Conjugation and hydrogen bonding in a curcumin analogue

The strongly yellow-coloured compound 5-(5-methyl-2,3,4,5-tetra­hydro­furan-2-yl)-1-phenyl­pent-4-ene-1,3-dione, C₁₆H₁₄O₃, has different substituents on each side of the dione group. The mol­ecule is essentially planar and the structure indicates a continuous conjugation from the furyl moiety to the carbonyl O atom in the enol group, as well as a strong conjugation within the enol group itself. The enol H atom is bonded to the O atom closest to the furyl group. Weak hydrogen bonds are indicated and comments about the colour of the compound are given.

S1. Comment

The fact that the coloured matters of the Curcuma lomga L (Zingiberaceae) are biologically active compounds (Ammon & Wahl, 1991) and contain a β-dicarbonyl group has made these molecules, as well as synthetic analogues (Pedersen et al., 1985; Mann et al., 1987; Mostad et al., 1983), interesting targets for structural study. Because of their strong colour, the family name, curcumin analogues, was introduced for diaryl and heteroaryl unsaturated open-chain fully enolized β-diketones (Arrieta et al., 1992), and their β,β-tricarbonyl analogues (Arrieta, 1993). These compounds may also be regarded as vinyl analogues of dibenzoylmethane. The title compound, (I), is new to the this class of compounds by containing a furyl ring at one end of the conjugated chain. \scheme

The molecule may exist in one of two different enol tautomeric structures according to the position of the H atom in the enol ring, and it was of interest to determine which one was preferred in the crystal phase. Finally, it was of interest to study the intermolecular interactions between the three types of ring stuctures. The geometry of β-diketones has been extensively studied, and lately it has been shown that the enol hydrogen in benzoylacetone isasymmetrically positioned in an enol ring otherwise symmetric within 2σ (Hellerup Madsen et al., 1998). In the present structure the position of the enol H atom was clearly identified in a difference map and refinements showed the atom to be attached to the O atom in the 3-position of the pentyl chain. This is supported by the significant variations in the bond lengths within the enol group as well as in the whole molecule. It thus appears that a long continuous conjugation from C15 through O1 is preferable to a conjugation from O2 to both sides of the molecule and consequently determines the position of the H atom. This conjugation also augments the asymmetry in the enol ring as compared to that reported for benzoylacetone. The intramolecular hydrogen bond, giving a O1···O2 distance of 2.45 Å is, according to Emsley (1984) classified as a strong hydrogen bond. The molecule is planar, the largest deviations from a least-squares plane through the molecule are 0.4 (H2) and 0.44 Å (H5). The only significant twists in the molecule are about the bonds C1—C7 and C9—C10: the torsion angles being 7.4 and −3.3°, respectively.

The molecules are packed in layers where the contacts between the planar units occur at an angle of about 80°. The shortest distance between the phenyl rings and the enol groups are 2.621 (18) Å [H2···O2(5/2 − x, 1/2 + y, 1/2 − z)] and 2.927 (17) Å [H3···C9(5/2 − x, 1/2 + y, 1/2 − z)]. Such contacts are frequently found between aromatic moieties. Furthermore, the molecules are arranged in such a way that the furyl and enol groups are stacked with an interplanar distance of 3.67 Å. Contacts between molecules in different layers occur over centres of symmetry where the distances H13···O1 and H11···O2 both are 2.6 Å. This type of interaction between curcuminoide molecules related by a centre of symmetry is also seen in other derivatives and may indicate a weak type of C—H···O hydrogen bond (Mostad, 1994; Arrieta et al., 1995). To our knowledge, there is no elaborated theory for the understanding of the colour of unsaturated enolized β-diketones. A first approach to explain the colour of the present compound requires us to consider the main chromophore in the molecule and this is assigned as an electronic interaction between O3 and O1, where the carbonyl C7=O1 bond acts as acceptor and O3 as donor. A similar concept has been proposed for curcumin and unsymmetrical curcuminoids (Arrieta, 1996)

S2. Experimental

The title compound was prepared by condensation from 5-methylfurfural and benzoylacetone (Arrieta et al., 1992). Plate-shaped crystals were grown from absolute ethanol.

