Synthesis of Sterically Fixed Phytochrome Chromophore Derivatives Bearing a 15E-Fixed or 15E-Anti-Fixed CD-Ring Component

To analyze the structure and function of phytochrome chromophores, we have been synthesizing natural and unnatural bilin chromophores of phytochromes. In this manuscript, we report the synthesis of sterically fixed 15E-fixed 18Et-biliverdin (BV) and 15E-anti-fixed 18Et-BV derivatives. The key reaction is the introduction of an sp3 carbon alkyl chain bearing a leaving group at the meso-position of the CD-ring component by using the corresponding Grignard reagents in the presence of LiCl.