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Article

Chemoenzymatic Stereoselective Synthesis of trans-Flavan-4-ols via Lipase-Catalyzed Kinetic Resolutions

by
Martín Soto
†,
Irene Sanz-Machín
†,
Humberto Rodríguez-Solla
* and
Vicente Gotor-Fernández
*
Organic and Inorganic Chemistry Department, University of Oviedo, Avenida Julián Clavería 8, 33006 Oviedo, Spain
*
Authors to whom correspondence should be addressed.
†
Authors contributed equally to this work.
Catalysts 2021, 11(11), 1296; https://doi.org/10.3390/catal11111296
Submission received: 4 October 2021 / Revised: 25 October 2021 / Accepted: 27 October 2021 / Published: 27 October 2021
(This article belongs to the Special Issue Biocatalytic Resolution of Chiral Molecules)
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Abstract

:
Flavan-4-ols are a subclass of flavonoids that are present in complex molecules with application in the industrial sector as pigments, antioxidants, or antimitotics, among many others. The most traditional way to achieve their synthesis is from naturally abundant flavanones, asymmetric transfer hydrogenation reactions or bioreduction being well known strategies, while their preparation from racemic flavan-4-ols has been less explored. In this article, we have focused on the synthesis of a series of trans-flavan-4-ols bearing different substitution patterns in the aromatic ring to explore later the potential of lipases as biocatalysts for stereoselective acylation reactions. Therefore, a series of flavanones have been chemically prepared, starting from the corresponding benzaldehydes by aldol condensation with 2′-hydroxyacetophenone in a strongly basic medium, and later transformed into the corresponding racemic trans-flavan-4-ols following a carbonyl reduction, Mitsunobu reaction, and ester deprotection sequence. A screening of lipases and optimization of the reaction conditions for the stereoselective acylation of racemic 2-phenylchroman-4-ol were performed before expanding the best reaction conditions to the kinetic resolution of other 2-arylchroman-4-ols. Interestingly, the combination of AK lipase from Pseudomonas fluorescens as enzyme and vinyl acetate as both acyl donor and solvent allowed the performance of highly asymmetric transformations (E > 200, 50–99% eeS and >99% eeP) under mild reaction conditions (30 °C and 250 rpm).

