Conformation of 1-aryl-2-ar­yloxy-1-propanols: threo-1-(4-hydr­oxy-3-methoxy­phen­yl)-2-(2-methoxy­phen­oxy)-1-propanol

The title compound, C₁₇H₂₀O₅, (I), is structurally related to naturally occurring 1-aryl-2-ar­yloxy-1-propanols. Such compounds are of interest as lignin models, and neolignans of this type have been detected in a large number of plants. In the crystal structure of (I), the mol­ecules adopt a conformation in which the aryl groups are far apart from each other. The O(ar­yl­oxy)-C-C-C(ar­yl) torsion angle is 177.76 (14)°. The conformation is compared with those of other compounds (neolignans and lignin model compounds) of the 1-aryl-2-ar­yloxy-1-propanol type (including some acetate derivatives). The comparison shows that in all the examined compounds the above-mentioned torsion angle is close to 180°, and the distance between the centers of the aromatic rings approaches the maximum achievable in most of the compounds. The hydrogen-bonding pattern of (I) is discussed in terms of graph-set theory.

S1. Comment

1-Aryl-2-aryloxy-1-propanols are of interest as lignin model compounds (Adler et al., 1966; Wallis et al., 1991; Stomberg et al., 1993; Li & Lundquist, 2001). Quite a few neolignans with this type of structure have been isolated from plants (Stomberg et al.,1993; Wallis, 1998; Lee & Ley, 2003; Kónya et al., 2004; Hanessian et al., 2006). Crystal structures of a number of 1-aryl-2-aryloxy-1-propanols have been reported (Jakobsons et al., 1986; Wallis et al., 1991; Stomberg et al., 1993; Wallis et al., 1996; Lee & Ley, 2003). The crystal structure of threo-1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol, (I), is reported in this paper. Jakobsons et al. (1986) have reported the crystal structure of the erythro form of this compound, (II). The synthesis of the threo and erythro forms of 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol by reduction of 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanone has been described by Adler et al. (1966). The erythro/threo ratio in the reaction products obtained depends on the particular reducing reagent used and the reaction conditions (Brunow et al., 1988). Reduction with borane dimethyl sulfide complex gives predominantly the threo form, (I) (Li & Lundquist, 2001).

A perspective drawing and the atom numbering of (I) are shown in Fig. 1. There are strong O—H···O hydrogen bonds (Fig. 2) present in the crystal structure of (I) (for geometrical details of hydrogen bonds see Table 1). Three of them are intramolecular, supporting the rigidity of the molecule, and one is intermolecular, forming C(8) chains on the first-level graph set, as defined by Bernstein et al. (1995) and Grell et al. (1999). There are also two weak C—H···O hydrogen bonds present in the crystal structure, consolidating the crystal framework, both of them forming C(5) chains on the first-level graph set (Fig. 3).

Geometric details of (I) are given in Table 2. For comparison, the corresponding geometric data for the related compounds (II), (III)–(VI),and acetate derivatives (VII)–(IX) (see Scheme 2) are included in the table. To enable comparison we have consistently considered the isomer with R-configuration at Cα. The torsion angles C(aryl)—O—Cβ—Cα and O(aryloxy)—C—C—C(aryl) [C11—O4—C9—C8 and O4—C9—C8—C1 for (I)], the distances between the centers of the aromatic rings and the angles between the aromatic ring mean planes are given in Table 2.

In all the compounds, the torsion angle O(aryloxy)—-C—C—C(aryl) is close to 180°. In most of the compounds, the distance between the centers of the aromatic rings is close to the maximum achievable. The erythro acetate (IX) constitutes an exception in the sense that the C(aryl)—O—Cβ—Cα torsion angle is comparatively small, and as a consequence of this the distance between the ring centers deviates notably from the maximum. The separation of the aromatic rings in compounds (I)–(IX) might be attributed to π–π electron repulsion (Hunter & Sanders, 1990). The orientations of the torsion angle C(aryl)—O—Cβ—Cα for the threo and erythro forms (I) and (II) differ considerably, which explains the larger distance between the aromatic ring centers for (II) (see Table 2). The aromatic plane angles of the acetate derivatives (VII)–(IX) deviate considerably from the average of those of compounds (I)–(VI).

