Continuous-flow synthesis of polysubstituted γ-butyrolactones via enzymatic cascade catalysis

Polysubstituted chiral γ-butyrolactones are the core structural units of many natural products and high value-added flavors and fragrances used in the food and cosmetic industry. Current enzymatic cascade synthesis of these molecules faces the problems of low enzyme activity and phase separation in batch reaction, resulting in low productivity. Herein, we report a new continuous-flow process to synthesize the optically pure Nicotiana tabacum lactone (3S,4S)-4a and whisky lactone (3R,4S)-4b from α,β-unsaturated γ-ketoesters. A new ene reductase (ER) from Swingsia samuiensi (SsER) and a carbonyl reductase (SsCR) were engineered by directed evolution to improve their activity and thermostability. The continuous-flow preparative reactions were performed in two 3D microfluidic reactors, generating (3S,4S)-4a (99% ee and 87% de) and (3R,4S)-4b (99% ee and 98% de) with space-time yields 3 and 7.4 times higher than those of the batch reactions. The significant enhancement in the productivity of enzyme cascade catalysis brought by cutting-edge continuous microfluidic technology will benefit the general multi-enzyme catalytic systems in the future.