Synthesis and spectroscopy of (14′‐13C)‐ and (15′‐13C)spheroidene

The preparation of two13C‐labelled isotopomers of spheroidene, the carotenoid bound in the photosynthetic reaction center ofRhodobacter sphaeroides, is described.All‐E(14′‐13C)‐spheroidene has been synthesized in high yield and purity and with more than 98%13C incorporation starting from (2‐13C)acetonitrile; similarly, all‐E(15′‐13C)spheroidene has been obtained starting from (I‐13C)acetonitrile. The spectroscopic properties of all‐Espheroidene and its (14′‐13C)‐ and (15′‐13C)isotopomers are discussed. The mass spectra of the labelled compounds reveal that toluene is eliminated mainly from the central part of the molecule. Analysis of the ′H‐NMR spectrum of all‐E(15′‐13C)spheroidene gives a value of 14.0 Hz for the1H ‐1H coupling constant of the 15‐15′ double bond.