The pure four stereoisomers of 3,ll-dimethyl-2-nonacosanone (1), the female-produced sex pheromone of the German cockroach, were synthesized starting from (R)-citronellol (2a) and ethyl (R)-3-hydroxybutanoate (3). The key step was the chromatographic separation of (5RS,6R)-6-hydroxy-5-methyl-2-heptanone (16a) to give pure (5R,6R- and) 5S,6R-isomers. All of the four stereoisomers of 1 were bioactive. © 1990.