Oxidative condensation-cyclization of aldehydes and aryl-2-pyridylmethylamines proceeded in the presence of a stoichiometric amount of elemental sulfur as an oxidant in the absence of catalyst. The reaction gave a variety of 1,3-diarylated imidazo[1,5-a]pyridines in good to high yields. The products showed fluorescence emission in a wavelength range of 454-524 nm. The quantum yields of 1,3-diarylated imidazopyridines were greatly improved compared to those of the parent 3-monosubstituted compounds.

Iodine-mediated, oxidative desulfurization promoted cyclization of N-2-pyridylmethyl thioamides serves as an efficient and versatile method for the preparation of 2-azaindolizines (imidazo[1,5-a]pyridines) and rare 2-azaindolizine sulfur-bridged dimers. The 2-azaindolizines prepared in this manner are readily converted to a variety of fluorescent compounds by using transition-metal-catalyzed cross-coupling reactions. [reaction: see text].

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Properties. -Various functionalized 2-azaindolizines are prepared from readily available thioamides. The reaction strongly depends on the time and the solvent allowing the preparation of sulfur-bridged dimers [cf. (III)] as well. Product (IIe) is transformed to fluorescent derivatives using transition metal-catalyzed cross-couplings. -(SHIBAHARA*, F.; KITAGAWA, A.; YAMAGUCHI, E.; MURAI, T.; Org. Lett. 8 (2006) 24, 5621-5624; Dep. Chem., Fac. Eng., Gifu Univ., Yanagido, Gifu 501-11, Japan; Eng.) -R. Steudel 14-159

Fused pyridine derivatives R 0450 Synthesis of 1,3-Diarylated Imidazo[1,5-a]pyridines with a Combinatorial Approach: Metal-Catalyzed Cross-Coupling Reactions of 1-Halo-3-arylimidazo[1,5-a]pyridines with Arylmetal Reagents. -Selective halogenation of a variety of substrates (I) is achieved. Only (Ig), bearing a highly electron-rich 4-methylaminophenyl group fails to provide the iodo derivative. Iodinated and brominated products are further applied to Kumada-Tamao-Corriu [→ cf. (IV)] and Suzuki-Miyaura [cf. (VII)] cross-coupling protocols. Photophysical properties, UV/vis and fluorescent spectra of the diarylated products are studied. -(SHIBAHARA*, F.; YAMAGUCHI, E.; KITAGAWA, A.; IMAI, A.; MURAI*, T.; Tetrahedron 65 (2009) 26, 5062-5073; Dep. Chem., Fac. Eng., Gifu Univ., Yanagido, Gifu 501-11, Japan; Eng.) -Y. Steudel 44-154