Spontaneous Enantiomeric Resolution of 1,3-Diols from the Naphtylidene Derivative of 2,4-Pentanedione

Abstract

Enantiomers (2S, 4S)- and (2R, 4R)-3-(naphthalene-1-ylmethyl) pentane-2,4-diols were synthesized by the reduction of (Z)-4-hydroxy-3-(naphthalene-1-ylmethyl) pent-3-en-2-one with NaBH₄ in methanol (MeOH). Crystallization in dichloromethane of this racemic mixture led to simple crystals with a crystalline habit with similar morphologies; however, in a group of them, it was possible to find a barely observable difference that allowed determining a crystal structure for each of the enantiomers, the 2S,4S, and the 2R,4R.

1. Introduction

Compounds containing the β-diketone function play an essential role in nature, being part of the structure of metabolites such as curcumin, a molecule to which countless biological properties are attributed, among which the anti-cancer activity stands out [1,2,3,4,5]. Through condensation with aldehydes, β-diketones are valuable synthetic intermediates that serve as building blocks for the synthesis of derivatives such as benzylidenes that show activities against breast cancer (MCF-7) and leukemia (K562), among others [6]. Furthermore, reducing the β-diketone function leads to 1,3–diols furnishing relevant synthetic scaffolds and providing a basic skeleton of the remarkable molecules v.gr. Rifamycin B (used for the treatment of HIV), Rosuvastatin (used to prevent cardiovascular diseases), and Compactin (used as a hypolipidemic agent) [7].

However, the 1,3-diols obtained after the reduction of the β-diketone function are isomeric mixtures whose separation can be a complex but necessary task since the convenience of making available optically pure compounds is well recognized [8]. In this regard, synthesizing aliphatic molecules of the 1,3-glycol type and its derivatives and separating their stereoisomers is a recurring problem. For this reason, one of the strategies used is chemical derivation to facilitate their separation [9]. Itoh et al. described the separation of various 1,1-Bis(1-Hydroxyalkyl)cyclopropanes via the formation of 1,3-dioxane derivatives by acetalization of the diols. In addition, Bianchini et al. described the obtention of a new optically pure compound (R)-(R)-3-benzyl-2,4-bis-(diphenylphosphino)pentane [10]. It should be noted that the obtention of this compound was achieved by prior synthesis of the molecule (S)-(S)-PhCH₂CH(CH(OH)CH₃)₂ by enantioselective reduction of 3-benzyl-2,4-pentanedione with [((S)-BINAP)Ru(p-cymene)Cl]Cl. Chemoenzymatic transformation is another recently used method for high-yield preparations of enantiomeric compounds. For instance, Bäckvall et al. reported the use of Candida Antarctica lipase B to transform α-substituted β-hydroxy ketones with yields up to 95% with good diastereoisomeric proportions. The direct crystallization of enantiomers from a racemic mixture is a method that receives constant attention. However, reports of successful cases of the efficient crystallization of enantiomers are scarce. Thus, trial and error remain a common practice in this field. [11] Thus, in this work, we describe the spontaneous resolution of 1,3-diols by crystallizing their enantiomers (R, R, and S, S). A discussion of the supramolecular arrangement and their Hirshfeld plots is carried out regarding their crystallographic characteristics.

2. Results and Discussion

2.1. X-ray Diffraction

The compounds 4a and 4b were synthesized by reduction with NaBH₄ in MeOH of 3 at 0 °C (Scheme 1) [12,13]. As indicated in Scheme 1, the reduction reaction gave rise to the formation of compounds 4a and 4b; however, they were not the only species obtained. Two other diastereomers corresponding to the 1,3-diol meso compound with the relative configuration 2S, 3r, 4R (2S, 3s, 4R), and additionally, a hydroxy ketone resulting from the reduction of a single carbonyl carbon, with a relative configuration syn or anti were obtained.

A portion of the diastereomeric mixture (racemate 4a/4b, 1,3-diol meso, and hydroxy ketone) was chromatographed using a ternary mixture of hexane-dichloromethane-methanol (Hex-CH₂Cl₂-MeOH) in a 60:35:5 ratio. The fraction containing 4a/4b spontaneously afforded the corresponding monocrystal. This was possible despite being a racemate and using a non-chiral and common organic solvent such as dichloromethane. The crystal structure of compounds 4a and 4b was carried out in single-crystal X-ray diffraction analysis (Figure 1). The crystallographic data are shown in

Table 1. Crystallographic data of compounds 4a and 4b.

