Highly Functionalized Cyclopentane Derivatives by Tandem Michael Addition/Radical Cyclization/Oxygenation Reactions
Abstrakt
Densely functionalized cyclopentane derivatives with up to four consecutive stereocenters are assembled by a tandem Michael addition/single-electron transfer oxidation/radical cyclization/oxygenation strategy mediated by ferrocenium hexafluorophosphate, a recyclable, less toxic single-electron transfer oxidant. Ester enolates were coupled with alpha-benzylidene and alpha-alkylidene beta-dicarbonyl compounds with switchable diastereoselectivity.
This pivotal steering element subsequently controls the diastereoselectivity of the radical cyclization step. The substitution pattern of the radical cyclization acceptor enables a switch of the cyclization mode from a 5-exo pattern for terminally substituted olefin units to a 6-endo mode for internally substituted acceptors.
The oxidative anionic/radical strategy also allows efficient termination by oxygenation with the free radical 2,2,6,6-tetramethyl-1-piperidinoxyl, and two C-C bonds and one C-O bond are thus formed in the sequence. A stereochemical model is proposed that accounts for all of the experimental results and allows the prediction of the stereochemical outcome.
Further transformations of the synthesized cyclopentanes are reported.