Alpinumisoflavone

The title compound, C20H16O5, {systematic name: 5-hydroxy-7-(4-hydroxyphenyl)-2,2-dimethyl-2H,6H-benzo[1,2-b:5,4-b′]dipyran-6-one}, was obtained by demethylation of the biologically active related compound, 4-O-methylalpinumisoflavone. The molecular structure of the title compound is characterized by a fused tricyclic system that contains an approximately planar benzopyrone ring fragment. The six membered pyran ring adopts a half-chair conformation. Both ring systems show an out-of-plane twist. The dihedral angle between the mean plane of the benzopyrone system and the benzene ring is 54.29 (3)°. The molecules are linked by O—H⋯O hydrogen bonds, forming a molecular tape running along the b axis.

The title compound, C 20 H 16 O 5 , {systematic name: 5-hydroxy-7-(4-hydroxyphenyl)-2,2-dimethyl-2H,6H-benzo[1,2-b:5,4-b 0 ]dipyran-6-one}, was obtained by demethylation of the biologically active related compound, 4-O-methylalpinumisoflavone. The molecular structure of the title compound is characterized by a fused tricyclic system that contains an approximately planar benzopyrone ring fragment. The six membered pyran ring adopts a half-chair conformation. Both ring systems show an out-of-plane twist. The dihedral angle between the mean plane of the benzopyrone system and the benzene ring is 54.29 (3) . The molecules are linked by O-HÁ Á ÁO hydrogen bonds, forming a molecular tape running along the b axis.
In the present work, single crystals of alpinumisoflavone suitable for X-ray diffraction were obtained by demethylation of 4-O-methylalpinumisoflavone using cold BCl 3 . The crystals isolated from the crude extract were usually of poor quality.
Therefore we decided to modify 4-O-methylalpinumisoflavone chemically by demethylation (Scheme 2) hoping that a new compound would yield crystals of a better quality. This turned to be true. The molecular structure of the title compound differs from 4-O-methylalpinumisoflavone only in the replacement of the methoxy group by the hydroxyl on the benzene ring D.
The molecular structure of the title compound is characterized by a tricyclic fused ring system, A/B/C, and a benzene ring D (Fig. 1). The benzopyrone ring fragment, B/C, is planar and it is twisted out of plane with respect to the benzene ring D. The outer six-membered ring A is deformed into a half-chair conformation, with Cremer & Pople (1975) parameters q 2 , q 3 and φ 2 of 0.2342 (9), -0.1148 (9) Å and 220.4 (2)°, respectively.
The presence of the hydroxyl group proximal to the keto group on the ring C permits the formation of a relatively stronger intramolecular O-H···O hydrogen bond (Table 1). This distance is comparable to the intramolecular contact distance equal to 1.724 (17) Å in 4-O-methylalpinumisoflavone which is the closest related compound in the studied series (Kingsford-Adaboh et al., 2001, 2006. The corresponding distances observed in other members of the series are longer (ca 2.3 Å; Kingsford-Adaboh et al., 2006).  (Table 1), where the terminal OH group of the benzene ring D serves as a proton donor to the carbonyl oxygen atom, is observed to play an important role in the molecular bonding in the crystal structure (Tab. 1, Fig. 2).

Experimental
Alpinumisoflavone was obtained from the demethylation of 4-O-methylalpinumisoflavone. Solvent extraction of 4-Omethylalpinumisoflavone from the pulverized root bark of Milletia thonningii followed similar procedure as described in our earlier work (Kingsford-Adaboh et al., 2001). A cold solution of BCl 3 in chloroform (-78 °C) was added slowly to supplementary materials sup-2 about 15 ml of a chloroform solution of 4-O-methylalpinumisoflavone (200 mg, 0.571 mmol) cooled to -78 °C using dry ice and acetone mixture. The solution was stirred for about 10 min under argon atmosphere. 30 ml of water was added slowly and the resulting yellowish mixture was extracted with chloroform three times. The combined extracts were washed with water twice and then dried over anhydrous sodium sulphate. After evaporation of the solvent under vacuum, the residue was chromatographed on a silica gel using petroleum ether and ethylacetate mixture in the ratio of between 8/1 and 5/1 as the mobile phase. The product was recrystallized from acetonitrile. The demethylation yield (159.6 mg about 80%, m.p. 486 K). The product was confirmed by 13 C NMR spectra of both the reactants and the product.)

Refinement
All the H atoms were located in the difference Fourier map. The H atoms that have been attached to the C atoms were constrained in idealized geometry while the hydroxyl H atoms were freely isotropically refined. C methyl -H= 0.98 Å allowing for rotation around the C-C bond with U iso (H methyl ) = 1.5U eq (C methyl ). C aryl -H = 0.95 Å with U iso (H aryl ) = 1.2U eq (C aryl ). Fig. 1. The molecular structure of the title compound. The displacement ellipsoids are drawn at the 50% probability level.

Special details
Geometry. All e.s. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Although there were present diffractions that violated the space-group-systematic absences the average I/σ values for h0l with l = 2n and for l =2n+1 were 25.6 and 0.8, respectively. This indicates presence of the c glide plane. Thus we selected P2 1 /c. We have also refined the structure with P2 1 . All atoms except H completely fit to the c and i symmetries. The reflections that should be absent for supplementary materials sup-4 P2 1 /c might be accidentally observed. Probably some of them were diffractions from small ice particles (frost) generated in the X-ray beam path.