Synthesis of an Ethynyl Carbamate and Application for Enantioselective Cyclocarbolithiation

The intramolecular trans-cyclocarbolithiation of the α-lithiated 4-substituted 5-hexynyl carbamate (1S,4RS)-14 employing lithiodestannylation is presented. The 5-exo-dig cyclization products cis-/trans-16a were formed exclusively. The highly enantioenriched organotin precursor (S)-11 was synthesized via an asymmetric deprotonation of the corresponding alkyl carbamate 10 by the chiral complex sec-butyllithium/(−)-sparteine and subsequent substitution with tributyltin chloride.