Related literature

For related literature, see: Adler et al. (1966); Brunow et al. (1988); Grell et al. (1999); Hanessian et al. (2006); Hunter & Sanders (1990); Jakobsons et al. (1986); Kónya et al. (2004); Lee & Ley (2003); Li & Lundquist (2001); Stomberg et al. (1993); Wallis (1998); Wallis et al. (1991, 1996).

S2. Experimental

Compound (I) was synthesized by reduction of 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanone with BH₃.S(CH₃)₂ in tetrahydrofuran solution (argon atmosphere) (Adler et al., 1966). Excess BH₃.S(CH₃)₂ was decomposed by addition of methanol. Work-up gave a product consisting primarily of (I) (¹H NMR). Crystals of melting point 386–387 K were obtained from acetone (Li & Lundquist, 2001).

S3. Refinement

Friedel pairs were averaged, and the R conformation was adopted for atoms C8 and C9. H atoms were constrained to an ideal geometry using an appropriate riding model, with C—H = 0.95–1.00 Å. For the hydroxy and methyl groups, the O—H (0.84 Å) (or C—H) distances and C—O—H (C—C—H or O—C—H for methyl groups) angles (109.5°) were kept fixed, while the torsion angles were allowed to refine with the starting positions based on the circular Fourier synthesis. U_iso(H) values were set at 1.5U_eq(O), 1.5U_eq(C_methyl) and 1.2U_eq(C) in other cases.

Computing details

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003) and SADABS (Sheldrick, 2003); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. A perspective drawing of (I), showing the atom-numbering scheme. Displacement ellipsoids are shown at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. Hydrogen bonds (broken lines) forming chains of molecules in (I). H atoms not included in the hydrogen-bonding pattern have been omitted for clarity. For symmetry codes, see Table 1.
	Fig. 3. A projection of the content of the unit cell along the c axis. Hydrogen bonds are shown as broken lines. H atoms not included in the hydrogen-bonding pattern have bee omitted for clarity.

threo-1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-propanol

Crystal data

C17H20O5	Dx = 1.286 Mg m^−3
Mr = 304.33	Melting point = 386–387 K
Orthorhombic, Pna21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2n	Cell parameters from 7726 reflections
a = 23.2847 (11) Å	θ = 2.4–31.6°
b = 12.1184 (6) Å	µ = 0.09 mm^−1
c = 5.5709 (3) Å	T = 153 K
V = 1571.96 (14) Å^3	Needle, colourless
Z = 4	1.00 × 0.17 × 0.07 mm
F(000) = 648

Data collection

Siemens SMART CCD area-detector diffractometer	3175 independent reflections
Radiation source: fine-focus sealed tube	2688 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.062
Detector resolution: 120 µm pixels mm^-1	θmax = 33.1°, θmin = 2.4°
ω scans	h = −34→35
Absorption correction: multi-scan (SADABS; Sheldrick, 2003)	k = −18→18
Tmin = 0.461, Tmax = 0.993	l = −8→8
27834 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.041	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.112	H-atom parameters constrained
S = 1.00	w = 1/[σ^2(Fo^2) + (0.0721P)^2 + 0.1222P] where P = (Fo^2 + 2Fc^2)/3
3175 reflections	(Δ/σ)max = 0.001
204 parameters	Δρmax = 0.37 e Å^−3
1 restraint	Δρmin = −0.18 e Å^−3

