rac-2-{3-[1-(Acet­yl­oxy)eth­yl]-2,2-di­methyl­cyclobut­yl}acetic acid

The title compound, C₁₂H₂₀O₄, was prepared from α-pinene in three steps. The ester and acid moieties are cis on the slightly folded cyclo­butane ring. In the crystal, carb­oxy­lic acid bound dimers form layers parallel to (202).

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Chemical scheme

Structure description

As part of a project on strained carbocycles (Detert & Schollmeyer, 2017; Herges et al., 2005), the title compound, C₁₂H₂₀O₄ (Fig. 1), was prepared from racemic α-pinene by permanganate oxidation, borohydride reduction of the pinonic acid to pinolic acid and acetyl­ation. The compound crystallizes in the monoclinic space group C2/c with the asymmetric unit containing eight mol­ecules. Two enanti­omeric mol­ecules are connected via two hydrogen bridges of the carb­oxy­lic acids, forming centrosymmetric dimers. The distance between the oxygen atoms forming the hydrogen bond is 2.6547 (13) Å. These dimers are arranged in layers parallel to the (02) plane (Table 1, Fig. 2). The central cyclo­butane ring is folded in a butterfly-like manner: the planes defined by C1,C2,C4 and by C2, C3, C4 subtend an angle of 24.61 (12)°, which is due to the bulky methyl groups at C2. However, it is significantly smaller than the ideal angle of 35° (Bucourt, 1974). The acetic acid substituent on C1 and the acet­oxy­ethyl on C3 are cis and on the open side of the folded cyclo­butane. The geminal methyl groups on C2 open an angle of 110.39 (10)° and provoke an elongation of the cyclo­butane bond lengths e.g. C1—C2 = 1.5697 (15) Å versus C1—C4 = 1.5467 (15) Å. A deviation of only 0.0193 (10) Å for O8 destroys the otherwise perfect planarity of the acetic acid unit O7,O8,C5,C6.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O8—H8O⋯O7^i	0.90 (2)	1.75 (2)	2.6547 (13)	178 (2)
Symmetry code: (i) .

Figure 1 View of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2 Part of the packing diagram. Hydrogen bonds are drawn with dashed lines. View along the [101] direction. The color of the mol­ecules corresponds to the generating symmetry operator.

Synthesis and crystallization

The title compound was prepared from α-pinene by phase-transfer-catalyzed oxidation with permanganate according to Hünig et al. (1979) (43% yield) followed by reduction with sodium borohydride according to Fernández et al. (2001) (94% yield). The resulting diastereomeric mixture of pinolic acids (2.00 g) was dissolved in benzene (5 ml), acetic acid (2.58 g) and toluene­sulfonic acid (0.47 g) were added. The mixture was refluxed for 3.5 h and water was separated using a Dean–Stark trap. The mixture was washed with water, the aqueous phase extracted with toluene and the combined organic layers were dried and the solvents removed in vacuo. The residue thus obtained was dissolved in heptane (5 ml), treated with active charcoal and filtered. Upon cooling, the mixture separated into two phases, the lower layer was dissolved in heptane (15 ml) and upon cooling for 3 days. The precipitated solid was recrystallized from heptane to yield 0.22 g (9%) of colorless crystals with m.p. = 360–362 K. Hergueta et al. (2003) report a melting point of the enanti­opure compound of 258-258 K. Their NMR data correspond well with the results from the racemate, except a general deep-field shift of all H-NMR signals and a high-field shift of ca 0.25 p.p.m. in C-NMR. The numbering of H- and C-signals follows IUPAC nomenclature. ¹H-NMR (300 MHz, CDCl₃): δ = 4.77 (dq, J = 10.2, 6.2 Hz, 1H, 1′′-H), 2.41–2.15 (m, 3H, 2-H, 1′-H), 2.14–1.93 (m, 2H, 3′-H, 4′-H), 2.00 (s, 3H, 4′′-H), 1.30–1.16 (m, 1H, 4′-H), 1.08 (s, 3H, 5′′-H), 1.06 (d, J = 6.2 Hz, 3H, 2′′-H), 0.88 (s, 3H, 6′′-H). ¹³C-NMR (101 MHz, CDCl₃): δ = 179.3 (C-1), 170.7 (C-3′′), 71.9 (C-1′′), 47.1 (C-3′), 40.0 (C-2′), 37.9 (C-1′), 35.0 (C-2), 30.5 (C-5′′), 26.5 (C-4′), 21.6 (C-4′′), 17.7 (C-2′′), 16.9 (C-6′′).

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	C12H20O4
Mr	228.28
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	120
a, b, c (Å)	9.8411 (4), 12.3319 (5), 21.0912 (10)
β (°)	94.254 (4)
V (Å^3)	2552.56 (19)
Z	8
Radiation type	Mo Kα
μ (mm^−1)	0.09
Crystal size (mm)	0.55 × 0.29 × 0.25
Data collection
Diffractometer	Stoe IPDS 2T
Absorption correction	Integration [X-RED32 (Stoe & Cie, 2020), absorption correction by Gaussian integration (Coppens, 1970)]
Tmin, Tmax	0.966, 0.982
No. of measured, independent and observed [I > 2σ(I)] reflections	6713, 3019, 2607
Rint	0.023
(sin θ/λ)max (Å^−1)	0.658
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.040, 0.110, 1.03
No. of reflections	3019
No. of parameters	215
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.35, −0.17
Computer programs: X-AREA WinXpose, Recipe and Integrate (Stoe & Cie, 2020), SHELXT2014 (Sheldrick, 2015a), SHELXL2019/2 (Sheldrick, 2015b) and PLATON (Spek, 2020).