Harnessing the Substrate Promiscuity of Dioxygenase AsqJ and Developing Efficient Chemoenzymatic Synthesis for Quinolones

Abstract

Nature has developed complexity-generating reactions within natural product biosynthetic pathways. However, direct utilization of these pathways to prepare compound libraries remains challenging because of limited substrate scopes, involvement of multiple-step reactions, and moderate robustness of these sophisticated enzymatic transformations. Synthetic chemistry offers an alternative approach to prepare natural product analogues. However, because of complex and diverse functional groups appended on the targeted molecules, dedicated design and development of synthetic strategies are typically required. Herein, by leveraging the power of chemoenzymatic synthesis, we report an approach to bridge the gap between biological and synthetic strategies in the preparation of quinolone alkaloid analogues. Leading by in silico analysis, the predicted substrate analogues were chemically synthesized. The AsqJ-catalyzed asymmetric epoxidation of these substrate analogues was followed by a Lewis acid-triggered ring contraction to complete the viridicatin formation. We evaluated the robustness of this method in gram-scale reactions. Lastly, through chemoenzymatic cascades, a library of quinolone alkaloids is effectively prepared.