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

Article abstract of DOI:10.1016/j.tet.2006.07.108

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 C₄-C₅ 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 °C.?The stereospecificity and absence of MeOH trapping adducts indicate that these reactions may not involve open or trappable dipolar intermediates.