Green asymmetric synthesis of Warfarin and Coumachlor in pure water catalyzed by quinoline-derived 1,2-diamines†
Simple enantiomerically pure C2-symmetric quinoline (isoquinoline)-derived 1,2-diamines were synthesized from the corresponding aldehydes via stereospecific diaza-Cope rearrangement with 2,2′-(1,2-diaminoethane-1,2-diyl)diphenol (HPEN). Efficient green synthesis of both enantiomers of the anticoagulant Warfarin and rodenticide Coumachlor was achieved in an aqueous medium via the asymmetric iminium-type Michael reaction in the presence of the catalysts 8e and (ent)-8e in combination with (R)- or (S)-mandelic acid, respectively. This procedure provides high enantioselectivity (up to 91% ee), which has never been attained for these bioactive compounds with the known catalysts under aqueous conditions. Nearly optically pure Warfarin (∼99% ee) was prepared via a green isolation procedure, which included acidic precipitation of the crude product from a basic aqueous solution followed by single recrystallization. Furthermore, unlike the known primary amine-derived organocatalysts, the developed aqueous catalytic system does not produce parasitic byproducts and can be recovered and reused in the asymmetric reaction.