A method for exploring the dipolar character of transition states. A general directing effect of polar substituents in olefin addition reactions

Diastereodivergent Hydroxyfluorination of Cyclic and Acyclic Allylic Amines: Synthesis of 4-Deoxy-4-fluorophytosphingosines

“…Increasing the amount of HBF 4 ·OEt 2 to 20 equiv promotes protonation of the m -CPBA, and this may serve to decrease the hydrogen-bond acceptor ability of the peracid, as well as introduce electrostatic repulsive interactions between the protonated peracid and the ammonium group. Non-hydrogen-bond-directed epoxidation, presumably occurring preferentially on the face of the olefin anti to the ammonium ion due to minimization of unfavorable steric interactions and/or electronic interactions (minimization of dipoles), may therefore predominate due to activation of the m -CPBA as a more potent electrophilic oxidant by protonation, thus resulting in a reversal of the sense of diastereofacial selectivity of epoxidation (Scheme ).…”

Highly Diastereoselective anti-Dihydroxylation of 3-N,N-Dibenzylaminocyclohex-1-ene N-Oxide

“…Stereoselective anti -epoxidation of 3 in the presence of 10 equiv of Cl 3 CCO 2 H suggests that the protonated N , N -dibenzylamino N -oxide group does not participate in hydrogen-bonded delivery of the oxidant to the syn -face of the olefin. , The stereochemical outcome is consistent with the oxidation proceeding either under steric control or through minimization of the dipoles of the protonated amine N -oxide and the peracid to give protonated anti -epoxide N -oxide 13 . This presumably resides in conformation 13A , with the protonated C(3)- N , N -dibenzylamino N -oxide group occupying a pseudoequatorial position.…”