(-)-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 inducti on has been achieved for the first time by fusing the concepts of the asymm etric deprotonation (A) with s-BuLi/(-)-sparteine (s-BuLi/1) and the intram olecular carbolithiation (B). Several 4-functionalized 5-hexynyl carbamates with different terminal substituents have been prepared and efficiently cy clized providing enantiopure highly substituted alkylidene cyclopentanes. T he presence of a sterically demanding substituent in the propargylic positi on 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 a n intramolecular carbolithiation whereas for the 6-trimethylsilyl-substitut ed alkynes a sub sequent migration of the O-carbamoyl group onto the vinyli c carbanionic center follows.