Enantioselective Synthesis
of Tryptophan Derivatives
by a Tandem Friedel–Crafts Conjugate Addition/Asymmetric Protonation
Reaction
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Abstract
The tandem Friedel–Crafts conjugate addition/asymmetric
protonation reaction between 2-substituted indoles and methyl 2-acetamidoacrylate
is reported. The reaction is catalyzed by (<i>R</i>)-3,3′-dibromo-BINOL
in the presence of stoichiometric SnCl<sub>4</sub>, and is the first
example of a tandem conjugate addition/asymmetric protonation reaction
using a BINOL·SnCl<sub>4</sub> complex as the catalyst. A range
of indoles furnished synthetic tryptophan derivatives in good yields
and high levels of enantioselectivity, even on a preparative scale.
The convergent nature of this transformation should lend itself to
the preparation of unnatural tryptophan derivatives for use in a broad
array of synthetic and biological applications