Novel reaction modes of biliverdin-IXα: synthesis and structure of bilatriene dimers

Novel reactions of biliverdin-IXα dimethyl ester 2 affording compounds carrying two bilatriene entities are reported. Thus compound 2 undergoes various acid-promoted dimerisations by regio-and site-selective reaction of the respective vinyl groups adjacent to the pyrrolinone carbonyls to give the cyclobutane, 4, the diastereoisomeric cyclohexenes 5a and 5b, the butene 6 and its corresponding methanol adducts 7a and 7b. The distribution of products can be controlled by varying the concentration of acid and temperature. The elucidation of structure, stereochemistry and conformation of the compounds synthesised is accomplished mainly using spectroscopic means. The 1,2-trans relation of the two biliverdin moieties in the cyclobutane 4 is established by an asymmetric synthesis. The mechanism of reactions and the differences in the reactivity of the two vinyl groups in compound 2 and other biliverdins are discussed.