Graphical Abstract

1. Introduction

Flavan-4-ols, also known as 2-arylchroman-4-ols or 3-deoxyflavoids, are privileged structures in flavonoid chemistry [1], since their structural cores are present in many biologically active compounds with interesting properties as anticarcinogenic, antidiabetic, antioxidant, antimitotic, and biocidal agents [2,3]. Their chemical structure, together with the ones from flavone and flavanone, is represented in Figure 1. Remarkably, the relative and absolute configurations of the C-2 and C-4 centers possess a key role in building their biological profiles [4,5,6,7,8,9]; for that reason, the development of highly (stereo)selective methods towards the production of optically active flavan-4-ols is nowadays considered an attractive synthetic field, organometallic chemistry and enzymes being useful catalysts for the development of asymmetric processes.
Traditionally, the most straightforward approaches to produce racemic flavan-4-ols consist of reductions in flavanones and flavones, selectively leading to the corresponding cis-isomers as major products. In this context, the use of sodium borohydride [10,11] or catalytic hydrogenation employing palladium on charcoal [12,13] have afforded a series of racemates in high yields. The preferential formation of cis-flavan-4-ols can be easily explained considering that the hydride or catalyst would approach the flavanone from the less hindered face, opposite the aromatic group at C-2, favoring the addition of the hydride or hydrogen to the carbonyl from that face. As mentioned before, the reduction in flavones to the corresponding racemic cis-flavan-4-ols is also possible but requires a stronger reducing agent such as NiCl2/NaBH4, also occurring with total control of the selectivity [14]. Alternatively, the synthesis of the trans-flavan-4-ols is also feasible but requires the formation of cis-isomers and subsequent inversion of the C-4 center configuration. Janeczko and co-workers described the synthesis of racemic trans-flavan-4-ol from its cis-isomer through a bimolecular nucleophilic substitution by alcohol activation with tosyl chloride in pyridine and subsequent hydrolysis under basic conditions in aqueous medium [15].
Similarly, the stereoselective synthesis of cis-flavan-4-ol enantiomers has received much more attention than the one of its trans-analogues, metal [16,17,18] and enzyme catalysis [15,19,20,21,22,23,24] facilitating the preparation of chiral flavan-4-ols with excellent stereoselectivity levels. Among the enzymatic methods, the desymmetrization of flavanones through bioreduction processes and the kinetic resolution of flavanols and their corresponding acetates using hydrolases are the most demanding strategies. The practical application of redox processes is usually hampered by the formation of complex product mixtures, low isolated yields, and working with very low substrate concentrations [13,17,18,19,22], while the use of lipases as biocatalysts have provided better results for practical applications [19,20,21]. For instance, Kasahara and co-workers reported the hydrolysis of 2-phenylchroman-4-yl acetate using the lipase from Pseudomonas cepacia (PSL) that allows the recovery of (2R,4R)-alcohol and (2S,4S)-acetate with high enantiomeric excess values (93% and 95% ee, respectively) [19]. Tanaka and co-workers found that the lipase AY is a highly selective catalyst for enantiocomplementary hydrolysis and acetylation procedures of racemic cis-flavan-4-ol and its acetate [20]. Finally, the lipase from Candida cylindracea also resulted to be an efficient biocatalyst, in this case, when applied to the stereoselective acetylation of a series of cis-flavan-4-ol, obtaining the corresponding (2R,4R)-4-acetoxyflavans and the remaining (2S,4S)-flavan-4-ols with moderate to excellent optical purities, depending on the substitution patterns at the different positions of both aromatic rings [21].
Interestingly, there is only one report about the stereoselective enzymatic synthesis of trans-flavan-4-ols, which was covered by Janeczko and co-workers [15], describing Rhodococcus rubra KCh 82 as the most suitable reductive enzyme to produce (2R,4S)-trans-alcohol (41% yield, >99% ee), (S)-flavanone (50% yield, 59% ee), and (2S,4S)-cis-alcohol (9% yield, 40% ee), while for the oxidation of the racemic trans-alcohol, Yarrowia lypolytica catalyzed a highly selective oxidation, forming (R)-flavanone (52% yield, 85% ee) and (2S,4R)-trans-alcohol (48% yield, 93% ee) after 6 days at 25 °C. Unfortunately, there are no reports about the action of hydrolases towards the asymmetric production of trans-flavan-4-ols, so based on the biological importance of the flavanols family, herein, we aim to develop a general methodology for the selective preparation of a series of racemic trans-flavan-4-ols, later studying their kinetic resolutions through lipase-catalyzed acetylation reactions (Scheme 1).

2. Results and Discussion

The synthesis of racemic trans-flavan-4-ol (trans-2a) will be described and reaction conditions optimized before extending this methodology to a representative number of trans-flavan-4-ols bearing different substitution patterns at the aromatic ring present in the C-2 position. Continuing with the selection of trans-2a as model substrate, its kinetic resolution will be carefully analyzed using different hydrolases, trying to find adequate conditions for the preparation of optically active products with good yields and high chemical and optical purities.

