The invention relates to a process for preparing phytofluene of the formula I,
where
a) a phosphonium salt of the formula II is condensed with an aldehyde of the formula III
in a Wittig reaction to give an acetal of the formula IV
b) the condensation product of the formula IV is subjected to an acid-catalyzed acetal hydrolysis to give the aldehyde of the formula V
c) and V is condensed in a further Wittig reaction with a phosphonium salt of the formula VI to give phytofluene
where the radicals R
1
, R
2
and R
7
, and X
−
and Y
−
have the meaning sated in the description.