Synthesis of poly-Z-isomers of 2,6,11,15-tetramethylhexadeca-2,6,8,10,-14-pentaene, a C20 analogue of phytoene. Re-examination of the stereo-chemistry of a new isomer of phytoene from Rhodospirillum rubrum

(Z)-3,7-Dimethylocta-2,6-dienyltriphenylphosphonium bromide (4) has been prepared, and its condensations with formaldehyde and citral have been shown to proceed with retention of configuration about the α-double bond in the salt. Reaction between compound (4) and (Z)-citral gives a mixture of (6Z,8Z,10Z)-(15) and (6Z,8E,10Z)-(14) pentaenes showing retention or configuration about both Z-bonds in the starting materials. In a similar manner, compound (4) and (E)-citral or the (E)-phosphonium salt (5) and (Z)-citral give the pentaenes (11) and (12), whereas compound (5) and (E)-citral lead to a mixture of (6E,8E,10E)-(9) and (6E,8Z,10E)-(10) pentaenes. Mixtures of (9) and (10) and of (14) and (15) were also obtained, from (E)- and (Z)-citral respectively, by reductive coupling with titanium trichloride–lithium aluminium hydride. The isomeric pentaenes (9)–(12), (14), and (15) have been separated and characterised. 13 C N.m.r. data for the isomers of the pentaene (9), the C20 analogue of phytoene, are presented and discussed. The stereochemistry of a new isomer of phytoene from Rhodospirillum rubrum is re-examined by using comparative 13C n.m.r. data.