Photochemical synthesis and ring-chain-ring tautomerism of benzo[4,5]imidazo[1,2-]cyclopenta[]pyridines.

UV-mediated approach for the preparation of benzo[4,5]imidazo[1,2-]cyclopenta[]pyridine derivatives from allomaltols containing a benzimidazole fragment was developed. The suggested method includes ESIPT-induced contraction of a 3-hydroxypyran-4-one core and further intramolecular trapping of unstable α-hydroxy-1,2-diketone. The distinctive feature of the obtained photoproducts is ring-chain-ring tautomerism in solution. Based on X-ray analysis, the obtained photoproducts in the solid state exist in one isomeric form. Various types of derivatization of the synthesized compounds allow both forms of tautomers to be trapped.