Cinchona‐Mediated Enantioselective Protonations

Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms

“…As such, peptide 2.94 was indeed able to effectively convert loratadine derivative 2.91c, in which urea directing group was incorporated in place of the native ethyl carbamate, to N-oxide 2.92c in 45% yield and 95:5 er (Figure 29c). Additionally, varenicline, the active ingredient in smoking cessation aid Chantix ® , contains a symmetrical quinoxaline ring, 94 which we hypothesized to be an excellent candidate for a peptide mediated desymmetrization. While oxidations of this more electron deficient heterocycle were slower, varenicline derivative 2.91d was successfully transformed into N-oxide 2.92d in 95:5 er and 61% yield (Figure 29d).…”

Deracemization of α-Aryl Hydrocoumarins via Catalytic Asymmetric Protonation of Ketene Dithioacetals

“…Asymmetric protonations of enolate equivalents, at least in principle, offer a highly straightforward approach to enantioenriched α-tertiary carbonyl compounds. Since pioneering investigations by Duhamel and Yoshikawa, , such reactions have typically relied on a stoichiometric chiral proton source, although catalytic asymmetric versions have also been described. − In the context of a different project, we were in need of enantiomerically pure α-aryl hydrocoumarins ( 1 ). Despite their high relevance as biologically active compounds, however, nonracemic α-aryl hydrocoumarins ( 1 ) have been entirely unknown.…”