Photogeneration of quinone methide-type intermediates from pyridoxine and derivatives

The photochemical generation of novel quinone methide-type intermediates has been observed upon photolysis of Vitamin B-6 (pyridoxine, 1) and its derivatives 7 and 8. Irradiation of 1 or 7 in 1: 1 CH3OH/H2O gives the corresponding methyl ethers (phi(p) = 0.18 and 0.21, respectively), consistent with nucleophilic attack of methanol on o-quinone methide-type intermediates. Similarly, photolysis in aqueous CH3CN with ethyl vinyl ether resulted in the regioselective formation of the respective chroman products through [4 + 2] cycloaddition. Although laser flash photolysis (LFP) studies point to formation of two different o-quinone methide-type intermediates upon irradiation of either 7 or 8 in neutral aqueous solution, only one such reactive species is observed in alkaline solutions. These intermediates necessarily arise from formal loss of water. The mechanism by which this occurs is dependent upon the pH of the solution. In alkaline solution formation of the quinone methide-type intermediate occurs by dehydroxylation of the excited state phenolate. In neutral solution the reactive species may be formed either by ESPT or by excited state intramolecular proton transfer (ESIPT) between the phenol and the benzylic alcohol, followed by loss of water. (C) 2002 Elsevier Science B.V. All rights reserved.