The Flavonoids of Combretum quadrangulare : Crystal structures of the Polymorphic Forms of 5-Hydroxy-2-(4'-hydroxy-3',5'-dimethoxyphenyl)-3,7-dimethoxy-4 H -1-benzopyran-4-one

The following substituted 2-phenyl-4H-1-benzopyran-4-ones have been isolated from the dried flowers of Combretum quadrangulare Kurz ( Combretaceae ): 5-hydroxy-3,3',4',5',7-pentamethoxy ( combretol ) (1), 3',5-dihydroxy-3,4',7-trimethoxy ( ayanin ) (2) and 4',5-dihydroxy- 3,3',5',7-tetramethoxy (3). The last substance (3) was obtained as a mixture of two polymorphic forms (a and ß) each of which was characterized by X-ray diffraction. Diffractometer data at 295 K were refined by full matrix least squares to residuals of 0.043 (1181 'observed' reflections) for the a-phase and 0.044 (1421) for the ß phase of (3). Crystals of the a-phase of (3) are triclinic, Pi, a 12.663(6), b 9.592(4), c 7.444(4) ?, a 102.48(3), ß 101.39(4), ? 91.72(4)°,Z 2. Crystals of the ß-phase of (3) are monoclinic P21/n, a 17.139(8), b 12.728(6), c 7.845(7) ?, ß 95.07(6)°, Z 4. An unambiguous synthesis of (3) was also achieved.