DOI: 10.1055/s-00000083

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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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