DIASTEREOSELECTIVE SYNTHESIS OF PHYCOCYANOBILIN CYSTEINE ADDUCTS

Methodology is presented for the synthesis of two diastereomers of a cysteine-linked phycocyanobilin derivative. The crucial reaction is a diastereoselective 1,6-Michael addition of cysteine methyl ester to an appropriate dihydropyrromethenone educt. The diastereomers so generated were then elaborated to two phycocyanobilin trimethyl esters. Definitive assignments of relative stereochemistry, double bond geometry, and solution conformation are accomplished by application of ROESY NMR experiments, while absolute stereochemical assignments are based on degradation to compounds of known chirality.