantioselective heterocyclic imine reduction [3][4][5][6][7][8][9][10][11][12][13][14] and at the same time is only known to be synthetically applied in water-organic biphasic or pure aqueous homogeneous reaction media, thus raising challenges in the subsequent extraction step.…”

“…Addressing these challenges, the aim of this work was to design a bioprocess being suitable for imine reductases, which allows on the one hand the biotransformation to be carried out in aqueous medium as favored solvent for enzyme catalysis while at the same time fully avoiding any extractive work-up step (Figure 1, Part B). Imine reductases (IREDs) are NAD(P)Hdependent oxidoreductases, which catalyze the asymmetric reduction of imines to the corresponding amines and emerged as a promising enzyme class for the synthesis of a broad range of in particular heterocyclic amines, [3][4][5][6][7][8][9][10][11][12][13][14] which are common structural elements in biologically active molecules and represent important building blocks in pharmaceutical and agricultural industry. [15] As a solution to overcome the above-mentioned challenges, we envisioned to embed the entire aqueous phase including enzymes and cofactors into an "immobilized phase", and we became interested to evaluate a superabsorber material for such a purpose.…”

Hot off the Press

“…Imine reductases have also been used for the enantioselective synthesis of thiomorpholines. 30 Again, a one-pot process was developed (Scheme 3), combining a modified Asinger reaction for the synthesis of the 3,6-dihydro-2 H -1,4-thiazine followed by the imine reductase step to access the optically active thiomorpholine.…”