Asymmetric Synthesis ofcis-1,2-Dialkenyl-Substituted Cyclopentanes via (−)-Sparteine-Mediated Lithiation and Cycloalkylation of a 9-Chloro-2,7-nonadienyl Carbamate

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Abstract
Herein we report our comprehensive results in enantioselective cyclopentane synthesis via stereogenic allyllithium compounds. The described cycloalkylation reaction starts with a (−)-sparteine-mediated asymmetric deprotonation of the 2,7-alkadienyl carbamate 7e and leads to the enantioenriched (80% ee) and diastereomerically pure (dr = 99:1) cis-1,2-divinyl-cyclopentane 8, by a subsequent cyclization and elimination of lithium chloride. The reaction mechanism has been investigated by silylation and lithiodestannylation experiments and was found to represent a completely regioselective anti-SN‘SE‘-reaction. Trapping of the vinyllithium intermediate 12 with various electrophiles under retention of the configuration at the double bond extends the field of application for this cyclization. We also applied this reaction as the key step in the enantioselective synthesis of (+)-dihydromultifidene (17).