Relative asymmetric induction in the intramolecular reaction between alkynes and cyclopropylcarbene-chromium complexes: Stereocontrolled synthesis offive-membered rings fused to oxygen heterocycles
Jb. Yan et al., Relative asymmetric induction in the intramolecular reaction between alkynes and cyclopropylcarbene-chromium complexes: Stereocontrolled synthesis offive-membered rings fused to oxygen heterocycles, J ORG CHEM, 64(4), 1999, pp. 1291-1301
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.