Propentdyopents [5-(2-oxo-2H-pyrrol-5-ylmethylene)pyrrol-2(5H)-ones] and related compounds. Part 2. The Z⇄E photoisomerisation of pyrromethenone systems

Abstract

The Z⇄E photoisomerisation of four α-unsubstituted pyrromethenones which are potential precursors of alkanol–propentdyopent adducts is studied using a direct n.m.r. spectroscopic approach in CD₃OD–CDCl₃. Chemical-shift assignments are made with the help of n.O.e. and decoupling experiments. For the polyalkylpyrromethenones the photostationary state contains ca. 25% of the E isomer: a system with a β-ethoxycarbonyl group leads to 43%E isomer at photoequilibrium. The E isomers can be manipulated in solution without appreciable change, but thermodynamic equilibration to the Z isomer (ca. 99%) occurs on dry silica. A similar photoisomerisation of (4Z,15Z)-bilirubin IIIα in ND₃–CD₃OD leads to a photoequilibrated mixture containing 16% of (4E,15Z)-bilirubin IIIα.