S3. Refinement

All H atoms were refined; O—H = 0.99 (3) Å and C—H = 0.930 (19)–1.00 (3) Å.

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997).

5-(methylfur-2-yl)-1-phenylpent-4-ene-1,3-dione

Crystal data

C16H14O3	Dx = 1.285 Mg m^−3
Mr = 254.27	Melting point: 363K K
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 7.0907 (14) Å	Cell parameters from 5595 reflections
b = 8.0793 (16) Å	θ = 2.7–26.4°
c = 23.070 (5) Å	µ = 0.09 mm^−1
β = 96.10 (3)°	T = 150 K
V = 1314.2 (5) Å^3	Transparent plates, yellow
Z = 4	0.3 × 0.3 × 0.2 mm
F(000) = 536

Data collection

Simens SMART diffractometer	2094 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.040
Graphite monochromator	θmax = 26.4°, θmin = 2.7°
Detector resolution: 8 pixels mm^-1	h = −8→8
w scan	k = −10→10
13412 measured reflections	l = −28→28
2684 independent 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)] = 0.040	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.098	All H-atom parameters refined
S = 1.07	w = 1/[σ^2(Fo^2) + (0.035P)^2 + 0.430P] where P = (Fo^2 + 2Fc^2)/3
2684 reflections	(Δ/σ)max = 0.005
228 parameters	Δρmax = 0.18 e Å^−3
0 restraints	Δρmin = −0.18 e Å^−3

Special details

Experimental. Data were collected over a hemisphere of reciprocal space by a combination of four sets of exposures, each set at a different ϕ angle for the crystal. Exposures of 45 s covered 0.6 ° in ω. The crystal detector distance was 5.0 cm.

