(-)-sparteine-mediated stereoselective intramolecular carbolithiation of 4-substituted 5-hexynyl carbamates.: Synthesis of enantiopure 1,3-difunctionalized alkylidene cyclopentanes

The stereoselective carbolithiation of alkynes with external chiral induction has been achieved for the first time by fusing the concepts of the asymmetric deprotonation (A) with s-BuLi/(-)-sparteine (s-BuLi/1) and the intramolecular carbolithiation (B). Several 4-functionalized 5-hexynyl carbamates with different terminal substituents have been prepared and efficiently cyclized providing enantiopure highly substituted alkylidene cyclopentanes. The presence of a sterically demanding substituent in the propargylic position is the essential feature of these cyclizations in order to suppress the abstraction of the remaining propargylic proton. Furthermore, in dependence on the terminal substituent, the 6-phenyl-substituted precursors undergo an intramolecular carbolithiation whereas for the 6-trimethylsilyl-substituted alkynes a sub sequent migration of the O-carbamoyl group onto the vinylic carbanionic center follows.