STEREOCONTROL OF BILIRUBIN CONFORMATION

Optically active, diastereomeric synthetic bilirubin analogs (1 and 2) with an alpha-methyl and a beta-methyl in each propionic acid side ch ain were synthesized. Both diastereomers adopt a folded, ridge-tile co nformation. The methyls force the pigment to adopt either a left-hande d (M) or right-handed (P) helicity. As evidenced by exciton-type circu lar dichroism spectra, in the (alpha S,alpha'S,beta S,beta'S) diastere omer (1) the M helicity ridge-tile enantiomer is strongly preferred; i n the (alpha R,alpha'R,beta S,beta'S) diastereomer (2),the P helicity conformation is preferred. Copyright (C) 1996 Elsevier Science Ltd