Tetra­hydro-2,6,6-tri­methyl-1,4-pyran-2,5-di­carboxyl­ic acid (cineolic acid)

In the title compound, C₁₀H₁₆O₅, the carboxyl­ate group at the C-2 position is axial, but that at C-5 is equatorial. These carboxyl­ate groups form hydrogen-bonded rings [average O...O′ distance = 2.64 (1) Å] across the inversion centers at (½, 0, 0) and (½, ½, ½), respectively, linking the mol­ecules into stepped chains along the bc diagonal of the cell.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• Mean [sigma](C-C) = 0.001 Å
• R factor = 0.037
• wR factor = 0.109
• Data-to-parameter ratio = 22.5

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

---

S1. Comment

Hydrogen bonding to give supramolecular arrays is a major factor in the design of the crystal structures of polycarboxylic acids and their salts with protonated cations (Jeffrey & Saenger, 1994). Network analysis of hydrogen bonding in these systems was introduced by Etter (1990) and extended by Bernstein et al. (1995). Structures of this type have been studied by many groups. Examples from this laboratory include di- tri- and tetracarboxylates (Barnes et al., 1988, 1991, 1996, 1998). Oxidation of 1,8-epoxy-p-menthane, (1,8-cineol), (I), was reviewed by Simonsen (1931). The first product is racemic cineolic acid, (II) (Fig. 1). During this reaction, the configuration of the ring O1···C6 changes from boat to chair as the bond between C8 and C11 is broken. The carboxylate group at C2 is still axial [C6—O1—C2—C8 = 76.17 (7)°], but that at C5 takes the equatorial position [O1—C6—C5—C11 = −178.54 (6)°]. The planes of the carboxylate groups C2/C8/O9/O10 and C5/C11/O12/O13 make angles of 89.00 (3) and 57.23 (3)° to the ring plane O1/C3/C4/C6. The torsion angles O9—C8—C2—C3 and C6—C5—C11—O13 are −16.61 (9) and −82.90 (8)°, respectively. All bond lengths and angles have typical values.

The carboxylate groups form typical R₂²(8) intermolecular hydrogen-bonded rings across the centres of inversion at (1/2, 0, 0) for C8 [O9···O10ⁱ = 2.666 (9) Å] and (1/2, 1/2, 1/2) for C11 [O13···O14ⁱⁱ = 2.623 (9) Å] [symmetry codes (i) 1 − x, −y, −z; (ii) 1 − x, 1 − y, 1 − z]. As a result of the (axial + equatorial) arrangement of the carboxylate groups the hydrogen-bonded chains (Fig. 2), which lie along the bc diagonal of the cell, form steps with the plane C2/C8/O9/O10, making an angle of 81.31 (3)° to C5/C11/O12/O13. The formation of hydrogen-bonded chains occurs commonly in dicarboxylic acids in which the acid groups are back to back across the molecule. In (II), the effect is a staircase in the a direction, unlike the alternate up and down molecules in the chains of camphoric acid (Barnes et al., 1991), the zigzag chains in trans-cyclohexane-1,4-dicarboxylic acid (Luger et al., 1972) or the flat chains in fumaric acid (Brown, 1966; Bednovitz & Post, 1966).

The structure of (II) was also determined from data collected at 296 K. There were no structural differences (a = 6.5441, b = 6.6087, c = 13.9443 Å, α = 87.3168, β = 88.8787, χ = 64.3207°; R1 0.0406 for 2140 reflections with I > 2σ(I)].

S2. Experimental

1,8-Cineol (10 ml) was oxidized with KMnO₄ as reported by Wallach et al. (1888). The cineolic acid was recrystallized from water.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON, Spek (1999); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The structure of (II), showing ellipsoids at the 50% probability level.
	Fig. 2. Hydrogen-bonded chain in (II).

tetrahydro-2,6,6-trimethyl-1,4-pyran-2,5-dicarboxylic acid

Crystal data

C10H16O5	F(000) = 232
Mr = 216.23	Dx = 1.362 Mg m^−3
Triclinic, P1	Melting point: 198 K
a = 6.4737 (3) Å	Mo Kα radiation, λ = 0.71073 Å
b = 6.5302 (3) Å	Cell parameters from 5956 reflections
c = 13.8836 (6) Å	θ = 2.9–33.7°
α = 86.935 (3)°	µ = 0.11 mm^−1
β = 88.987 (3)°	T = 100 K
γ = 64.078 (3)°	Block, colorless
V = 527.11 (4) Å^3	0.46 × 0.32 × 0.20 mm
Z = 2