Special details

Experimental. Data were collected at 153 K using a Siemens SMART CCD diffractometer equipped with LT-2 A cooling device. A full sphere of reciprocal space was scanned by 0.3° steps in ω with a crystal–to–detector distance of 3.97 cm, 20 s per frame. Preliminary orientation matrix was obtained from the first 100 frames using SMART (Bruker, 2003). The collected frames were integrated using the preliminary orientation matrix which was updated every 100 frames. Final cell parameters were obtained by refinement on the position of 7726 reflections with I>10σ(I) after integration of all the frames data using SAINT (Bruker, 2003).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.78121 (5)	0.38521 (9)	1.3188 (3)	0.0316 (3)
O2	0.75452 (5)	0.23189 (9)	0.9902 (3)	0.0319 (3)
H2	0.7812	0.2337	1.0916	0.048*
O3	0.64633 (5)	0.69179 (8)	0.7416 (3)	0.0284 (3)
H3	0.6319	0.7553	0.7403	0.043*
O4	0.55540 (5)	0.75094 (8)	1.0324 (2)	0.0233 (2)
O5	0.57727 (5)	0.88639 (9)	0.6754 (3)	0.0284 (3)
C1	0.66931 (6)	0.53329 (11)	0.9765 (3)	0.0228 (3)
C2	0.71051 (6)	0.51306 (11)	1.1516 (3)	0.0230 (3)
H2A	0.7190	0.5682	1.2675	0.028*
C3	0.73931 (6)	0.41258 (11)	1.1581 (3)	0.0214 (3)
C4	0.72700 (6)	0.33110 (11)	0.9870 (3)	0.0222 (3)
C5	0.68638 (7)	0.35174 (12)	0.8126 (3)	0.0247 (3)
H5	0.6780	0.2969	0.6960	0.030*
C6	0.65743 (7)	0.45266 (12)	0.8061 (3)	0.0252 (3)
H6	0.6296	0.4662	0.6850	0.030*
C7	0.80036 (8)	0.46919 (15)	1.4790 (4)	0.0340 (4)
H7A	0.8103	0.5355	1.3873	0.051*
H7B	0.8343	0.4431	1.5665	0.051*
H7C	0.7697	0.4867	1.5933	0.051*
C8	0.63948 (6)	0.64411 (11)	0.9746 (3)	0.0240 (3)
H8	0.6585	0.6932	1.0951	0.029*
C9	0.57537 (6)	0.63680 (11)	1.0338 (3)	0.0224 (3)
H9	0.5551	0.5943	0.9053	0.027*
C10	0.56255 (9)	0.58584 (15)	1.2757 (4)	0.0365 (4)
H10A	0.5847	0.6241	1.4001	0.055*
H10B	0.5732	0.5076	1.2735	0.055*
H10C	0.5214	0.5928	1.3103	0.055*
C11	0.51113 (6)	0.77356 (11)	0.8765 (3)	0.0217 (3)
C12	0.52213 (6)	0.84637 (12)	0.6865 (3)	0.0229 (3)
C13	0.47874 (7)	0.87181 (13)	0.5254 (4)	0.0299 (3)
H13	0.4858	0.9216	0.3971	0.036*
