Diastereoselective Osmylations Diastereoselective Osmylations (z) olefins (z) Olefins Me, h PhSO280:20 OBn TBSo 62: 38 I Addition occurs anti to the allylic heteroatom functionality because both EDG and EWG provide the same sense of induction (E)Olefins ■ Kishi's Empirical Mode 人 Arrange olefin in the stable ground state conformer OsO attacks anti to the allylic oxygen substituent Works for both(4) and(E) allylic ethers(alchols) (E)Olefins 22:78 Kishi Model Aco Vedejs Model a Allylic oxygen protecting group: oBn81:19 H, Bn, SiR3, acetonides-all work well OsO4 Acetates give lower selectivity Kishi& Co-workers Tetrahedron Lett. 1983. 24. 3943 and Tetrahedron. 1984. 40. 2247. X= OTBS OAC X=SR, SO2R, SiR3 James oup Seminar, Osmium Mediated Dihydroxylation E. Vedejs&C. McClure JACS, 1986, 108, 1094 10A-11-Osmylations 10/8/00 8: 14 PME. Vedejs & C. McClure JACS, 1986, 108, 1094. syn anti anti syn Vedejs Model OsO4 X = OTBS, OAC X = SR, SO2R, SiR3 OsO4 Kishi Model OsO4 OsO4 34 : 66 22 : 78 33 : 67 61 : 39 62 : 38 selectivity PhMe2Si PhSO2 PhS TBSO AcO X (E) Olefins (z) Olefins (E) Olefins (z) Olefins ■ Vedejs argues that hyperconjugative effects are not important because both EDG and EWG provide the same sense of induction ■ Addition occurs anti to the allylic heteroatom functionality X PhMe2Si PhSO2 PhS TBSO AcO selectivity 78 : 22 80 : 20 83 : 17 62 : 38 70 : 30 OsO4 Diastereoselective Osmylations James Barrow, Evans Group Seminar, "Osmium Mediated Dihydroxylation: Mechanism and Application" April 21, 1993. Kishi & Co-workers Tetrahedron Lett., 1983, 24, 3943 and Tetrahedron, 1984, 40, 2247. Acetates give lower selectivity ■ Allylic oxygen protecting group: H, Bn, SiR3, acetonides→all work well Works for both (Z) and (E) allylic ethers (alchols) OsO4 attacks anti to the allylic oxygen substituent Arrange olefin in the stable ground state conformer ■ Kishi's Empirical Model: OsO4 OsO4 OsO4 X Me OBn selectivity 60 : 40 81 : 19 50 : 50 86 : 14 selectivity Me OBn X Diastereoselective Osmylations OsO4 Me OH Me X OH X Me H C C H H BnO OH X R OH OH BnO BnO C Me H C OH R X OH H Me BnO OH X X H Me OH Me Me X OH OH OH Me OH Me C (Me)H (H)Me C H H CH2OBn OBn X Me Me Me OH Me H X X OH Me C C H Me H X OH Me C Me H H C Me X H Me 10A-11-Osmylations 10/8/00 8:14 PM