. . . "The reaction of a compound of formula III wherein Ya is hydrogen or halogen, especially iodine or more especially chlorine or bromine, and Xa is halogen, while Ra and Rb are as defined for compounds of formula III, in the presence of a base, with elimination of hydro-halic acid HX, to form an olefin of formula VII is preferably carried out in the presence of a base selected from a nitrogen base, e" .