Continuous Flow-Facilitated CB2 Agonist Synthesis, Part 2: Cyclization, Chlorination, and Amination

A new route to the cannabinoid receptor type 2 agonist, RG7774, has been developed circumventing an alkylation with poor regioselectivity as the final step. In the new synthetic route, this side chain is incorporated from the beginning. In this article, the development of the final four transformations is detailed, using a combination of batch and flow processing. Due to poor solubility, an N-pivaloylation was performed in batch, followed by cyclization at up to 200 degrees C, enabled by flow processing. The following chlorination and SNAr steps were examined in both batch and flow for improved handling of hazardous reagents and intermediates, as well as enhanced heat transfer. A workup between these two steps was found to be vital in preventing side product formation from residual dimethylamine. To achieve this on a laboratory scale, a continuous solid phase treatment was developed, whereby two cation exchange columns (containing SCX-2 silica) were cycled, with monitoring by UV/vis analysis. These four steps were demonstrated with a combined yield of 72%, which serves as a significant positive contribution to the new route's high overall yield of 53%.