Process for preparing 2,7-dimethyl-2,4,6-octatrienal monoacetals

A process for preparing 2,7-dimethyl-2,4,6-octatrienal monoacetals of the general formula I,

• in which the substituents R1 and R2 may, independently of one another, be C1-C8-alkyl or, together with the oxygen atoms and the carbon atoms to which they are bonded, form a 1,3-dioxolane or 1,3-dioxane ring of the following structures
• in which R3 and R4, and R5 may each, independently of one another, be hydrogen or C1-C4-alkyl,
• which comprises
• a) condensing an ester phosphonium salt of the general formula II or an ester phosphonate of the general formula III,
• in which the substituents have, independently of one another, the following meaning:
• R6 C1-C8-alkyl;
• R7 aryl;
• X− an anion equivalent of an inorganic or organic acid;
• R8 and R9
• C1-C8-alkyl;
• with an aldehyde of the formula IV
• in a Wittig or Wittig-Horner reaction to give an acetal ester of the general formula V,
• in which the substituents R1, R2 and R6 in the compounds IV and V have the abovementioned meaning,
• b) reducing the ester of the formula V to an acetal alcohol of the general formula VI,
• and
• c) oxidizing the alcohol to 2,7-dimethyl-2,4,6-octatrienal monoacetals of the general formula I.