The asymmetric Michael addition of 4-hydroxycoumarin to a , b -unsaturated ketones on water without organic co-solvents is reported to be catalysed by organic primary amines. The application of enantiome- rically pure ( S , S )-diphenylethylenediamine affords a series of important pharmaceutically active com- pounds in good to excellent yields (73–98%) and with good enantioselectivities (up to 76% ee ) via reactions accelerated by ultrasound. In particular, our developments led to an efficient protocol for the ‘solids on water’ formation of the anticoagulant warfarin in both enantiomeric forms. The presented scal- able and environmentally friendly organocatalytic approach affords the target drug in enantiomerically pure form.