ANIONIC CYCLIZATION OF OLEFINIC ALKYLLITHIUMS - RING-CLOSURE OF TERMINALLY SUBSTITUTED 5-HEXENYLLITHIUMS

A series of 5-hexenyllithiums having a phenyl, trimethylsilyl, or cyclopropyl substituent at the terminal [C(6)] alkene carbon have been prepared from the corresponding iodides by lithium-iodine exchange with t-butyllithium at -78 degrees C. Although 6-alkyl-substituted 5-hexenyllithiums do not isomerize to five-membered rings upon warming, terminally substituted 5-hexenyllithiums bearing a moderately activating phenyl or trimethylsilyl group cleanly undergo a totally regiospecific 5-exo cyclization at sub-ambient temperatures to afford five membered rings bearing a CHRLi moiety that may be trapped with an electrophile to deliver high yields of functionalized product. Cyclization of 6-cyclopropyl-5-hexenyllithium is accompanied by ring opening of the three-membered ring.