STERICALLY CROWDED HETEROCYCLES .8. PREPARATIVE PHOTOISOMERIZATION OFSOME IMIDAZO[1,2-A]PYRIDINES

UV-Illumination of zo[1,2-a]pyridin-3-yl)-1,3-diphenylprop-2-en-1-one (1a) or its 6-methyl and 6-iodo derivatives 1c and 1g resulted in mino r (E)-isomers 2a, 2c, and 2g and prevailing (E,Z)-mixtures of -2-yl)-[ (3,5-diphenylfuran-2-yl)phenylmethylidene] amines 3a, 3c, and 3g while only corresponding furanoic derivatives 3b and 3d-3f were obtained fr om 5-methyl (1b), 7-methyl (1d), 8-methyl (1e), and 5-phenyl (1f) deri vatives of the (Z)-ketone 1a. VIS-Illumination of (Z)-1,3-diphenyl-3-( 2-phenylimidazo [1,2-a]benzo[h]quinolin-3-yl)prop-2-en-1-one (4) led t o )-[(3,5-diphenylfuran-2-yl)phenylmethylidene]amine (5). Photoisomers 2a, 2c, 2g, 3a-3g, and 5 were isolated and the molecular structure of 3c was X-ray determined. Mechanism of the photoisomerization is discu ssed using semiempirical quantum chemical calculations and compared wi th mass spectra of compounds 1a, 2a, 3a, 1g, 2g, and 3g.