Synthesis and spectroscopy of chemically modified spheroidenes

The syntheses and spectroscopic properties of the all-E isomers of 11′,12′-dihydrospheroidene (2), 3,4,11′,12′-tetrahydrospheroidene (3), 3,4-dihydrospheroidene (4), 3,4,5,6-tetrahydrospheroidene (5), 3,4,7,8-tetrahydrospheroidene (6) and 15,15′-didehydrospheroidene (7) are described. Spheroidenes 2-6 have the same overall shape as native spheroidene (1), which is the carotenoid bound in the photosynthetic reaction centre of Rhodobacter sphaeroides 2.4.1, but they have instead polyene chromophores of nine (4), eight (2, 5) or seven (3, 6) conjugated double bonds. In 7, the central double bond is substituted by a triple bond. The observed λmax values of 2-6 are generally in good agreement with those calculated via the Fieser-Kuhn rules. In the mass spectra of 2-6, fragments due to bis-allylic fragmentation occur, whereas 1 and 7 do not show this type of fragmentation. A detailed analysis of the 1H- and 13C-NMR spectra of 1-7 has been achieved by mutual comparison.

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