Synthesis and spectroscopic characterization of the doubly locked 9E,11Z retinal model systems 7E,13E‐11,19–10,20‐dimethanoretinal and its 13Z isomer

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Abstract
7E,13E‐11,19‐10,20‐dimethanoretinal (1) and its 13Z isomer 2 were prepared from β‐ionone, using the novel synthon 2‐(diethoxyphosphinyl)‐5,5‐dimethoxyhexanenitrile. This synthon was also used to prepare 7E,9E,13E‐10,20‐methanoretinal (3) and its 13Z isomer 4 in high yield, starting from β‐ionone. Spectroscopic analysis (mass, 1H and 13C NMR and UV/Vis) of these compounds is discussed. The introduction of the methano bridges leads to minimal steric and electronic changes. The photostationary state reached from 1 and 2 has the 13Z form 2 as the main constituent. This is one of the very few with a Z form as the main constituent of the photostationary state. 1 and 2 are very sensitive to acid‐catalyzed isomerization of the 13‐C14‐C double bond. The presence of the 11,19‐methano bridge is responsible for this efficient Z‐E isomerization.