2,2′-Dimethoxy-4,4′-[rel-(2R,3S)-2,3-dimethylbutane-1,4-diyl]diphenol

The title molecule, C20H26O4, commonly known as meso-dihydroguaiaretic acid, is a naturally occurring lignan extracted from Larrea tridentata and other plants. The molecule has a noncrystallographic inversion center situated at the midpoint of the central C—C bond, generating the meso stereoisomer. The central C—C—C—C alkyl chain displays an all-trans conformation, allowing an almost parallel arrangement of the benzene rings, which make a dihedral angle of 5.0 (3)°. Both hydroxy groups form weak O—H⋯O—H chains of hydrogen bonds along [100]. The resulting supramolecular structure is an undulating plane parallel to (010).

The title molecule, C 20 H 26 O 4 , commonly known as mesodihydroguaiaretic acid, is a naturally occurring lignan extracted from Larrea tridentata and other plants. The molecule has a noncrystallographic inversion center situated at the midpoint of the central C-C bond, generating the meso stereoisomer. The central C-C-C-C alkyl chain displays an all-trans conformation, allowing an almost parallel arrangement of the benzene rings, which make a dihedral angle of 5.0 (3) . Both hydroxy groups form weak O-HÁ Á ÁO-H chains of hydrogen bonds along [100]. The resulting supramolecular structure is an undulating plane parallel to (010).
listed for chaparral (Tyler & Foster, 1999). For instance, L. tridentata has been shown to be active against Mycobacterium tuberculosis, with a minimum inhibitory concentration of 200 µg/ml (Camacho-Corona et al., 2008). We are currently working on the full characterization of the main active compounds found in the chloroform extract of that plant.
Previous phytochemical studies carried out on L. tridentata showed that it contains a series of lignans (Konno et al., 1990;Gnabre et al., 1995), one of which being the title molecule. This molecule, commonly called meso-dihydroguaiaretic acid, crystallizes in the space group P2 1 2 1 2 1 , with the molecule placed on a non-crystallographic inversion center (Fig. 1).
As a consequence, the relative stereochemistry for chiral C atoms is (R,S). The central aliphatic chain is stabilized in an all-trans conformation, and peripheral benzene rings are almost parallel, making a dihedral angle of 5.0 (3)°.
The crystal structure features weak O-H···O hydrogen bonds involving all hydroxy groups. Infinite chains are formed along the short axis [100], with OH functionalities serving both as donor and acceptor groups. As a result, a two-dimensional supramolecular framework is formed, parallel to plane (010) in the crystal (Fig. 2).

Experimental
Aerial parts of L. tridentata were collected in April 2006, at Galeana (Nuevo León, Mexico) and identified by Biologist Marcela González Álvarez. A voucher specimen (024772) is available in the botanic department of the Biology Faculty (UNL, Monterrey, Mexico). After grinding, the dry material (500 g) was placed in an Erlenmeyer vessel filled with hexane (1 l) and left at 298 K for 24 h. The preparation was then filtered and the resulting vegetal material soaked with chloroform for 72 h. The chloroform extract was then filtered and concentrated in vacuo, affording 89 g of extracts. The chloroform extract (80 g) was chromatographed on silica-gel (1600 g) using mixtures of chloroform/ethanol as eluent, giving 13 fractions. The third fraction, eluted with pure CHCl 3 , afforded colorless crystals, which were separated by filtration. After recrystallization from hexane/ethyl acetate (80:20), pure meso-dihydroguaiaretic acid was obtained (730 mg, m.p. 360 K). Spectroscopic data are consistent with the X-ray structure (see archived CIF). The full characterization by NMR also allowed to confirm that this lignan was early isolated by Waller & Gisvold (1945) from the same plant.

Refinement
As no significant anomalous scattering effects are present in the crystal, measured Friedel pairs (1325) were merged for refinement. Hydroxyl H atoms, H2 and H14, were found in a difference map and refined freely, although O-H bond lengths were restrained to 0.85 (2) Å. Other H atoms were placed in idealized positions and refined as riding to their parent C atom, with bond lengths fixed to 0.93 (aromatic CH), 0.97 (methylene CH 2 ) or 0.96 Å (methyl CH 3 ). Methyl groups supplementary materials sup-2 were considered as rigid groups free to rotate about their C-C bonds. Isotropic displacement parameters for H atoms were calculated as U iso (H) = 1.5U eq (carrier atom) for methyl and hydroxyl groups and U iso (H) = 1.2U eq (carrier atom) otherwise. Fig. 1. The title compound, with displacement ellipsoids at the 30% probability level.   (5) 124 (4)