Process for the stereoselective preparation of insecticide 6,7,8,9,10-10-hexahalo-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin-3-oxide

A process for the stereoselective preparation of insecticide 6,7,8,9,10,10-hexahalo-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin-3-oxide of the general formula I:wherein X is a halogen such as fluorine, chlorine or bromine. The process comprises reacting 1,4,5,6,7,7-hexahalo-5-norbornene-2,3-dimethanol of the general formula IV:wherein X is as defined above, with a cyclic sulfite ring forming reagent optionally in an inert organic solvent at ambient to 139° C., wherein the reaction is carried out in the presence of a stereo isomer directing agent comprising an isomer of the compound of the formula I different from the desired isomer. The molar ratio of the stereoisomer directing agent to the dimethanol compound of the formula IV is at least 0.07.