Iodocarbocyclization reaction

The "halocarbocyclization reaction", which involves an intramolecular attack of a carbon nucleophile on a double bond activated by an electrophilic halogenating reagent, has so far been uncommon. We report here that the "iodocarbocyclization reaction" of alkenyl or alkynylmalonate derivatives proceeded in good yields in the presence of Ti(OR)₄ and I₂ to give cyclized products in a highly regio- and stereospecific manner. The diastereoselective iodocarbocyclization with 4-pentenylmalonate derivatives having a chiral center at the allylic or homoallylic position and allylmalonate having a chiral auxiliary in the ester part has also been investigated. Furthermore, the reaction of 4-pentenylmalonate using a catalytic amount of a chiral titanium alkoxide in the presence of I₂ and 2,6-dimethoxypyridine proceeded with high enantioselectivity to give good yields of cyclopentane derivatives. The origin of enantioselectivity in this catalytic asymmetric reaction is also described.