Complex	4a (CCDC 2189445)	4b (CCDC 2189446)
Empirical formula	C16H20O2	C16H20O2
Formula weight	244.32	244.32
T(K)	150(2)	150(2)
Crystal system	Orthorombic	Orthorombic
Space group	P212121	P212121
a (Å)	7.6041(6)	7.6127(5)
b (Å)	7.9171(6)	7.9115(5)
c (Å)	21.5553(17)	21.5467(13)
α (°)	90	90
β (°)	90	90
γ (°)	90	90
V (Å3)	1297.68(18)	1297.71(14)
Z	4	4
ρCalc (Mg m−3)	1.251	1.251
μ (mm−1)	0.635	0.635
F(000)	528.0	528.0
Crystal size (mm3)	0.393 × 0.299 × 0.265	0.331 × 0.247 × 0.174
Wavelength (Å)	1.54178	1.54178
2θ range for data collection	8.204 to 158.194°	8.206 to 159.5°
Index ranges	−9 ≤ h ≤ 9, −10 ≤ k ≤ 9, −26 ≤ l ≤ 27	−9 ≤ h ≤ 9, −10 ≤ k ≤ 10, −27 ≤ l ≤ 26
Reflections collected	29301	22941
Independent reflections	2783 [Rint = 0.0354, Rsigma = 0.0155]]	2797 [Rint = 0.0325, Rsigma = 0.0179]
Data/restraints/parameters	2783/0/171	2797/0/171
Goodness of fit (GOF) on F2	1.075	1.064
Final R indices [I≥2σ(I)]	R1 = 0.0301, wR2 = 0.0811	R1 = 0.0315, wR2 = 0.0797
Final R indices [all data]	R1 = 0.0305, wR2 = 0.0815	R1 = 0.0318, wR2 = 0.0801
Largest difference peak/hole (e Å-3)	0.22 and −0.17	0.24 and −0.16
Flack parameter	−0.02(5)	−0.01(6)

, demonstrating that both enantiomers had similar cell parameters and crystallized in the orthorhombic P2₁2₁2₁ space group (see Supplementary Materials). The principal intramolecular interaction observed in both enantiomers is the hydrogen bond (Figure 2) with an OH···O distance of 1.881 and 1.915 Å for 4a and 4b, respectively.

To understand the underlying intermolecular interactions that stabilized the crystal, we further examined its crystal structure, revealing two important intermolecular bondings; an OH···H and a C-H···π interaction was observed in both compounds, as reported in

Table 2. Intermolecular interactions in compound 4a.

Contact	Distance H···A (Å)	Distance D···A (Å)	Angle D-H···A (°)	Symmetry Code
C9H9···Cg *	2.856	3.473	123.65	−1/2 + x, 1.5 − y, 1 − z
C11···H5B-C5	2.891	3.662	136.24	−1/2 + x, 1/2 − y, 1 − z
O2···H1-O1	1.909	2.777	165.23	1 − x, −1/2 + y, 1.5 − z

* Cg = C11, C12, C13, C14, C15, C16.

and

Table 3. Intermolecular interactions in compound 4b.

Contact	Distance H···A (Å)	Distance D···A (Å)	Angle D-H···A (°)	Symmetry Code
C9H9···Cg *	2.855	3.475	123.89	−1/2 + x, 1/2 − y, 1 − z
C11···H5B-C5	2.888	3.660	136.26	−1/2 + x, 1.5 − y, 1 − z
O2···H1-O1	1.918	2.778	166.76	1 − x, −1/2 + y, ½ − z

* Cg= C11, C12, C13, C14, C15, C16.

. It should be emphasized that a closer look at the crystal structures presented the same molecular and supramolecular arrangements; for this reason, we only describe 4a. The first interaction was (O2···H1-O1); this was different from the intramolecular hydrogen bonding mentioned above in which both hydroxyl groups acted as acceptor and donor, forming intermolecular interactions that extended in one direction (Figure 3a). The second interaction was from naphthalene–naphthalene interactions (C9-H9···Cg, 2.856 Å), as depicted in Figure 3b. All intermolecular distances and angles were within the expected range reported previously [14,15,16].

