Crystal structures of 4-methoxy-6-methyl- and 4-hydroxy-6-methyl-2-pyrones.

interesting results because of the regular arrangement of the molecules in the environment.1–3 We previously reported on the photocycloaddition reaction of 4-methoxy-6-methyl-2-pyrone (1a) (Fig. 1) with maleimide (2) in the solid state to give a highly stereoselective [2 + 2] cycloadduct4 owing to the formation of two sets of 1:1 complex crystals between (1a) and (2) (Fig. 1).5 The crystal showed a hydrogen bond between maleimide (NH) and the 2-pyrone (O=C). Since 4-hydroxy-6methyl-2-pyrone (1b) (Fig. 1), having a similar structure to that of (1a), did not undergo a solid-state photoreaction with (2), Xray diffraction studies of the title compounds were carried out to clarify the crystal structures and the photochemical behaviors. (1a), prepared by a method described in the literature,6 and commercially available (1b) were recrystallized from a petroleum ether–ethanol–hexane (1:1:1) mixture and acetonitrile, respectively, to give colorless plate crystals. Data collections were performed at 93 K with graphitemonochromated Mo Kα radiation on a Rigaku RAXIS-RAPID Imaging Plate diffractometer (λ = 0.71069 Å). Lowtemperature experiments were carried out in order to increase the number of observable reflections of (1a), which was a very thin platelet crystal. Lorentz and polarization corrections were applied to the intensity data. The structure was solved by direct methods using SIR 92.7 The structure was refined by a fullmatrix least-squares method. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms of (1a) were found by using Fourier techniques, but could not be refined. Hydrogen atoms of (1b) were refined isotropically. Table 1 gives the crystal data and experimental conditions of (1a) and (1b). The molecular structure of (1a) is shown in Fig. 2. The final positional parameters are given in Table 2, and selected bond lengths and angles are listed in Table 3. The bond lengths and angles of (1a) and (1b) are normal and similar to those with related structures.8 The molecules of (1a) are arranged without hydrogen bond and stacked with each other (Fig. 3). On the other hand, the molecules of (1b) are linked by an OH···O=C intermolecular hydrogen bond (H···O(2) 1.84(4)Å) (Fig. 4), whose value is similar to that of the NH···O=C hydrogen bond (H···O(2) 1.83(4)Å) between (2) and (1a).5 The crystal of (1b) has a more close-packed structure than (1a) because of the formation of the hydrogen bond; the calculated density of (1b) is 1.499 g cm–3 and that of (1a) is 1.425 g cm–3. In addition, the intermolecular hydrogen bonds of (1b) were also estimated in solution from a 1H NMR analysis because of showing an OH signal downfield (δ 11.6 in DMSO-d6). It was clarified from X-ray studies that a 1:1 complex crystal of (1a) with (2) was generated from a newly-formed hydrogen 613 ANALYTICAL SCIENCES MAY 2002, VOL. 18 2002 © The Japan Society for Analytical Chemistry