Fig. 1. In nature, carbocation chemistry in the biosynthesis of terpenes and steroids is accomplished with cation–π interactions (red circles indicate the position of π-basic amino-acid residues in the cation–π enzyme for substrate 1). The complementary enolate chemistry in polyketide biosynthesis and the beginning of both pathways fails in solution because decarboxylation of 2 (solid arrows) dominates over enolate addition (dashed arrows). In this report, selective acceleration of this disfavored but relevant process is achieved with anion–π interactions (blue background) and explained with the discrimination between non-planar tautomers (2) and planar tautomers (4/5; BH⁺: protonated base, E = electrophilic carbon).