An acid inaccessible to aldol products The aldol reaction is a venerable and widely applicable method for making carbon-carbon bonds. Ironically, it is most challenged by the simplest substrates. The trouble is that the product looks a lot like one of the reactants, and so it can latch onto the coupling partner instead. Schreyer et al. report that a bulky phosphorus-based acid catalyst alleviates this problem. The acidic site is buried in a pocket that is too small to activate the product for further reaction. The chiral geometry of the catalyst also induces high enantioselectivity. Science , this issue p. 216