Synthesis and Cytokinin Activity of Fluorescent 7-Phenylethynylimidazo[4,5-b]pyridine and Its Riboside

7-Phenylethynylimidazo[4,5-b]pyridine and its riboside have been newly developed as fluorescent carbon-substituted cytokinin analogues. Palladium-catalyzed coupling of 7-iodo-3-(tri-O-acetyl-β-d-ribofuranosyl)imidazo[4,5-b]pyridine with phenylacetylene followed by ammonolysis afforded the 7-phenylethynyl riboside via its tri-O-acetate. Acid hydrolysis of the riboside provided its free base, which showed a marked enhancement in fluorescence intensity in an aqueous alkaline solution. The free base and its riboside were more active than the corresponding 6-phenylethynylpurine and its riboside, respectively, in Amaranthus betacyanin and tobacco callus bioassays. Surprisingly, the imidazo[4,5-b]pyridine base exhibited strong cytokinin activity comparable to that of N 6-benzyladenine in the tobacco callus bioassay. This compound would be useful for studying localization and transport of cytokinins in cells or tissues of plants. Keywords: 7-Alkynylimidazo[4,5-b]pyridine; 6-alkynyl-1-deazapurine; fluorescent cytokinin analogue; tobacco callus growth; betacyanin biosynthesis