Mechanism of a Dually Catalyzed Enantioselective Oxa-Pictet-Spengler Reaction and the Development of a Stereodivergent Variant.

Enantioselective oxa-Pictet-Spengler reactions of tryptophol with aldehydes proceed under weakly acidic conditions utilizing a combination of two catalysts, an indoline HCl salt and a bisthiourea compound. Mechanistic investigations revealed the roles of both catalysts and confirmed the involvement of oxocarbenium ion intermediates, ruling out alternative scenarios. A stereochemical model was derived from density functional theory calculations, which provided the basis for the development of a highly enantioselective stereodivergent variant with racemic tryptophol derivatives.