Retinoids and related compounds. Part 26. Synthesis of (11Z)-8,18-propano- and methano-retinals and conformational study of the rhodopsin chromophore

Abstract

In order to clarify the conformation of the chromophore in rhodopsin, especially the effect of the torsional angle around the C6–C7 single bond on the CD spectrum, 8,18-propano- and methano-retinal 4 and 5 were prepared via the palladium-catalyzed coupling reaction of vinyl triflates derived from bicyclic ketones 10 and 27 with methyl (E)-3-(trimethylstannyl)but-2-enoate. In a binding experiment with bovine opsin, retinal analogs 4 and 5 afforded the new rhodopsin analogs. Although the opsin shifts of these pigments were similar to that of the native rhodopsin, CD spectra exhibited the characteristic feature owing to the locked structures of retinal analogs. This fact strongly indicates that the CD spectra were significantly influenced by the torsional angles around the 6–7 and 8–9 single bonds of the chromophore in the protein.