Remarkable asymmetric induction is observed in the alkylation of enolates derived from enantiomerically pure amides, even though these enolates seem to have no apparent chirality (see scheme). The ...proposed origin and mechanism for this transfer of chirality is discussed. LTMP=lithium tetramethylpiperidide.
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The synthesis of enantiomerically enriched (−)-(
R)-
N,
N′-diisopropyl-2-phenylpropanamide was achieved in up to 69% enantiomeric excess by symmetrisation of the corresponding racemic ...amide by addition of
sec-BuLi (to give the corresponding achiral lithium enolate) and subsequent desymmetrisation by the addition of a chiral
C-based proton source. We discuss potential factors that may be responsible for this observed enantioselectivity and comment on the role of the chiral acid.
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The synthesis of enantiomerically enriched (
R)-2-methyl-1-tetralone
1
(64% e.e.) was achieved through protonation of its lithium enolate
3
using a
C
2-symmetrical bis-sulfonamide
5d
...as an internal proton source. Access to the complementary (
S)-enantiomer
1
(45% e.e.) can be achieved using an external quench strategy involving acetic acid as the external proton source.