2.2. Hirshfeld Surface

The Hirshfeld surfaces were plotted to visualize the van der Waals (VDW) distances and determine the interaction sites [17]. The surface was drawn using Crystal Explorer software and was based on our results from the X-ray studies in CIF format [18]. The plots are shown in Figure 4; the corresponding fingerprints were plotted and are shown in

Table 4. Two-dimensional fingerprint plots of compounds 4a and 4b.

Contacts	4a	4b
All contacts 100%
H···H68.0%
O···H/H···O9.8%
C···H/H···C18.4%

. The conspicuous red regions on the surface (close contacts) are due to O···H/H···O contacts accounting for 9.8% of all contacts in each compound; in fact, the two enantiomers had exactly the same fingerprints (Table 4) in which contacts H···H and C···H/H···C were 68.0 and 18.4%, respectively.

3. Materials and Methods

All chemicals were available commercially, and the solvents were purified using conventional methods before use. Melting points were determined in an Electrothermal Engineering IA9100X1 melting point apparatus and are reported uncorrected. IR absorption spectra were recorded in the range of 4000–230 cm⁻¹ as KBr pellets on a BRUKER Tensor 27 spectrophotometer. The ¹H, ¹³C, and 2D spectra were recorded in CDCl₃ on a 500 MHz Bruker spectrometer using TMS as an internal reference. NMR spectra were processed with MestReC 12.0.0 software.

Electro Ionization Impact mode mass spectra were recorded in a JEOL, SX 102 A by JEOL, an AccuTOF JMS-T100LC using the Direct Analysis in Real Time (DART) mode, and an MStation JMS-700 in fast atom bombardment (FAB) mode. Single crystal X-ray diffraction analyses were performed on a Bruker D8 Venture κ-geometry diffractometer using CuKα radiation with a combination of ω- and φ-scans.

Data were corrected for absorption (semi-empirical from equivalents method) and polarization. Structures were solved using direct methods, and models were refined by the full-matrix least-squares method on F² against all reflections. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters.

Synthesis of 3-(naphthalene-1-ylmethylene)pentane-2,4-dione (2): 1.5 mL of 2,4-pentanodione and 1.6 mL of 1-naphthaldehyde in 20 mL of dichloromethane (CH₂Cl₂) reacted with 0.05 mL of piperidine and 0.05 mL of glacial acetic acid (CH₃COOH) in the presence of molecular sieves, which was removed by filtration once the reaction was completed. The reaction solvent was removed under reduced pressure and the product was extracted with a 1:1 mixture of water–dichloromethane. The organic layer was dried over anhydrous Na₂SO₄, the excess solvent was removed, and the sample was kept in a high vacuum. The product was purified by flash column chromatography using Hex-AcOEt-MeOH at 80:15:5 ratios. Yield: 53%., m.p. 66 °C. ¹H NMR (500 MHz, CDCl₃): δ(ppm) 8.29 (s, 1H), 7.99 (dd, J = 8.10, 0.92 Hz, 1H), 7.91 (m, 2H), 7.61 (ddd, J = 8.43, 6.89, 1.69 Hz, 1H), 7.58 (ddd, J = 8.17, 6.86, 1.49 Hz, 1H), 7.44 (m, 2H), 2.50 (s, 3H), 2.05 (s, 3H). FTIR (KBr, cm⁻¹): 3055.05, 3004.01, 1692.04, 1659.04. EI+: (m/z): 238.

