Complete mapping of the stereochemical course of retentive deprotonation/alkylation of 1H-benzo[e][1,4]diazepin-2(3H)-ones.
Enantiopure amino-acid derived 1H-benzo[e][1,4]diazepin-2(3H)-ones (BZDs) undergo highly retentive deprotonation/alkylation reactions. To confirm the role of stereolabile, axially chiral intermediates in these reactions and to determine the precise stereochemical course of deprotonation and alkylation, a new alanine-derived BZD (S)-1d was prepared. Because of slow diazepine ring inversion of the C3-alkylated derivatives of 1d, it proved possible to determine that electrophiles react at the concave face of the enolate derived from 1d. Furthermore, an enantiopure silyl enol ether derivative of (S)-1d was prepared and characterized by X-ray crystallography, confirming that deprotonation resulted in an (M)-configured axially chiral enolate. Activation parameters for diazepine ring inversion in the potassium enolate of 1d were determined experimentally and are well-matched by density functional calculations. Finally, the factors leading to concave-face alkylation of the enolate derived from 1d are analyzed based on calculated alkylation transition structures. Minimization of torsional effects at the BZD ring fusion and maximization of imine and amide resonance are proposed to favor concave-face alkylation.
Hsu
Danny C
DC
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, USA.
Lam
Polo C-H
PC
Slebodnick
Carla
C
Carlier
Paul R
PR