2.1. Chemical Synthesis of trans-Flavan-4-ols

Flavanone 1a was selected as a commercial starting material for the synthesis of trans-flavan-4-ol (trans-2a) following the strategy depicted in Scheme 2, which involves the reduction in the carbonyl group of ketone 1a to afford the corresponding cis-2a, followed by Mitsunobu inversion of the stereogenic center located at the C-4 position, and final chemical hydrolysis of the protected alcohol.
Firstly, the chemical reduction was performed at a very low temperature to achieve a total selectivity towards the cis-isomer formation, so flavanone (1a) was reacted with LiAlH4 in anhydrous THF at −78 °C. Remarkably, the alcohol cis-2a was obtained after 12 h with complete control of the selectivity and in 95% yield after column chromatography purification. The relative cis-configuration was confirmed by comparison of its NMR data with the ones already described in the literature [10,11,12,13,14,15,16,17,18,19,20]. The next step consists of selective inversion of the relative configuration of the hydroxyl group at the C-4 position in order to obtain the trans-2a isomer. Without any doubt, the Mitsunobu reaction is, nowadays, one of the most straightforward pathways to invert the configuration of a determined stereocenter [25,26,27,28,29], and it is based on the conversion of the starting alcohol into a better leaving group, usually an ester, following a SN2 mechanism. To achieve this, triphenylphosphine (PPh3), a carboxylic acid as nucleophile and an alkyl azodicarboxylate are employed, generally diethyl azodicarboxylate (DEAD) or diisopropyl azodicarboxylate (DIAD).
The acidic strength is closely related to the efficiency of the Mitsunobu reaction [30], so four acids with variable pKa values (acetic acid, p-nitrobenzoic acid, chloroacetic acid and p-toluensulfonic acid) were employed in the presence of DEAD or DIAD as disclosed in Table 1. Although there was no influence about the use of one or another azodicarboxylate (even vs. odd entries), the use of the acidic source resulted to be a key factor, since no reaction was observed with acetic acid (entries 1 and 2), while p-nitrobenzoic acid, chloroacetic acid and p-toluensulfonic acid led to high conversions into the corresponding protected trans-alcohol 3. The best results were found with the strongest acids, chloroacetic acid (entries 5 and 6) and p-toluensulfonic acid (PTSA, entries 7 and 8), next treating the reaction crudes with sodium carbonate in tetrahydrofuran and the presence of water and methanol. Thus, the trans-2a was obtained after liquid–liquid extraction and column chromatography with high isolated yields (86–88%) and similar selectivities (76-84% dr). The trans:cis ratios were unequivocally calculated by 1H-NMR analyses of the reaction crudes by comparison of the two signals corresponding to the CHs at C-2 and C-4 positions, observing their chemical shifts at 5.10 and 5.18 ppm for the starting cis-2a, while the signals shift to 4.88 and 5.33 for the trans-2a.
Trying to obtain the alcohol trans-2a with total selectivity, another approach was attempted by reacting racemic cis-2a with trifluoroacetic anhydride in the presence of pyridine in order to generate the corresponding triflate, and further hydrolysis led to trans-2a. However, no improvement was achieved in terms of selectivity (79% yield, 90/10 trans/cis ratio). The impossibility to increase the trans:cis ratio over 9:1 may be explained as a consequence of the occurrence of an epimerization process, which has been observed in esters derived from flavan-4-ols in acidic media [31].
Since only the flavanone 1a was commercially available, the synthesis of flavanones 1b–j bearing different substituents, such as electron-withdrawing and electron-donor groups, in the aromatic ring at the C-2 position was chemically carried out (Scheme 3), which will allow us to have a deeper understanding on the reactivity of lipases with the corresponding trans-flavan-4-ol substrates. The strategy consisted of the aldol condensation between 2′-hydroxyacetophenone (4) and the corresponding benzaldehydes 5b–j in strongly basic media such as in the presence of potassium hydroxide for 3 h at 60 °C, but also similar results were found for 24 h at room temperature with sodium hydroxide, followed by an acid-promoted intramolecular Michael-type addition of the resulting 2′-hydroxychalcone 6b–j intermediates using refluxing glacial acetic acid as both catalyst and solvent for 72 h [32,33]. Results are depicted in Table 2, the flavanones 1b–j being isolated after column chromatography in moderate yields (36–47%). Interestingly, no significant differences were observed when considering electron-withdrawing (entries 2–6) or electron-donor groups (entries 7–10) at different positions of the C-2 aromatic ring.
Next, flavanones 1b–j were reduced to the racemic alcohols cis-2b–j using LiAlH4 in THF at −78 °C, which were obtained in very high yields (90–96%) and with total control of the selectivity independently of the nature of their substituents and their substitution pattern (>98/2 dr). Finally, cis-flavan-4-ols 2b–j were reacted under the best reaction conditions previously found for substrate 2a (Method A: PPh3, DEAD and PTSA; Method B: PPh3, DIAD and chloroacetic acid) and after hydrolysis with aqueous Na2CO3 in THF/MeOH, the corresponding trans-flavan-4-ols 2b–j were obtained in good to high yields and good to high selectivities in all the cases. It must be mentioned that Mitsunobu reactions led to complex mixtures; therefore, the isolated yields were sometimes highly dependent on the efficiency of the purification process performed by column chromatography technique on silica gel.