C14	0.42464 (8)	0.82384 (14)	0.5529 (4)	0.0339 (4)
H14	0.3951	0.8401	0.4406	0.041*
C15	0.41360 (7)	0.75319 (15)	0.7409 (4)	0.0345 (4)
H15	0.3766	0.7213	0.7587	0.041*
C16	0.45694 (7)	0.72855 (13)	0.9050 (4)	0.0285 (3)
H16	0.4493	0.6809	1.0365	0.034*
C17	0.59077 (8)	0.95625 (15)	0.4767 (4)	0.0356 (4)
H17A	0.5816	0.9180	0.3265	0.053*
H17B	0.6318	0.9743	0.4798	0.053*
H17C	0.5682	1.0243	0.4875	0.053*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0307 (5)	0.0264 (5)	0.0377 (7)	0.0092 (4)	−0.0159 (5)	−0.0100 (5)
O2	0.0349 (6)	0.0206 (5)	0.0402 (7)	0.0090 (4)	−0.0117 (6)	−0.0093 (5)
O3	0.0246 (5)	0.0184 (5)	0.0423 (7)	0.0008 (4)	0.0041 (5)	0.0065 (5)
O4	0.0241 (5)	0.0187 (4)	0.0271 (6)	0.0043 (3)	−0.0059 (4)	−0.0003 (4)
O5	0.0242 (5)	0.0257 (5)	0.0354 (7)	−0.0003 (4)	0.0012 (5)	0.0089 (5)
C1	0.0204 (6)	0.0178 (5)	0.0301 (8)	0.0017 (4)	−0.0039 (6)	−0.0010 (6)
C2	0.0222 (6)	0.0188 (6)	0.0282 (8)	0.0020 (5)	−0.0053 (6)	−0.0050 (6)
C3	0.0189 (6)	0.0192 (6)	0.0262 (7)	0.0015 (4)	−0.0030 (6)	−0.0029 (6)
C4	0.0208 (6)	0.0167 (5)	0.0289 (7)	0.0007 (4)	−0.0009 (6)	−0.0036 (5)
C5	0.0269 (7)	0.0203 (6)	0.0270 (7)	−0.0001 (5)	−0.0047 (6)	−0.0067 (6)
C6	0.0256 (6)	0.0227 (6)	0.0275 (8)	0.0020 (5)	−0.0076 (6)	−0.0016 (6)
C7	0.0303 (7)	0.0336 (8)	0.0381 (10)	0.0034 (6)	−0.0150 (8)	−0.0108 (8)
C8	0.0230 (6)	0.0174 (6)	0.0314 (8)	0.0004 (4)	−0.0059 (6)	−0.0009 (6)
C9	0.0228 (6)	0.0178 (5)	0.0264 (8)	0.0027 (5)	−0.0021 (6)	0.0025 (5)
C10	0.0432 (9)	0.0330 (8)	0.0332 (10)	0.0083 (7)	0.0044 (8)	0.0130 (8)
C11	0.0218 (6)	0.0193 (6)	0.0241 (7)	0.0032 (5)	−0.0007 (6)	−0.0011 (5)
C12	0.0213 (6)	0.0207 (6)	0.0266 (7)	0.0036 (5)	−0.0001 (6)	0.0001 (6)
C13	0.0325 (8)	0.0277 (7)	0.0294 (9)	0.0078 (6)	−0.0043 (7)	0.0014 (6)
C14	0.0281 (7)	0.0365 (8)	0.0370 (10)	0.0075 (6)	−0.0124 (7)	−0.0038 (8)
C15	0.0221 (7)	0.0365 (8)	0.0448 (11)	−0.0016 (6)	−0.0053 (7)	−0.0047 (8)
C16	0.0238 (6)	0.0254 (6)	0.0364 (9)	−0.0016 (5)	−0.0002 (7)	0.0007 (7)
C17	0.0384 (9)	0.0351 (8)	0.0334 (9)	−0.0043 (7)	0.0077 (8)	0.0079 (8)

