Thallium(III) reagents supported on montmorillonite clay minerals in oxythallation process

In an oxythallation process for the rearrangement of a cinnamaldehyde to the corresponding phenylmalondialdehyde tetramethyl bisacetal by reaction with an electrophilic thallium(III) salt, the improvement comprising the use of a supported thallium reagent containing from about 0.5 to 50% by weight thallium at least 75% of which is in the +3 oxidation state, said supported reagent obtained by contacting a thallium(III) salt selected from the group consisting of thallium(III) trinitrate, thallium(III) sulfate, thallium(III) perchlorate, thallium(III) acetate, thallium(III) phosphate and thallium(III) trifluoroacetate with a montomorillonite clay mineral having a surface area greater than about 200 m.sup.2 /g in a mixture of methanol and trimethylorthoformate.