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 alpha,beta-unsaturated N-enoyloxazolidinones using Bu3SnH as chain carrier and Et3B/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, CH2Cl2-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.