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 F²are 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	1.40232 (15)	0.44540 (14)	0.14142 (4)	0.0418 (3)
O2	1.10575 (16)	0.49405 (14)	0.07803 (4)	0.0384 (3)
O3	0.51191 (14)	0.84032 (12)	0.07622 (4)	0.0325 (3)
C1	1.4325 (2)	0.6696 (2)	0.27951 (6)	0.0353 (4)
C2	1.5615 (2)	0.6906 (2)	0.32879 (7)	0.0375 (4)
C3	1.7399 (2)	0.6205 (2)	0.33153 (7)	0.0394 (4)
C4	1.7920 (3)	0.5301 (2)	0.28470 (8)	0.0450 (4)
C5	1.6644 (2)	0.5095 (2)	0.23531 (7)	0.0404 (4)
C6	1.4827 (2)	0.57759 (18)	0.23230 (6)	0.0316 (3)
C7	1.3503 (2)	0.54835 (19)	0.17850 (6)	0.0326 (3)
C8	1.1726 (2)	0.62980 (19)	0.16814 (6)	0.0332 (3)
C9	1.0554 (2)	0.60045 (18)	0.11708 (6)	0.0319 (3)
C10	0.8739 (2)	0.68233 (19)	0.10420 (7)	0.0334 (3)
C11	0.7644 (2)	0.65873 (19)	0.05353 (6)	0.0316 (3)
C12	0.5835 (2)	0.73296 (18)	0.03742 (6)	0.0309 (3)
C13	0.4577 (2)	0.7199 (2)	−0.01133 (7)	0.0355 (4)
C14	0.3020 (2)	0.8232 (2)	−0.00273 (7)	0.0403 (4)
C15	0.3394 (2)	0.89405 (19)	0.05026 (7)	0.0357 (4)
C16	0.2361 (3)	1.0122 (2)	0.08449 (9)	0.0455 (4)
H1	1.307 (2)	0.718 (2)	0.2792 (7)	0.044 (5)*
H8	1.131 (2)	0.707 (2)	0.1959 (7)	0.038 (4)*
H10	0.835 (2)	0.753 (2)	0.1332 (7)	0.034 (4)*
H13	0.474 (2)	0.656 (2)	−0.0446 (7)	0.036 (4)*
H11	0.809 (2)	0.585 (2)	0.0243 (7)	0.035 (4)*
H2	1.525 (2)	0.751 (2)	0.3626 (7)	0.040 (4)*
H3	1.834 (2)	0.634 (2)	0.3665 (7)	0.040 (4)*
H5	1.698 (2)	0.450 (2)	0.2035 (8)	0.053 (5)*
H4	1.917 (3)	0.482 (2)	0.2858 (8)	0.055 (5)*
H16A	0.205 (3)	0.967 (3)	0.1206 (10)	0.080 (7)*
H14	0.183 (3)	0.837 (2)	−0.0284 (8)	0.053 (5)*
H16B	0.312 (3)	1.114 (3)	0.0962 (10)	0.087 (8)*
H16C	0.124 (4)	1.056 (3)	0.0608 (12)	0.110 (9)*
HO2	1.233 (4)	0.454 (3)	0.0931 (11)	0.100 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0429 (6)	0.0475 (7)	0.0350 (6)	0.0133 (5)	0.0031 (5)	−0.0090 (5)
O2	0.0409 (6)	0.0432 (7)	0.0313 (6)	0.0093 (5)	0.0043 (5)	−0.0049 (5)
O3	0.0323 (6)	0.0322 (6)	0.0331 (5)	0.0026 (4)	0.0040 (4)	−0.0025 (4)
C1	0.0385 (9)	0.0360 (8)	0.0327 (8)	0.0021 (7)	0.0096 (6)	0.0006 (7)
C2	0.0471 (9)	0.0371 (9)	0.0293 (8)	−0.0044 (7)	0.0088 (7)	−0.0010 (7)
C3	0.0464 (10)	0.0368 (9)	0.0337 (8)	−0.0047 (7)	−0.0019 (7)	0.0029 (7)
C4	0.0413 (9)	0.0437 (10)	0.0486 (10)	0.0080 (8)	−0.0024 (7)	−0.0015 (8)
C5	0.0432 (9)	0.0398 (9)	0.0382 (9)	0.0086 (7)	0.0040 (7)	−0.0053 (7)
C6	0.0381 (8)	0.0285 (8)	0.0289 (7)	0.0011 (6)	0.0062 (6)	0.0025 (6)
C7	0.0372 (8)	0.0312 (8)	0.0304 (7)	0.0015 (6)	0.0090 (6)	0.0013 (6)
C8	0.0370 (8)	0.0340 (8)	0.0296 (8)	0.0057 (7)	0.0074 (6)	−0.0023 (6)
C9	0.0372 (8)	0.0304 (8)	0.0292 (7)	0.0019 (6)	0.0095 (6)	0.0015 (6)
C10	0.0363 (8)	0.0337 (8)	0.0311 (8)	0.0035 (6)	0.0082 (6)	−0.0010 (7)
C11	0.0345 (8)	0.0291 (8)	0.0322 (8)	0.0006 (6)	0.0076 (6)	0.0006 (6)
C12	0.0354 (8)	0.0272 (7)	0.0312 (8)	0.0002 (6)	0.0093 (6)	−0.0006 (6)
C13	0.0385 (9)	0.0375 (9)	0.0308 (8)	−0.0008 (7)	0.0044 (6)	−0.0023 (7)
C14	0.0342 (9)	0.0472 (10)	0.0388 (9)	0.0034 (7)	0.0006 (7)	−0.0008 (7)
C15	0.0312 (8)	0.0358 (8)	0.0406 (9)	0.0021 (6)	0.0062 (6)	0.0021 (7)
C16	0.0425 (10)	0.0459 (11)	0.0495 (11)	0.0093 (8)	0.0112 (8)	−0.0036 (9)

Geometric parameters (Å, º)

