Preparation of indolenines

A process for preparing an indolenine of the formula I ##STR7## where R.sup.1, R.sup.2 and R.sup.3 are independently of one another hydrogen, C.sub.1 -C.sub.20 -alkyl, C.sub.2 -C.sub.20 -alkenyl, C.sub.3 -C.sub.8 -cycloalkyl, phenyl, 1-naphthyl, 2-naphthyl, C.sub.7 -C.sub.20 -alkylphenyl, C.sub.7 -C.sub.20 -phenalkyl, or R.sup.1 and R.sup.2 or R.sup.1 and R.sup.3 together are an unsubstituted or C.sub.1 -C.sub.12 -alkyl-substituted C.sub.3 -C.sub.8 -alkylene chain,

• X and Y are independently of one another hydrogen, C.sub.1 -C.sub.8 -alkyl, C.sub.1 -C.sub.8 -alkoxy, C.sub.2 -C.sub.8 -alkoxyalkyl, C.sub.2 -C.sub.8 -alkoxycarbonyl, C.sub.1 -C.sub.4 -alkylsulfonyl, halogen, nitro or cyano,
• by reacting a 4-methylene-1,3-dioxolan-2-one of the formula II ##STR8## where R.sup.1, R.sup.2 and R.sup.3 are each as defined above, with an aniline of the formula III ##STR9## where X and Y are each as defined above, in the presence of a Lewis acid and in the presence or absence of a halide of main group 1 or 2 of the Periodic Table at from 100.degree. to 350.degree. C. and at from 1 to 50 bar, the improvement wherein the reaction is carried out in the presence of a polar solvent.