Synthesis of (Z)-4-hydroxy-3-(naphthalen-1-ylmethyl)pent-3-en-2-one (3): 516.3 mg of 1 and 10% mmol Pd/C-10% as catalysts were placed in 40 mL of AcOEt and allowed to react under hydrogen atmosphere at room temperature for 2 hours. The reaction was monitored by TLC and quenched by filtration using a sintered glass funnel with packed celite to retain the catalyst. Then, the filtrated solution was concentrated in a vacuum to give compound 3. Yield: 76%; m.p. 85°C. ¹H NMR (500 MHz, CDCl₃): δ(ppm) enol tautomer 16.95 (s, 1H), 8.13 (ddt, J = 8.43, 1.40, 0.82, 0.82 Hz, 1H), 7.91 (ddt, J = 8.04, 1.41, 0.63, 0.63 Hz, 1H), 7.76 (d, J = 7.74 Hz, 1H), 7.59 (ddd, J = 8.29, 6.76, 1.31 Hz, 1H), 7.54 (m, 1H), 7.41 (m, 1H), 7.18 (ddd, J = 7.12, 2.48, 1.17 Hz, 1H), 4.07 (s, 2H), 2.04 (s, 6H); ketone tautomer 7.96 (m, 0.82H), 7.87 (ddt, J = 8.00, 1.39, 0.62, 0.62 Hz, 0.83H), 7.74 (d, J = 7.60 Hz, 0.89H), 7.56 (m, 1H), 7.51 (ddd, J = 8.08, 6.80, 1.32 Hz, 0.82H), 7.28 (dd, J = 7.01, 1.15 Hz, 0.76H), 4.20 (t, J = 7.29, 7.29 Hz, 0.76H), 3.62 (d, J = 7.29 Hz, 1.57H), 2.10 (d, J = 0.39 Hz, 4.61H). FTIR (KBr, cm⁻¹): 3049.58, 3012.45, 2919.42, 2860.93, 1699.61. EI+: (m/z): 240.

Synthesis of (2S, 4S)- and (2R, 4R)-3-(naphthalene-1-ylmethyl) pentane-2,4-diols (4a, 4b): 533 mg of 3 in 40 mL of methanol (MeOH) were reacted with 224.9 mg of NaBH₄ for 1h at 0 °C. After the reaction was complete, ice was added to the reaction mixture and 3% HCl was added until effervescence ceased, indicating neutralization of free hydride. The product was extracted in AcOEt–H₂O at a 1:1 ratio. The organic phase was dried over anhydrous Na₂SO₄ and concentrated in a vacuum. Yield: 98%. The diastereomer mixture was separated on preparative layer chromatography using Hex-CH₂Cl₂-MeOH at 60:35:5 ratios. The constitution of the diastereomeric mixture was composed following the order of polarity (from least to most polar): 1,3-diol meso ca. 40%, racemate 4a/4b ca. 30%, and the hydroxyketone ca. 30%.

Compounds (4a, 4b): The racemic mixture crystallized from CH₂Cl₂ at room temperature and the independent single crystals of the enantiomers were obtained simultaneously. The X-ray diffraction results showed that a single enantiomer was found, which led to a search for the complementary one. The crystals were morphologically almost identical, with minimal perceptible differences in their crystalline habit. This led to the findings reported in the present work. The physical and spectroscopic data were as follows: m.p. 101 °C; ¹H NMR (500 MHz, CDCl₃): δ(ppm) 8.05 (dq, J = 8.59, 1.06, 0.93, 0.93 Hz, 1H), 7.86 (dd, J = 7.64, 1.82 Hz, 1H), 7.73 (dd, J = 7.45, 2.00 Hz, 1H), 7.51 (ddd, J = 8.45, 6.84, 1.72 Hz, 1H), 7.48 (ddd, J = 8.08, 6.79, 1.52 Hz, 1H), 7.39 (m, 2H), 4.36 (qd, J = 6.50, 6.50, 6.49, 1.83 Hz, 1H), 3.91 (qd, J = 6.46, 6.44, 6.44, 3.86 Hz, 1H), 3.30 (dd, J = 14.19, 9.74 Hz, 1H), 3.23 (dd, J = 14.22, 5.62 Hz, 1H), 1.83 (dddd, J = 9.61, 5.65, 3.85, 1.83 Hz, 1H), 1.46 (d, J = 6.49 Hz, 3H), 1.24 (d, J = 6.46 Hz, 3H). FTIR (film, cm⁻¹): 3326.29, 3047.60, 2969.91, 2930.50, 2899.48. ${[\propto ]}_D^{25}$ = −0.001 (c 1.5 mg/mL CHCl₃). EI+: (m/z): calculated for C₁₆H₂₀O₂ 244.1463; found: 244.1464 [M+].

4. Conclusions

Suitable crystals of compound 4 were successfully crystallized and analyzed by single crystal X-ray diffraction analysis. The crystals of the two compounds were carefully separated by microscope optical inspection affording the two relative configurations, which were found to be the enantiomeric forms. Hirshfeld’s fingerprints showed that two enantiomers had the same fingerprints.