Data collection

Bruker SMART APEX CCD diffractometer	3044 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.013
Graphite monochromator	θmax = 31.0°, θmin = 2.9°
Detector resolution: 273070 pixels mm^-1	h = −9→9
ϕ and ω scans	k = −9→9
6332 measured reflections	l = −20→20
3260 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)] = 0.037	Hydrogen site location: mixed
wR(F^2) = 0.109	H atoms treated by a mixture of independent and constrained refinement
S = 1.11	w = 1/[σ^2(Fo^2) + (0.0704P)^2 + 0.0601P] where P = (Fo^2 + 2Fc^2)/3
3260 reflections	(Δ/σ)max = 0.017
145 parameters	Δρmax = 0.46 e Å^−3
0 restraints	Δρmin = −0.36 e Å^−3

Special details

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. H atoms attached to C atoms were placed in calculated positions and allowed to ride during the refinement. Isotropic displacement parameters were constrained to be 1.3U_eq of the parent C atom. H atoms of carboxylate groups were located on a difference synthesis. The positional and isotropic displacement parameters of these H atoms were allowed to refine.

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	0.07959 (9)	0.14029 (9)	0.24929 (4)	0.01655 (12)
C2	0.05200 (12)	0.25974 (12)	0.15780 (5)	0.01581 (14)
C3	−0.00178 (12)	0.50922 (12)	0.16866 (5)	0.01725 (14)
H3A	0.0029	0.5806	0.1044	0.022*
H3B	−0.1591	0.5905	0.1943	0.022*
C4	0.16736 (12)	0.53527 (12)	0.23585 (5)	0.01675 (14)
H4A	0.1215	0.6988	0.2448	0.022*
H4B	0.3231	0.4683	0.2076	0.022*
C5	0.16837 (12)	0.41281 (11)	0.33316 (5)	0.01507 (14)
H5	0.0086	0.4864	0.3590	0.020*
C6	0.22892 (12)	0.15750 (11)	0.32169 (5)	0.01515 (14)
C7	−0.14785 (13)	0.23880 (15)	0.10951 (6)	0.02270 (16)
H7A	−0.1111	0.0773	0.1048	0.030*
H7B	−0.1736	0.3132	0.0447	0.030*
H7C	−0.2869	0.3125	0.1481	0.030*
C8	0.26083 (12)	0.14213 (12)	0.09246 (5)	0.01556 (14)
O9	0.33431 (10)	0.24932 (9)	0.03871 (4)	0.01928 (13)
O10	0.34183 (10)	−0.08175 (9)	0.09485 (4)	0.02072 (13)
H10A	0.457 (2)	−0.140 (2)	0.0501 (9)	0.027*
C11	0.32371 (12)	0.43900 (11)	0.40606 (5)	0.01605 (14)
O12	0.49789 (10)	0.46767 (10)	0.37291 (4)	0.02248 (14)
H12A	0.575 (2)	0.501 (2)	0.4224 (9)	0.029*
O13	0.28112 (10)	0.43468 (11)	0.49405 (4)	0.02419 (14)
C14	0.16507 (14)	0.05625 (13)	0.41217 (5)	0.02135 (16)
H14A	0.0046	0.1520	0.4285	0.028*
H14B	0.2648	0.0495	0.4657	0.028*
H14C	0.1847	−0.0979	0.4005	0.028*
C15	0.48170 (13)	0.01545 (12)	0.29747 (5)	0.01884 (15)
H15A	0.5066	−0.1390	0.2833	0.024*
H15B	0.5777	0.0081	0.3526	0.024*
H15C	0.5229	0.0863	0.2410	0.024*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0196 (2)	0.0188 (2)	0.0137 (2)	−0.0108 (2)	0.00169 (17)	−0.00089 (17)
C2	0.0154 (3)	0.0182 (3)	0.0140 (3)	−0.0074 (2)	0.0015 (2)	−0.0016 (2)
C3	0.0170 (3)	0.0161 (3)	0.0160 (3)	−0.0048 (2)	0.0000 (2)	−0.0004 (2)
C4	0.0203 (3)	0.0150 (3)	0.0149 (3)	−0.0078 (2)	0.0003 (2)	0.0001 (2)
C5	0.0166 (3)	0.0141 (3)	0.0140 (3)	−0.0062 (2)	0.0010 (2)	−0.0017 (2)
C6	0.0175 (3)	0.0143 (3)	0.0130 (3)	−0.0064 (2)	0.0019 (2)	−0.0011 (2)
C7	0.0191 (3)	0.0304 (4)	0.0217 (3)	−0.0135 (3)	−0.0010 (3)	−0.0026 (3)
C8	0.0166 (3)	0.0178 (3)	0.0126 (3)	−0.0077 (2)	0.0001 (2)	−0.0020 (2)
O9	0.0213 (3)	0.0186 (2)	0.0178 (3)	−0.0087 (2)	0.00453 (19)	−0.00102 (18)
O10	0.0260 (3)	0.0172 (2)	0.0192 (3)	−0.0095 (2)	0.0079 (2)	−0.00434 (19)
C11	0.0173 (3)	0.0139 (3)	0.0159 (3)	−0.0058 (2)	0.0008 (2)	−0.0016 (2)
O12	0.0221 (3)	0.0273 (3)	0.0220 (3)	−0.0144 (2)	0.0010 (2)	−0.0020 (2)
O13	0.0245 (3)	0.0337 (3)	0.0153 (3)	−0.0133 (2)	0.0016 (2)	−0.0039 (2)
C14	0.0307 (4)	0.0183 (3)	0.0154 (3)	−0.0113 (3)	0.0052 (3)	−0.0003 (2)
C15	0.0179 (3)	0.0168 (3)	0.0185 (3)	−0.0045 (2)	0.0012 (2)	−0.0019 (2)

