Relative asymmetric induction in the intramolecular reaction between alkynes and cyclopropylcarbene-chromium complexes: Stereocontrolled synthesis offive-membered rings fused to oxygen heterocycles

Synthesis of cyclopentenone derivatives fused to oxygen heterocycles by mea ns of the intramolecular coupling of alkynes and cyclopropylcarbene-chromiu m complexes has been examined for a variety of cases in which the tethering chain features a stereogenic center. In some cases, this process proceeds with a very high degree of stereoselectivity; however, the extent and direc tion of relative asymmetric induction is very dependent upon the position o f the chiral atom within the tethering chain and the length of the tetherin g chain. In the best case, featuring a two-carbon tether and a stereogenic center at the homopropargylic position (complex 1N), the heterocyclic ring was produced with 97:3 selectivity for the cis heterocycle (3N). In the wor st case, featuring a three-carbon tether and a stereogenic center at the ho mopropargylic position (complexes 1F and ii), no stereoselectivity was obse rved. Improvement in stereoselectivity was noticed when terminal alkynes we re replaced by silylated alkynes and when proton sources were eliminated fr om the reaction.