Syntheses of 13C2-labelled 11Z-retinals

To enable solid-state NMR investigations of the rhodopsin chromophore and its photointermediates, a series of 11Z-retinal isotopomers have been synthesised containing pairs of adjacent 13C labels at C9/C10, C10/C11 or C11/C12, respectively. The C9 labelled carbon atom was introduced through the Heck reaction of a 13C-labelled Weinreb acrylamide derivative, and the label at the C12 position derived from a 13C-containing ethoxy Bestmann–Ohira reagent. The 13C labels at C10 and C11 were introduced through the reaction of ?-ionone with labelled triethyl phosphonoacetat