Development of a Practical Synthesis of Functionalized Azaxanthene-Derived Nonsteroidal Glucocorticoid Receptor Modulators

An efficient route to two functionalized 2-aryl-5H-chromeno[2,3-b]pyridines (azaxanthenes) is reported. The addition of lithiated 2,6-dichloropyridine to salicylaldehyde followed by cyclization was a key process improvement identified for the formation of the azaxanthene core. Further elaboration of 2-chloro-5H-chromeno[2,3-b]pyridin-5-ol at the 5 position was accomplished via Lewis acid -catalyzed coupling with commercially available ((1-methoxy-2-methylprop-1-en-1-yl)oxy)-trimethylsilane. A partial classical resolution coupled with a preparative chiral supercritical fluid chromatography (SFC) separation was used to isolate the desired enantiomer of the azaxanthene carboxylic acid that is a common intermediate for both compounds 1 and 2. Suzuki-Miyaura cross-coupling with appropriately substituted boronic acids, followed by condensation with 2-amino-1,3,4-thiadiazole, provided the target compounds with an overall yield of approximately 10%. The use of stable, amorphous materials to support clinical comparison of functionalized azaxanthenes 1 and 2 is also discussed.