Geometric parameters (Å, º)

O1—C3	1.3647 (19)	C7—H7C	0.9800
O1—C7	1.425 (2)	C8—C9	1.531 (2)
O2—C4	1.3624 (16)	C8—H8	1.0000
O2—H2	0.8400	C9—C10	1.512 (3)
O3—C8	1.430 (2)	C9—H9	1.0000
O3—H3	0.8400	C10—H10A	0.9800
O4—C11	1.3755 (18)	C10—H10B	0.9800
O4—C9	1.4593 (16)	C10—H10C	0.9800
O5—C12	1.3739 (18)	C11—C16	1.384 (2)
O5—C17	1.428 (2)	C11—C12	1.402 (2)
C1—C6	1.390 (2)	C12—C13	1.386 (2)
C1—C2	1.390 (2)	C13—C14	1.396 (3)
C1—C8	1.5120 (19)	C13—H13	0.9500
C2—C3	1.3905 (19)	C14—C15	1.377 (3)
C2—H2A	0.9500	C14—H14	0.9500
C3—C4	1.402 (2)	C15—C16	1.394 (3)
C4—C5	1.379 (2)	C15—H15	0.9500
C5—C6	1.397 (2)	C16—H16	0.9500
C5—H5	0.9500	C17—H17A	0.9800
C6—H6	0.9500	C17—H17B	0.9800
C7—H7A	0.9800	C17—H17C	0.9800
C7—H7B	0.9800
C3—O1—C7	117.44 (12)	O4—C9—C8	104.75 (11)
C4—O2—H2	109.5	C10—C9—C8	114.09 (14)
C8—O3—H3	109.5	O4—C9—H9	109.5
C11—O4—C9	115.54 (11)	C10—C9—H9	109.5
C12—O5—C17	116.73 (14)	C8—C9—H9	109.5
C6—C1—C2	119.52 (13)	C9—C10—H10A	109.5
C6—C1—C8	121.87 (14)	C9—C10—H10B	109.5
C2—C1—C8	118.60 (13)	H10A—C10—H10B	109.5
C1—C2—C3	120.37 (14)	C9—C10—H10C	109.5
C1—C2—H2A	119.8	H10A—C10—H10C	109.5
C3—C2—H2A	119.8	H10B—C10—H10C	109.5
O1—C3—C4	114.89 (12)	O4—C11—C16	122.16 (15)
O1—C3—C2	125.07 (14)	O4—C11—C12	117.74 (13)
C4—C3—C2	120.02 (14)	C16—C11—C12	120.09 (15)
C5—C4—O2	119.46 (14)	O5—C12—C13	125.00 (15)
C5—C4—C3	119.44 (12)	O5—C12—C11	115.28 (13)
O2—C4—C3	121.09 (14)	C13—C12—C11	119.72 (14)
C4—C5—C6	120.56 (14)	C12—C13—C14	119.63 (17)
C4—C5—H5	119.7	C12—C13—H13	120.2
C6—C5—H5	119.7	C14—C13—H13	120.2
C1—C6—C5	120.09 (15)	C15—C14—C13	120.70 (17)
C1—C6—H6	120.0	C15—C14—H14	119.6
C5—C6—H6	120.0	C13—C14—H14	119.6
O1—C7—H7A	109.5	C14—C15—C16	119.81 (16)
O1—C7—H7B	109.5	C14—C15—H15	120.1
H7A—C7—H7B	109.5	C16—C15—H15	120.1
O1—C7—H7C	109.5	C11—C16—C15	120.01 (17)
H7A—C7—H7C	109.5	C11—C16—H16	120.0
H7B—C7—H7C	109.5	C15—C16—H16	120.0
O3—C8—C1	108.31 (14)	O5—C17—H17A	109.5
O3—C8—C9	109.14 (12)	O5—C17—H17B	109.5
C1—C8—C9	113.25 (12)	H17A—C17—H17B	109.5
O3—C8—H8	108.7	O5—C17—H17C	109.5
C1—C8—H8	108.7	H17A—C17—H17C	109.5
C9—C8—H8	108.7	H17B—C17—H17C	109.5
O4—C9—C10	109.21 (13)
C6—C1—C2—C3	−0.6 (2)	C11—O4—C9—C8	122.83 (14)
C8—C1—C2—C3	−179.38 (15)	O3—C8—C9—O4	−61.50 (15)
C7—O1—C3—C4	172.78 (16)	C1—C8—C9—O4	177.76 (14)
C7—O1—C3—C2	−5.7 (3)	O3—C8—C9—C10	179.13 (13)
C1—C2—C3—O1	178.58 (16)	C1—C8—C9—C10	58.39 (19)
C1—C2—C3—C4	0.2 (2)	C9—O4—C11—C16	65.80 (19)
O1—C3—C4—C5	−178.31 (16)	C9—O4—C11—C12	−115.35 (15)
C2—C3—C4—C5	0.2 (2)	C17—O5—C12—C13	−2.4 (2)
O1—C3—C4—O2	1.7 (2)	C17—O5—C12—C11	176.81 (14)
C2—C3—C4—O2	−179.79 (15)	O4—C11—C12—O5	0.95 (19)
O2—C4—C5—C6	179.77 (16)	C16—C11—C12—O5	179.81 (14)
C3—C4—C5—C6	−0.2 (3)	O4—C11—C12—C13	−179.80 (14)
C2—C1—C6—C5	0.6 (3)	C16—C11—C12—C13	−0.9 (2)
C8—C1—C6—C5	179.32 (15)	O5—C12—C13—C14	178.59 (16)
C4—C5—C6—C1	−0.2 (3)	C11—C12—C13—C14	−0.6 (2)
C6—C1—C8—O3	−53.49 (19)	C12—C13—C14—C15	1.3 (3)
C2—C1—C8—O3	125.25 (15)	C13—C14—C15—C16	−0.4 (3)
C6—C1—C8—C9	67.7 (2)	O4—C11—C16—C15	−179.40 (15)
C2—C1—C8—C9	−113.55 (17)	C12—C11—C16—C15	1.8 (2)
C11—O4—C9—C10	−114.57 (16)	C14—C15—C16—C11	−1.1 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O1	0.84	2.23	2.6813 (18)	114
O2—H2···O3^i	0.84	1.95	2.7438 (18)	157
O3—H3···O4	0.84	2.41	2.7606 (17)	106
O3—H3···O5	0.84	2.07	2.8781 (15)	162
C2—H2A···O2^ii	0.95	2.42	3.3545 (19)	168
C10—H10A···O3^iii	0.98	2.52	3.491 (3)	171

Symmetry codes: (i) −x+3/2, y−1/2, z+1/2; (ii) −x+3/2, y+1/2, z+1/2; (iii) x, y, z+1.