Environmentally Friendly Synthesis of Indoline Derivatives using Flow-Chemistry Techniques

Flow chemistry proved to be a valuable technique to improve the synthesis route to melanin-concentrating hormone receptor 1 (MCHr1) antagonists with the 1H,2H,3H,4H,5H-[1,4]diazepino[1,7-a]indole scaffold. A one-step route for the heterogeneous catalytic hydrogenation of ethyl 4-(2-nitrophenyl)-3-oxobutanoate for the synthesis of ethyl 2-(2,3-dihydro-1H-indol-2-yl)acetate was developed, and the use of common reducing chemicals was avoided. N-Alkylation of the indoline nitrogen atom was also optimized by using a purpose-built flow reactor and by design of experiment (DoE). Applying an optimal set of parameters allowed us to decrease the amount of carcinogenic 1,2-dibromoethane used by a factor of 10. Additionally, nearly complete conversion was achieved in a fraction of the original reaction time (30 min vs. 4 d); therefore, the productivity (space-time yield) of the flow-reactor system was proven to be ca. 200 times higher than that of the batch process.