Abstract: There is described a process for the production of a compound of formula I, ##STR1## in which at least one each of R.sub.1, R.sub.2 and R.sub.3, and of R.sub.4, R.sub.5 and R.sub.6, represents an electron withdrawing group, or one of R.sub.1, R.sub.2 and R.sub.3 together with one of R.sub.4, R.sub.5 and R.sub.6 form a --CO-- group, and the remainder of R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6, which may be the same or different, each represent an electron withdrawing group, alkyl, cycloalkyl, phenyl or phenylalkyl,or R.sub.1 and R.sub.2, or R.sub.4 and R.sub.5 , and R.sub.1 and R.sub.4, may together form a --(CH.sub.2).sub.n --chain, in which n is a whole number up to 5,the alkyl, cycloalkyl, phenyl or phenylalkyl group or the --(CH.sub.2).sub.n --chain optionally being substituted, which comprises reacting a compound of formula II, ##STR2## in which R.sub.1 a, R.sub.2 a and R.sub.3 a have the same significances as R.sub.1, R.sub.2, and R.sub.