Unsaturated oxo-nitriles: Stereoselective, chelation-controlled conjugate additions

Addition of Grignard reagents to the unsaturated oxo-nitrile 10 (4-hydroxy- 4-methyl-6-oxocyclohex-1-enecarbonitrile) provides conjugate addition produ cts with virtually complete stereocontrol. Mechanistic evidence supports a chelation-controlled conjugate addition via alkylmagnesium alkoxide inter m ediates. Diverse Grignard reagents having sp(3)-, sp(2)-, and sp-hybridized carbons react with comparable efficiency, with even sterically demanding n ucleophiles adding with complete stereocontrol. Unsaturated oxo-nitriles th at are incapable of chelation afford diastereomeric conjugate addition prod ucts through an unusual boatlike transition state. Collectively, these reac tions illustrate the complementary stereoselectivity of chelation-controlle d conjugate additions to hydroxylated, unsaturated oxo-nitriles and stereoe lectronically controlled conjugate additions to enones.