Free Radical-Mediated Intermolecular Conjugate Additions. Effect of the Lewis Acid, Chiral Auxiliary, and Additives on Diastereoselectivity

We have developed a highly diastereoselective method for the conjugate addition of carbon radicals to chiral α,β-unsaturated N -enoyloxazolidinones using Bu 3 SnH as chain carrier and Et 3 B/O 2 as radical initiator. Lewis acids have been screened, and Yb(OTf) 3 proved to give optimized results for both chemical yield (88% for 1a and 94% for 1b ) and diastereoselectivity (25:1 for 1a and 46:1 for 1b ). The selectivity is solvent-dependent, CH 2 Cl 2 -THF being an ideal combination. Scrupulously dry solvents or reaction conditions were not required. Substoichiometric amounts of Yb(OTf) 3 provided efficient reaction with minimal sacrifice in diastereoselectivity. Carbon radicals with reasonable nucleophilicity were generally successful, including functionalized radicals such as acetyl or methoxymethyl. Electrophilic radicals were not successful. A model which accounts for most of our observations is presented.