Geometric parameters (Å, º)

O1—C2	1.4262 (8)	C7—H7A	0.9800
O1—C6	1.4487 (8)	C7—H7B	0.9800
C2—C3	1.5253 (10)	C7—H7C	0.9800
C2—C7	1.5288 (10)	C8—O9	1.2231 (9)
C2—C8	1.5374 (10)	C8—O10	1.3186 (9)
C3—C4	1.5212 (10)	O10—H10A	0.922 (13)
C3—H3A	0.9900	C11—O13	1.2491 (9)
C3—H3B	0.9900	C11—O12	1.2943 (9)
C4—C5	1.5313 (9)	O12—H12A	0.945 (13)
C4—H4A	0.9900	C14—H14A	0.9800
C4—H4B	0.9900	C14—H14B	0.9800
C5—C11	1.5066 (9)	C14—H14C	0.9800
C5—C6	1.5536 (9)	C15—H15A	0.9800
C5—H5	1.0000	C15—H15B	0.9800
C6—C14	1.5224 (10)	C15—H15C	0.9800
C6—C15	1.5291 (10)
C2—O1—C6	119.95 (5)	C14—C6—C5	110.59 (5)
O1—C2—C3	111.53 (5)	C15—C6—C5	112.81 (6)
O1—C2—C7	104.83 (6)	C2—C7—H7A	109.5
C3—C2—C7	110.92 (6)	C2—C7—H7B	109.5
O1—C2—C8	111.95 (6)	H7A—C7—H7B	109.5
C3—C2—C8	111.38 (6)	C2—C7—H7C	109.5
C7—C2—C8	105.89 (6)	H7A—C7—H7C	109.5
C4—C3—C2	112.03 (6)	H7B—C7—H7C	109.5
C4—C3—H3A	109.2	O9—C8—O10	123.46 (6)
C2—C3—H3A	109.2	O9—C8—C2	122.40 (6)
C4—C3—H3B	109.2	O10—C8—C2	113.97 (6)
C2—C3—H3B	109.2	C8—O10—H10A	109.4 (8)
H3A—C3—H3B	107.9	O13—C11—O12	122.79 (7)
C3—C4—C5	108.72 (6)	O13—C11—C5	120.24 (6)
C3—C4—H4A	109.9	O12—C11—C5	116.97 (6)
C5—C4—H4A	109.9	C11—O12—H12A	111.5 (8)
C3—C4—H4B	109.9	C6—C14—H14A	109.5
C5—C4—H4B	109.9	C6—C14—H14B	109.5
H4A—C4—H4B	108.3	H14A—C14—H14B	109.5
C11—C5—C4	112.41 (6)	C6—C14—H14C	109.5
C11—C5—C6	111.12 (5)	H14A—C14—H14C	109.5
C4—C5—C6	111.57 (5)	H14B—C14—H14C	109.5
C11—C5—H5	107.1	C6—C15—H15A	109.5
C4—C5—H5	107.1	C6—C15—H15B	109.5
C6—C5—H5	107.1	H15A—C15—H15B	109.5
O1—C6—C14	102.44 (6)	C6—C15—H15C	109.5
O1—C6—C15	111.16 (5)	H15A—C15—H15C	109.5
C14—C6—C15	110.16 (6)	H15B—C15—H15C	109.5
O1—C6—C5	109.20 (5)
O9—C8—C2—O1	−142.26 (7)	O1—C6—C5—C11	−178.54 (5)
C8—C2—O1—C6	76.16 (7)	C6—C5—C11—O13	−82.90 (8)
C2—O1—C6—C5	49.82 (7)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O10—H10A···O9^i	0.922 (13)	1.745 (13)	2.6656 (8)	177.3 (12)
O12—H12A···O13^ii	0.945 (13)	1.680 (13)	2.6234 (8)	177.1 (12)

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