Novel Formation of Iodobenzene Derivatives from 1-(Methylthio)-3-tosylhexa-1,3-dien-5-ynes via Iodine-Induced Intramolecular Cyclization

The reaction of (1E,3E)-1-(methylthio)-3-tosylhexa-1,3-dien-5-ynes (3) with iodine to form iodine-substituted benzenes (4) is reported. The reaction of 3 with iodine proceeded very slowly, but UV irradiation
accelerates the reaction to give 4 in high-to-excellent yields. Irradiation induces the cis-trans isomerization
of the C1−C2 double bond, leading to the (1Z,3E)-geometric isomer (3‘), which easily reacts with iodine
to afford 4. This reaction is applicable to 3-(methoxycarbonyl)-1-(methylthio)-6-phenylhexa-1,3-dien-5-yne (11), which is synthesized as a geometric mixture. Interestingly, this mixture can be used as the
starting material. Irradiation of the mixture (the geometric isomer ratio = 50:28:5:17) with iodine resulted
in the formation of methyl 3-iodo-4-phenylbenzoate (12) in 80% yield.