Reactions of urocanic acid (UCA) methyl esters with singlet oxygen and 4-methyl-1,2,4-triazoline-3,5-dione (MTAD)

摘要

Singlet oxygen adds to the imidazole ring of cis- and trans-methyl urocanate (MUC) to yield the corresponding 2,5-endoperoxides, which are modestly stable at low temperature but decompose upon warming to form complex reaction mixtures. MTAD, a singlet oxygen mimic, reacts with cis- and trans-MUC to yield stereospecific [4+2] reaction products involving the olefinic side chain and the C-4-C-5 double bond of the imidazole ring. trans-MUC forms a 1:2 MTAD adduct while the cis isomer yields only the 1: 1 adduct at 25 degrees C. The stereospecificity and absence of MeOH trapping adducts indicate that these reactions may not involve open or trappable dipolar intermediates. (c) 2006 Elsevier Ltd. All rights reserved.