Development of Asymmetric Reactions Catalyzed by Bronsted Acid in Aqueous Media

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We have found that Mannich-type reactions took place smoothly under the influence of Bronsted acid, typically aq. HBF_4, to afford the corresponding (β-amino carbonyl compounds in high yields in aqueous organic solvents. Furthermore, the HBF_4-catalyzed Mannich-type reaction took place smoothly in the presence of sodium dodecyl sulfate in water under organic solvent-free conditions. Aza-Diels Alder reactions in aqueous media have been successfully achieved. Anti and syn stereoselectivity on the HBF_4-catalyzed Mannich-type reaction of ketene silyl acetal derived from α-oxy esters with aldimines were investigated. Whereas use of ketene silyl acetal derived from aryl ester in aqueous 2-propanol gave anti β-amino-α-siloxy ester with excellent stereoselectivity, use of ketene silyl acetal derived from methyl ester in water in the presence of sodium dodecyl sulfate gave the syn isomer preferentially.