Compounds of the formula I
(see formula I) where R1 is H or C1-C4-alkyl, R2 is C1-C4-alkyl and R3 is an ether, silyl ether or acetal protective group which can be converted into a hydroxyl group by hydrolysis, in particular one of the radicals (see formula IV) or (see formula V) and a process for preparing these compounds by reacting an alkenyne of the formula II
(see formula II) in an inert solvent in the presence of lithium amide with a cyclohexenone of the formula III
(see formula III) and the use of the compounds of the formula I for preparing astaxanthin, are described.
