Synthesis of the key intermediate ( 5) of Antillatoxin ( 1) is presented. The synthesis is based on the indium-mediated allylation of 2,4,6,6-tetramethyl-2,4-heptadienal ( 3) and methyl (Z)-2-(bromomethyl)-2-butenoate ( 4) in saturated ammonium chloride catalyzed by lanthanide triflate which afforded the corresponding product 2 in good yield and high syn selectivity.