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

In order to clarify, the conformation of the chromophore in rhodopsin, espe cially the effect of the torsional angle around the C6-C7 single bond on th e CD spectrum, 8,18-propano- and methano-retinal 4 and 5 were prepared via the palladium-catalyzed coupling reaction of vinyl triflates derived from b icyclic ketones 10 and 17 with methyl (E)-3-(trimethylstannyl)but-2-enoate. In a binding experiment with bovine opsin, retinal analogs 4 and 5 afforde d the new rhodopsin analogs. Although the opsin shifts of these pigments wa re similar to that of the native rhodopsin, CD spectra exhibited the charac teristic feature owing to, the locked structures of retinal analogs. This f act strongly indicates that the CD spectra were significantly influenced by the torsional angles around the 6-7 and 8-9 single bonds of the chromophor e in the protein.