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

Synthesis of cyclopentenone derivatives fused to oxygen heterocycles by means of the intramolecular coupling of alkynes and cyclopropylcarbene-chromium 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 direction of relative asymmetric induction is very dependent upon the position of the chiral atom within the tethering chain and the length of the tethering 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 worst case, featuring a three-carbon tether and a stereogenic center at the homopropargylic position (complexes 1F and ii), no stereoselectivity was observed. Improvement in stereoselectivity was noticed when terminal alkynes were replaced by silylated alkynes and when proton sources were eliminated from the reaction.