Dihydromyricetin hexaacetate

In the title compound, C27H24O14, also known as 2,3-diacetoxy-5-[(2RS,3RS)-3,5,7-triacetoxy-4-oxochromen-2-yl]phenyl acetate, the heterocyclic ring adopts a distorted half-chair conformation, with two C atoms displaced by 0.1775 (16) and −0.5950 (16) Å from the mean plane of the other four atoms. The dihedral angle between the aromatic rings is 57.81 (8)°. In the crystal, the molecules interact by C—H⋯O bonds, aromatic π–π stacking [centroid–centroid separation = 3.6206 (9) Å] and C—H⋯π interactions.

The compound is a strong anti-oxidant with many reported health-promoting properties and, therefore, is emerging as a promising functional ingredient for use in a number of pharmaceutical and food applications (Li et al., 2006). However, the compound is poorly soluble in both water and fat, which limits its applications (Gao et al., 2009). Acetylation could be one way how to improve its lipid solubility. Although the single-crystal structure of dihydromyricetin itself is known (Xu et al., 2007), none of its derivatives have to date been structurally characterized. Herein we report the structure of the hexaacetate of dihydromyricetin (Fig. 1).
Though the title structure is of the biological origin, it crystallizes in a structure where both enantiomers are present. Moreover, there are also present π-electron ring -π-electron ring interactions between the adjacent rings C1//C2//C3//C4// C5//C9 [symmetry code: 1-x,1-y, -z] as indicates the distance between the centroids that equals to 3.6206 (9)Å.

Experimental
The crystals of dihydromyricetin (1 g), prepared as described by Xu et al. (2007), were added by parts to a mixture of acetic anhydride (6 ml) and pyridine (1 ml) maintained at 75°C in the water bath. Each addition was done after the crystals of dihydromyricetin that had been added previously completely dissolved. The mixture was stirred for 30 min and upon addition of chilled water (120 ml), yielded an oily precipitate, which solidified in 15 min. After decanting the supernatant and washing the precipitate with water, the precipitate was collected and dried at 55°C for 24 h to afford a light yellow solid. A portion (50 mg) was dissolved in warm methanol, and yielded, on standing for several days at ambient temperature, colourless plates of (I) (m.p. 436 -440 K).

Refinement
All the H atoms were located in the difference electron density map. However, the H atoms were placed into the idealized positions with d(C-H) = 0.95 Å for the aryl, 0.980 Å for the methyl and 1.000 Å for the methine hydrogens. The U iso (H) values were constrained to be 1.2U eq for the aryl and methine H atoms while 1.5U eq for the methyl H atoms.
supplementary materials sup-2 Figures Fig. 1. View of the (R,R) enantiomer of (I). The displacement ellipsoids shown at 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 R-factors (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.