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In the absence of an acidic counteranion,
the non-protonated 9-amino-9-deoxy--hydroquinine
is still able to promote the sulfa-Michael addition, albeit with
lower reactivity, presumably by activating the nucleophilic component through Brønsted-base catalysis;
however, the observed low optical purity (8% ee) together
with reversal in the stereochemistry supports an iminium ion activation
mode of catalysis when the chiral salt is
employed. The iminium ion activation path is also corroborated by
the lower reactivity observed when more encumbered α-substituted
enones are employed. For similar mechanistic considerations on this type
of catalyst salt, see: Wang X, Reisinger CM, List B,
J.
Am. Chem. Soc.. 2008; 130 6070
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