O1—C7	1.2754 (18)	C7—C8	1.419 (2)
O2—C9	1.3217 (17)	C8—C9	1.388 (2)
O2—HO2	0.99 (3)	C8—H8	0.961 (17)
O3—C15	1.3738 (18)	C9—C10	1.450 (2)
O3—C12	1.3812 (17)	C10—C11	1.346 (2)
C1—C2	1.392 (2)	C10—H10	0.942 (16)
C1—C6	1.396 (2)	C11—C12	1.429 (2)
C1—H1	0.968 (17)	C11—H11	0.977 (16)
C2—C3	1.381 (2)	C12—C13	1.363 (2)
C2—H2	0.978 (17)	C13—C14	1.414 (2)
C3—C4	1.386 (2)	C13—H13	0.942 (16)
C3—H3	0.996 (16)	C14—C15	1.351 (2)
C4—C5	1.388 (2)	C14—H14	0.984 (18)
C4—H4	0.965 (19)	C15—C16	1.481 (2)
C5—C6	1.395 (2)	C16—H16A	0.96 (2)
C5—H5	0.930 (19)	C16—H16B	1.00 (3)
C6—C7	1.494 (2)	C16—H16C	0.98 (3)
C9—O2—HO2	106.0 (15)	O2—C9—C10	117.08 (13)
C15—O3—C12	106.71 (11)	C8—C9—C10	122.32 (14)
C2—C1—C6	120.13 (15)	C11—C10—C9	122.06 (14)
C2—C1—H1	118.6 (10)	C11—C10—H10	121.2 (9)
C6—C1—H1	121.2 (10)	C9—C10—H10	116.8 (9)
C3—C2—C1	120.45 (15)	C10—C11—C12	125.81 (14)
C3—C2—H2	119.1 (9)	C10—C11—H11	119.2 (9)
C1—C2—H2	120.4 (10)	C12—C11—H11	115.0 (9)
C2—C3—C4	119.93 (15)	C13—C12—O3	109.27 (13)
C2—C3—H3	121.5 (9)	C13—C12—C11	132.27 (14)
C4—C3—H3	118.6 (9)	O3—C12—C11	118.46 (13)
C5—C4—C3	119.91 (16)	C12—C13—C14	106.89 (14)
C5—C4—H4	119.2 (11)	C12—C13—H13	125.9 (10)
C3—C4—H4	120.9 (11)	C14—C13—H13	127.2 (10)
C4—C5—C6	120.77 (15)	C15—C14—C13	107.23 (15)
C4—C5—H5	120.7 (11)	C15—C14—H14	124.6 (11)
C6—C5—H5	118.6 (11)	C13—C14—H14	128.1 (11)
C5—C6—C1	118.81 (14)	C14—C15—O3	109.91 (13)
C5—C6—C7	118.34 (13)	C14—C15—C16	134.05 (16)
C1—C6—C7	122.86 (14)	O3—C15—C16	116.04 (14)
O1—C7—C8	120.30 (14)	C15—C16—H16A	112.9 (14)
O1—C7—C6	117.34 (13)	C15—C16—H16B	112.8 (13)
C8—C7—C6	122.36 (13)	H16A—C16—H16B	104.1 (19)
C9—C8—C7	120.30 (14)	C15—C16—H16C	110.7 (16)
C9—C8—H8	118.5 (9)	H16A—C16—H16C	112 (2)
C7—C8—H8	121.2 (9)	H16B—C16—H16C	104 (2)
O2—C9—C8	120.60 (14)
C6—C1—C2—C3	−0.2 (2)	C7—C8—C9—C10	−179.14 (14)
C1—C2—C3—C4	0.8 (2)	O2—C9—C10—C11	−3.4 (2)
C2—C3—C4—C5	−0.4 (3)	C8—C9—C10—C11	177.13 (14)
C3—C4—C5—C6	−0.5 (3)	C9—C10—C11—C12	179.43 (14)
C4—C5—C6—C1	1.1 (2)	C15—O3—C12—C13	−0.30 (15)
C4—C5—C6—C7	−178.78 (15)	C15—O3—C12—C11	179.66 (13)
C2—C1—C6—C5	−0.8 (2)	C10—C11—C12—C13	−179.35 (16)
C2—C1—C6—C7	179.13 (14)	C10—C11—C12—O3	0.7 (2)
C5—C6—C7—O1	8.7 (2)	O3—C12—C13—C14	0.16 (17)
C1—C6—C7—O1	−171.25 (14)	C11—C12—C13—C14	−179.80 (16)
C5—C6—C7—C8	−171.57 (15)	C12—C13—C14—C15	0.05 (19)
C1—C6—C7—C8	8.5 (2)	C13—C14—C15—O3	−0.24 (18)
O1—C7—C8—C9	−1.2 (2)	C13—C14—C15—C16	179.9 (8)
C6—C7—C8—C9	179.06 (13)	C12—O3—C15—C14	0.33 (16)
C7—C8—C9—O2	1.4 (2)	C12—O3—C15—C16	−179.85 (14)