Two-Step Synthesis of a Dolutegravir Intermediate DTG-6 in a Microfluidized Bed Cascade System: Route Design and Kinetic Study

In the existing two-step method for the preparation of DTG-6 (i.e., an important intermediate of the anti-HIV drug Dolutegravir (DTG)), a strong base is required to neutralize the homogeneous strong acid catalyst of the first step to make the reaction solution weakly acidic for the DTG-5 cyclization in the second step. The DTG-6 yield in the two-step synthesis is affected by the reaction of the strong base with the carboxyl group on the generated intermediate DTG-5. In this article, a solid acid catalyst, titanium cation-exchanged montmorillonite (Ti⁴⁺-mont), was used in the microfluidized bed to catalyze the conversion of DTG-4 to DTG-5. DTG-5 can be directly cyclized with (R)-3-aminobutanol (RABO) to form DTG-6 without the introduction of a strong base into the reaction solution. After the parametric screening on the flow rate, solid acid type, temperature, residence time, and solvent type, the DTG-6 yield increased from 90% (in our previous work) to 95% in the microfluidized bed cascade system. Due to the easy separation of heterogeneous catalyst, the utilization of a microfluidized bed not only simplified operations, but also improved synthetic efficiency. Moreover, the kinetics of the cyclization of unstable intermediate DTG-5 with RABO was investigated and verified by means of experimental data.