a reoxidant such as cupric chloride the above discussed carbonylation reaction is preferably conducted in the presence of a polar organic solvent that is inert to the reactants employed in the carbonylation reaction i e carbon monoxide and the vinylmercuric halide the precise polar organic solvent employed is not critical and any of the conventional ones may be utilized such as ethyl alcohol methyl alcohol ether acetone and the like it should however be understood that while the utilization or an organic polar solvent in the carbonylation lactonization second step reaction is preferred if desired the reaction can be carried out in the presence of aqueous solvent medium it is preferred that the carbonylation reaction be conducted by adding the noble metal halide the lithium chloride and the polar organic solvent together in a reaction flask thereafter the reaction mixture may be cooled to low temperatures as low as 78 u00b0 c and purged with a carbon monoxide atmosphere while purging with carbon monoxide is occurring the vinylmercuric halide is added after the entire amount of the vinylmercuric halide is added to the reaction mixture the reaction is thereafter allowed to gradually rise to room temperature while low temperatures in the neighborhood of 20 u00b0 c to 78 u00b0 c are preferred it should be understood that they are not critical and that.