Inter-action between maleic acid and N-R-furfuryl-amines: crystal structure of 2-methyl-N-[(5-phenyl-furan-2-yl)meth-yl]propan-2-aminium (2Z)-3-carb-oxy-acrylate and N-[(5-iodo-furan-2-yl)meth-yl]-2-methyl-propan-2-aminium (2Z)-3-carb-oxy-prop-2-enoate.

The title mol-ecular salts, C15H20NO(+)·C4H3O4(-), (I), and C9H15INO(+)·C4H3O4(-), (II), have very similar mol-ecular geometries for both cation and anion. The anions of both (I) and (II) are practically planar (r.m.s. deviations = 0.062 and 0.072 Å, respectively) and adopt a rare symmetrical geometry with the hy-droxy H atom approximately equidistant from the two O atoms. In their crystals, the cations and anions in both (I) and (II) form tight ionic pairs via strong N-H⋯O hydrogen bonds, with a roughly perpendicular disposition of the anion to the furan ring of the cation. This ion-pair conformation appears to correlate with the lack of reactivity of these salts in [4 + 2] cyclo-addition reactions. In the extended structures of (I) and (II), the ion pairs form hydrogen-bonded chains propagating along [010] and [001], respectively, via N-H⋯O hydrogen bonds.