Stereochemistry of the halolactonisation reaction

The stereochemistry of the reaction of cis - and trans -stilbene-2-carboxylic acids with chlorine and bromine, yielding the two diastereoisomeric 3-phenyl-4-halo-3:4-dihydro iso coumarins (III) and (IV) has been investigated. The reactions are entirely stereospecific and involve intramolecular attack by the carboxyl group on an intermediate carbonium or halonium ion. The lactones (III) and (IV) yield the two 3-(α-hydroxybenzyl) phthalides (V) and (VI), with retention of configuration. Thionyl chloride transforms (V) and (VI) into the 3-(α-chlorobenzyl) phthalides (IX) and (X), with inversion. Steric effects are mainly responsible for the alternative formation of halolactones or normal dihalogenated derivatives in the reactions between unsaturated acids and halogens.