2.2. Lipase-Catalyzed Kinetic Resolution of trans-Flavan-4-ols 2a–j

Once the preparation of trans-flavan-4-ols 2a–j was achieved, the potential of hydrolytic enzymes including a series of lipases and an acylase was explored, in order to produce optically active alcohols and esters via a classical kinetic resolution through the stereoselective acetylation of the racemic alcohols. Based on the previous successful studies dealing with the resolution of cis-alcohols using lipases [19,20,21], the alcohol trans-2a was selected as a model substrate to find a selective hydrolase, which preferentially would acetylate one of its enantiomers using standard conditions for this type of process such as 3 equivalents of vinyl acetate (VinOAc) as activated ester, THF as solvent, 30 °C and a 100 mM substrate concentration, using orbital shaking (250 rpm) to preserve the stability of the selected immobilized biocatalyst.
The set of hydrolases was composed by Candida antarctica lipase type A (CAL-A), Candida antarctica lipase type B (CAL-B), Pseudomonas cepacia lipase (PSL), Rhizomucor miehei lipase (RML), lipase AK from Pseudomonas fluorescens, and acylase from Aspergillus melleus (Acylase). In all cases a substrate:enzyme ratio 1:1 (weight:weight) was initially used and the reactions were performed for 24 h, searching for active and stereoselective enzymes (Table 3). In terms of selectivity, CAL-B, RML, and Acylase displayed poor results (entries 2, 4, and 6), leading to a broad conversion range (15–48%), while CAL-A, PSL, and lipase AK led to moderate to excellent enantioselectivities (entries 1, 3, and 5), highlighting the perfect stereodiscrimination displayed by AK lipase (E > 200). Therefore, the use of this lipase was selected for further reaction intensification.
After having determined that Amano lipase AK from Pseudomonas fluorescens could be a suitable lipase for performing the kinetic resolution of trans-2a, the influence of other parameters that affect the enzymatic catalysis was investigated. Therefore, different temperatures (30 and 45 °C), enzyme:substrate ratios (1:1 and 2:1 w/w), and loading of VinOAc (3–109 equiv.) were studied in order to discover optimal conditions for kinetic resolution purposes, which means the achievement of around 50% conversion and the recovery of the resulting acetate and the remaining alcohol with high optical purities. Reactions were carried out taking aliquots every 24 h for 4 days, which were analyzed by chiral HPLC (see conditions in the Experimental section). To compare the biotransformations, the data obtained after 72 h have been summarized in Table 4, since no significant improvements were achieved from this point, and in some cases, the perfect selectivity attained after 3 days decreased in some extent after prolonged reaction times. Starting from the initial conditions achieved in the enzyme screening and displayed in Table 3 (entry 5), the temperature was increased to 45 °C, observing a significant improvement in the conversion value (27%, entry 1), although doubling the enzyme loading (entry 2) with respect to the reaction described in Table 3 led to a more significant improvement, reaching 41% conversion after 72 h and, again, excellent selectivity.
Continuing at 30 °C but doubling the activated ester ratio from three to six equivalents (entry 3), the conversion reached 44%, which was later much more improved, selecting VinOAc as both acyl donor and solvent (109 equivalents for a 100 mM substrate concentration, entry 4). In this case, a perfect 50% conversion into the enantiopure acetate (2S,4R)-3a and the remaining alcohol (2R,4S)-2a was possible after 48 h (entry 4). Final hydrolysis of the enantiopure acetate (2S,4R)-3a using an aqueous solution of Na2CO3 in a mixture of MeOH/THF at room temperature for 2 h led to the corresponding alcohol (2S,4R)-2a in 91% isolated yield without loss of the optical purity, affording, in this manner, both alcohol enantiomers through a chemoenzymatic strategy.
After comparison of the optical rotation values of the products obtained by lipase-catalyzed kinetic resolution (see experimental section), the absolute configuration of the so-obtained enantiopure trans-2a was assigned as (2R,4S) [34,35], while for the acetate trans-2a, (2S,4R)-stereochemistry was proposed. This assignment is also in agreement with the one proposed when applying Kazlauskas’ rule for the resolution of secondary alcohols [36]. Finally, under optimized conditions, i.e., 100 mM trans-3a in VinOAc, 30 °C and 250 rpm, the resolutions of racemic alcohols trans-2b–j were developed, finding, in all cases, excellent selectivities but different activity values depending on the position of the substitution of the phenyl ring at the C-2 position (Table 5). In all cases, those reactions with substrates bearing para-substitutions were completely selective (entries 4, 5, 6, 9, and 10), noticing slower reactions when considering flavanols with substituents at the ortho-position (entries 2 and 7). Satisfyingly, the biotransformation of model substrate trans-2a was scaled up to 250 mg of substrate, achieving very high isolated yields in both enantiopure alcohol and acetate after column chromatography purification.

