Hydrogen-Bonding as a Stereocontrolling Element in Free-Radical C-Allylation Reactions: Vicinal, Proximal, and Remote Asymmetric Induction in the Amino Acid Series

摘要

Stereocontroiled bond-forming reactions by free-radical processes in acyclic molecules are currently among the more challenging areas in asymmetric synthesis. Recent advances in this area have generally focused on the utilization of derivatives of carboxylic acids with appropriate chiral auxiliaries and on exploiting the presence of beta-alkoxy substituents and other resident groups. A model for the transition state in such reactions involving 1,2-asymmetric induction has been proposed by Hart and Krishnamurthy and further elaborated upon by Curran and co-workers. The general consensus is that, with few exceptions, the high diastereoselectivity is the result of a combination of minimized allylic 1,3-strain and torsional strain and of stereoelectronic effects. The notion that intramolecular H-bonding might play a role in the C-deuteration and C-allylation of beta-hydroxycarbalkoxy and amide a-radicals has been alluded to and experimentally tested with unrewarding or inconclusive results.