Catalyst-Free Cyclization- and Curtius Rearrangement-Induced Functional Group Transformation: An Improved Synthetic Strategy of First-in-Class ATX Inhibitor Ziritaxestat (GLPG-1690)

A practical and highly efficient protocol for the production of Autotaxin (ATX) inhibitor Ziritaxestat (1) was described. The procedure began with a catalyst-free bicomponent cyclization for the construction of the imidazo[1,2-a]pyridine skeleton 16. Subsequently, a typical Curtius rearrangement of carboxylic acid 17 followed by nucleophilic attacking of 3,5-dichlorobenzyl alcohol 18f led to the carbamate analogue 19f. N-methylated 20 was readily deprotected through HBr/HOAc treatment, which further conveniently took part in an alternative K2CO3-induced N-alkylation reaction with 9 to give 10. 10 coupled directly with piperazine to furnish 13, which ideally circumvent the removal of the Boc group. As a result, the hypertoxic KCN and the hypertoxic and costly isonitrile 3 involved in the tricomponent cyclization were carefully avoided. Ultimately, a novel, scalable, and cost-effective route was favorably developed to afford Ziritaxestat in a 20.4% overall yield.