3. Materials and Methods

Chemical reagents were purchased from Sigma-Aldrich (Steinheim, Germany), VWR International (Barcelona, Spain), and Thermo Fisher Scientific (Waltham, MA, USA) and used as received, except tetrahydrofuran, which was dried under an inert atmosphere using the sodium-benzophenone system. Regarding the enzymes employed in this contribution, Pseudomonas cepacia (PSL, 23,000 U/g), lyophilized lipase AK from Pseudomonas fluorescens (AK, 23,700 U/g), and Acylase from Aspergillus melleus (>0.5 U/g) were also purchased from Sigma-Aldrich (Steinheim, Germany), Candida antarctica type A lipase (CAL-A) was obtained from ChiralVision (3000 U/g, Den Hoorn, Netherlands), while Candida antartica type B lipase (CAL-B, Novozym-435, 7300 PLU/g) and Rhizomucor miehei lipase (RML, 150 IUN/g) were kindly donated by Novozymes company (Bagsværd, Denmark).
NMR spectra were recorded on a Bruker AV300 MHz spectrometer (Bruker Co., Faellanden, Switzerland) including 1H, 13C, and 19F NMR monodimensional experiments. All chemical shifts (δ) are given in parts per million (ppm) and referenced to the residual solvent signal as internal standard. High performance liquid chromatography (HPLC) analyses were performed on an HP 1100 chromatograph (Agilent Technologies, Inc., Wilmington, DE, USA) equipped with a VIS–UV detector using different chiral columns (25 cm × 4.6 mm, 5 μm particle size, Chiral Technologies, Mainz, Germany) for the measurement of the corresponding alcohol and ester enantiomeric excess values. HPLC injections were made using a 0.8 mL/min flow, mixtures of hexane and 2-propanol as eluent, 30 °C column temperature, and 210 and 214 nm as wavelengths (see chromatograms in the Supplementary Material section). Measurement of the op-tical rotation values was carried out at 590 nm on an Autopol IV Automatic polarime-ter (Rudolph Research Analytical, Hackettstown, NJ, USA).
Melting points were measured in a Gallenkamp apparatus, introducing the samples in open capillary tubes and the measurements are uncorrected. IR spectra were recorded on a Jasco FT/IR-4700 spectrophotometer (Jasco-Spain, Madrid, Spain), and νmax values are given in cm−1 for the main absorption bands. High resolution mass spectra (HRMS) experiments were carried out by electrospray ionization in positive mode (ESI+) using a VG AutoSpecQ high-resolution mass spectrometer (Fision Instrument, Mildford, MA, USA). Thin-layer chromatography (TLC) was conducted with Merck Silica Gel 60 F254 precoated plates (Merck KGaA, Darmstadt, Germany) and visualized with a UV lamp, plus either potassium permanganate or vanillin stains. Column chromatographies were performed using silica gel 60 (230–240 mesh) (Merck KGaA, Darmstadt, Germany).

