Syntheses of Bile Pigments. Part 18 . Synthesis and conformational studies of oxa‐ and thia‐deaza‐biliverdin analogues

Following the conventional methodology used for the synthesis of bile pigments, some oxa‐ and thia‐deazabiliverdin analogues were synthetized for the first time. Both UV/VIS spectroscopic and 1 H{ 1 H}‐NOE difference studies reveal that the oxa‐deaza‐biliverdin analogue 8a occurs in apolar solvents in a partly ‘stretched’ conformation, whereas the corresponding thia derivative 8b behaves rather like a genuine bile pigment. Unexpectedly, the helical‐shaped conformation is also preponderant in a trioxa‐trideaza‐biliverdin analogue, which could be only characterized in its protonated form 14 .