Syntheses of Bile Pigments. Part 16 . Synthesis of a vinyl‐substituted 2,3‐Dihydrobilinedione: Possible role of this new class of bile pigments in phycobilin biosynthesis

The total synthesis of racemic cis ‐2,3,18 1 , 18 2 , ‐tetrahydroprotobiliverdin IXα dimethyl ester ( 19b ), which is identical with the dimethyl ester of rac ‐ 4 , is described ( Scheme 2 ). Under virtually neutral conditions, in solution, this bile pigment isomerized within a few min to racemic Z ‐phycocyanobilin dimethyl ester ( rac ‐ 5b ). Likewise, acid‐catalyzed allyl rearrangement of 3‐vinyl‐substituted cis ‐ and trans ‐2,3‐dihydrodipyrrin‐1(10 H )‐ones 11c and 13c , respectively, yielded the corresponding ethylidene derivatives. In this case, however, the E ‐isomer was formed stereo selectively from both substrates. The above results prove that, if protobiliverdin IXα ( 2 ) is transformed enzymatically to its 2,3,18 1 , 18 2 ‐tetrahydro derivative, the latter would isomerize spontaneously to phycocyanobi‐lin. The biosynthesis of bacteriochlorophyll a and b from a common precursor bearing a vinyl group at C(8)may be straightforwardly explained in the same way.