3.1. Synthesis of Flavanones 1a–j

An aqueous solution of KOH (4.9 g, 122.4 mmol in 12 mL of water) was carefully added to a solution of 2′-hydroxyacetophenone (4, 2.0 g, 14.7 mmol) in ethanol (30 mL), a yellow precipitate usually being formed. The mixture was stirred for 5–10 min. After this time, the corresponding benzaldehyde 5a–j (14.7 mmol) dissolved in ethanol (7 mL) was added, observing that the color of the solution turned from yellow to intense red. The reaction was stirred for 3 h at 60 °C, and at this point, the reaction was quenched by pouring it onto ice. The reaction mixture was acidified to pH = 2 with an aqueous concentrated HCl solution, and the desired 2′-hydroxychalcone was extracted with EtOAc (3 × 20 mL). The organic layers were combined, dried over Na2SO4, filtered, and concentrated under reduced pressure. A solution of the recovered crude 2′-hydroxychalcone 6a–j was dissolved in glacial acetic acid (7 mL for each mmol of crude chalcone) was refluxed for 72 h. After this time, the reaction was quenched by pouring it on water (10 mL for each mmol of crude chalcone), observing a brown precipitate of the corresponding flavanone 1a–j, which was extracted with dichloromethane (3 × 30 mL). The organic phases were combined, washed with brine (3 × 30 mL), dried over Na2SO4, and concentrated under reduced pressure. Since some acetic acid remained in the round-bottom flask, toluene (3 × 10 mL) was added to co-evaporate the remaining AcOH. The final product was purified by column chromatography on silica gel using hexane:EtOAc 5:1 as eluent (36–47% isolated yield, Table 2).
2-Phenylchroman-4-one (1a). White solid. Mp: 77–78 °C. Rf (hexane:EtOAc 5:1): 0.45. 1H NMR δ (300 MHz, CDCl3): 7.94 (dd, J = 8.1, 1.8 Hz, 1H), 7.58–7.33 (m, 6H), 7.12–7.01 (m, 2H), 5.49 (dd, J = 13.2, 3.0 Hz, 1H), 3.10 (dd, J = 16.9, 13.2 Hz, 1H), 2.90 (dd, J = 16.9, 3.0 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 192.09, 161.69, 138.87, 136.33, 128.98 (2C), 128.91, 127.18, 126.28 (2C), 121.75, 121.07, 118.26, 79.73, 44.80 ppm. Data are in agreement with those from the commercial source.
2-(2-Fluorophenyl)chroman-4-one (1b). Yellow liquid. Rf (hexane:EtOAc 5:1): 0.49. 1H NMR δ (300 MHz, CDCl3): 7.78 (dd, J = 7.5, 1.7 Hz, 1H), 7.69–7.63 (m, 1H), 7.53 (ddd, J = 10.0, 4.9, 1.9 Hz, 1H), 7.42–7.37 (m, 1H), 7.30–7.18 (m, 3H), 5.93 (dd, J = 13.1, 3.2 Hz, 1H), 3.21 (dd, J = 16.9, 13.1 Hz, 1H), 3.07 (dd, J = 16.9, 3.2 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 191.68, 161.63, 159.73 (d, J = 247.7 Hz), 136.31, 130.37 (d, J = 8.6 Hz), 127.58 (d, J = 3.5 Hz), 127.26, 126.30 (d, J = 12.8 Hz), 124.73 (d, J = 3.5 Hz), 121.92, 121.05, 118.20, 115.87 (d, J = 21.3 Hz), 73.98 (d, J = 3.1 Hz), 43.83 ppm. 19F NMR δ (282 MHz, CDCl3): –118.33 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C15H12FO2: 243.0821; found 243.0813.
