REGIOSELECTIVITY, DIASTEREOSELECTVITY, AND CHEMOSELECTIVITY IN THE DIOXIRANE OXIDATION OF ACYCLIC AND CYCLIC ALLYLIC ALCOHOLS BY METHYL(TRIFLUOROMETHYL)DIOXIRANE (TFD) - A COMPARISON WITH DIMETHYLDIOXIRANE

The solvent-dependent shift in the regioselectivity of the geraniol ep oxidation by methyl(trifluoromethyl) dioxirane (TFD) reveals that as f or the less reactive dimethyldioxirane (DMD), hydrogen bonding stabili zes the transition state of the epoxidation. In protic media, the hydr ogen bonding is exerted intermolecularly by the solvent, whereas in un polar, non-hydrogen-bonding solvents intramolecular assistance through the adjacent hydroxy functionality comes into the play and the attack on the allylic alcohol moiety is favored. For chiral allylic alcohols , additional steric interactions control the pi-facial selectivity in the conformationally fixed transition state. Analogous to DMD, the pre ferred dihedral angle in the hydrogen-bonded transition state of the T FD epoxidation constitutes approximately 130 degrees, but contrary to DMD and for synthetic purposes important, the allylic alcohols and der ivatives 1 and 3-5 investigated here are chemoselectively epoxidized b y TFD without formation of the corresponding enones.