HIGHLY STEREOSELECTIVE TANDEM CYCLIZATIONS OF 5-HEXENYLLITHIUMS - PREPARATION OF ENDO-2-SUBSTITUTED BICYCLO[2.2.1]HEPTANES AND 3-SUBSTITUTED TRANS-BICYCLO[3.3.0]OCTANES

Tandem cyclization of diolefinic alkyllithiums, derived from acyclic diolefinic alkyl iodides by low-temperature lithium-iodine exchange, proceeds via two highly stereoselective and totally regiospecific 5-exo-trig ring closures to deliver bicyclic alkyllithiums. Trapping of the organolithium product by addition of an electrophile cleanly affords functionalized bicyclic molecules in good yield. In this way both endo-2-substituted bicyclo[2.2.1]heptanes and 3-substituted trans-bicyclo[3.3.0]octanes have been prepared in isolated yields of 65-80% from the readily available 3-(2-iodoethyl)-1,5-hexadiene (5) and 7-iodo-4-ethenyl-1-heptene (9), respectively. Attempts to effect tandem cyclization of 5,10-undecadienyllithium (11), which would be mediated by a secondary alkyllithium species, were unsuccessful. The results suggest that tandem anionic cyclization provides a convenient route to a variety of bicyclic systems not readily available by other approaches.