2-(3-Fluorophenyl)chroman-4-one (1c). Colourless oil. Rf (hexane:EtOAc 5:1): 0.45. 1H NMR δ (300 MHz, CDCl3): 7.94 (dd, J = 8.1, 1.8 Hz, 1H), 7.53 (ddd, J = 8.4, 7.2, 1.8 Hz, 1H), 7.47–7.34 (m, 1H), 7.30–7.19 (m, 2H), 7.15–7.02 (m, 3H), 5.49 (dd, J = 12.9, 3.3 Hz, 1H), 3.05 (dd, J = 16.9, 12.9 Hz, 1H), 2.91 (dd, J = 16.9, 3.3 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 191.55, 163.12 (d, J = 246.9 Hz), 161.37, 141.42 (d, J = 7.3 Hz), 136.45, 130.61 (d, J = 8.2 Hz), 127.22, 121.98, 121.73 (d, J = 3.0 Hz), 121.04, 118.23, 115.75 (d, J = 21.2 Hz), 113.34 (d, J = 22.7 Hz), 78.88, 44.77 ppm. 19F NMR δ (282 MHz, CDCl3): –111.80 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C15H12FO2: 243.0821; found, 243.0817.
2-(4-Fluorophenyl)chroman-4-one (1d). Yellow solid. Mp: 97–99 °C. Rf (hexane:EtOAc 5:1): 0.41. 1H NMR δ (300 MHz, CDCl3): 7.93 (dd, J = 7.7, 1.4 Hz, 1H), 7.59–7.41 (m, 3H), 7.19–6.99 (m, 4H), 5.47 (dd, J = 13.2, 3.0 Hz, 1H), 3.06 (dd, J = 16.9, 13.1 Hz, 1H), 2.87 (dd, J = 16.8, 3.0 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 191.80, 162.94 (d, J = 247.6 Hz), 161.50, 136.39, 134.73 (d, J = 3.2 Hz), 128.16 (d, J = 8.3 Hz, 2C), 127.20, 121.88, 121.01, 118.20, 115.92 (d, J = 21.7 Hz, 2C), 79.04, 44.78 ppm. 19F NMR δ (282 MHz, CDCl3): –112.79 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C15H12FO2: 243.0821; found, 243.0816.
2-(4-Chlorophenyl)chroman-4-one (1e). Pale yellow solid. Mp: 87–89 °C. Rf (hexane:EtOAc 5:1): 0.46. 1H NMR δ (300 MHz, CDCl3): 7.93 (dd, J = 7.7, 1.3 Hz, 1H), 7.52 (ddd, J = 9.0, 7.6, 1.8 Hz, 1H), 7.48–7.34 (m, 4H), 7.12–7.01 (m, 2H), 5.47 (dd, J = 13.0, 3.1 Hz, 1H), 3.04 (dd, J = 16.9, 13.0 Hz, 1H), 2.88 (dd, J = 16.9, 3.2 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 191.64, 161.41, 137.38, 136.42, 134.70, 129.17 (2C), 127.63 (2C), 127.21, 121.94, 121.01, 118.21, 78.94, 44.72 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C15H12ClO2: 259.0526; found, 259.0511.
2-(4-Bromophenyl)chroman-4-one (1f). Pale yellow solid. Mp: 114–117 °C. Rf (hexane:EtOAc 5:1): 0.53. 1H NMR δ (300 MHz, CDCl3): 7.93 (dd, J = 7.7, 1.3 Hz, 1H), 7.65–7.46 (m, 3H), 7.37 (d, J = 8.3 Hz, 2H), 7.13–7.01 (m, 2H), 5.45 (dd, J = 12.9, 3.1 Hz, 1H), 3.04 (dd, J = 16.9, 13.0 Hz, 1H), 2.88 (dd, J = 16.9, 3.2 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 191.59, 161.40, 137.92, 136.43, 132.14 (2C), 127.93 (2C), 127.22, 122.84, 121.96, 121.02, 118.22, 78.98, 44.70 ppm. ESI-TOF-HRMS: [M + Na]+ calcd. for C15H11BrO2Na: 324.9835; found, 324.9833.
