Poppy acid: Synthesis and crystal chemistry

It had long ago been reported that poppy acid crystals encapsulate and orient a great variety of molecules during solution growth and in so doing seem to egregiously violate the principle of isomorphism. To comprehend this surprising host-guest chemistry, and exploit it for measuring anisotropic molecular properties, we attempted to carry out the oft-used literature synthesis of poppy acid, (3-hydroxy-2,6-dicarboxy-gamma-pyrone), but discovered that the standard procedures did not produce the title compound. We instead obtained a constitutional isomer as the potassium salt of 2-oxaloate-3-hydroxy-5-carboxyfuran. Therefore, we designed and carried out the first total synthesis of poppy acid. It crystallizes as either of two polymorphs, an orthorhombic form (Pbca) and a monoclinic form (C2/c), both characterized by weakly bonded layers consistent with perfect cleavages. The great majority of the dyes tested, 15 of 19, produced poppy acid crystals colored in particular growth sectors displaying strong linear dichroism. The observation of pronounced absorption anisotropy is consistent with a general mixed crystal growth mechanism in which the dyes substitute for poppy acid molecules within the layers and are further oriented in the direction of hydrogen bound rows of molecules within layers.