2-(2-Methoxyphenyl)chroman-4-one (1g). Viscous yellow solid. Rf (hexane:EtOAc 5:1): 0.39. 1H NMR δ (300 MHz, CDCl3): 7.95 (dd, J = 7.7, 1.8 Hz, 1H), 7.64 (dd, J = 7.7, 1.7 Hz, 1H), 7.50 (ddd, J = 8.6, 5.5, 1.8 Hz, 1H), 7.46–7.29 (m, 1H), 7.12–7.04 (m, 3H), 6.92 (dd, J = 8.4, 1.1 Hz, 1H), 5.86 (dd, J = 12.0, 4.2 Hz, 1H), 3.83 (s, 3H), 3.05–2.83 (m, 2H) ppm. 13C NMR δ (75 MHz, CDCl3): 192.75, 162.10, 155.83, 136.05, 129.47, 127.55, 127.12, 126.47, 121.46, 121.09, 120.95, 118.20, 110.57, 74.73, 55.41, 43.78 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C16H15O3: 255.1021; found, 255.1016.
2-(3-Methoxyphenyl)chroman-4-one (1h). Yellow solid. Mp: 80–83 °C. Rf (hexane:EtOAc 5:1): 0.41. 1H NMR δ (300 MHz, CDCl3): 7.93 (dd, J = 8.1, 1.8 Hz, 1H), 7.42 (d, J = 8.6 Hz, 2H), 7.10–7.01 (m, 4H), 6.92 (ddd, J = 8.3, 2.5, 1.0 Hz, 1H), 5.45 (dd, J = 13.2, 3.0 Hz, 1H), 3.84 (s, 3H), 3.08 (dd, J = 16.9, 13.2 Hz, 1H), 2.89 (dd, J = 16.9, 3.1 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 192.01, 161.59, 160.03, 140.40, 136.30, 130.05, 127.14, 121.74, 121.02, 118.41, 118.24, 114.17, 111.97, 79.57, 55.41, 44.81 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C16H15O3: 255.1021; found, 255.1010.
2-(4-Methoxyphenyl)chroman-4-one (1i). Yellow solid. Mp: 92–95 °C. Rf (hexane:EtOAc 5:1): 0.30. 1H NMR δ (300 MHz, CDCl3): 7.93 (dd, J = 8.3, 1.8 Hz, 1H), 7.54–7.45 (m, 1H), 7.43 (s, 1H), 7.40 (s, 1H), 7.11–7.00 (m, 2H), 7.00–6.88 (m, 2H), 5.43 (dd, J = 13.3, 2.9 Hz, 1H), 3.83 (s, 3H), 3.11 (dd, J = 16.9, 13.3 Hz, 1H), 2.86 (dd, J = 16.9, 2.9 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 192.36, 161.77, 160.10, 136.28, 130.89, 127.86 (2C), 127.16, 121.65, 121.04, 118.26, 114.33 (2C), 79.48, 55.49, 44.59 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C16H15O3: 255.1021; found, 255.1010.
2-(4-Methylphenyl)chroman-4-one (1j). Yellow solid. Mp: 80–82 °C. Rf (hexane:EtOAc 5:1): 0.36. 1H NMR δ (300 MHz, CDCl3): 7.94 (dd, J = 8.1, 1.8 Hz, 1H), 7.50 (ddd, J = 8.8, 7.3, 1.8 Hz, 1H), 7.38 (d, J = 8.1 Hz, 2H), 7.25 (d, J = 8.4 Hz, 2H), 7.09–6.99 (m, 2H), 5.44 (dd, J = 13.3, 2.9 Hz, 1H), 3.09 (dd, J = 16.9, 13.3 Hz, 1H), 2.87 (dd, J = 16.9, 3.0 Hz, 1H), 2.39 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 192.19, 161.69, 138.75, 136.21, 135.83, 129.56 (2C), 127.09, 126.27 (2C), 121.59, 120.99, 118.21, 79.58, 44.61, 21.28 ppm. ESI-TOF-HRMS: [M + H]+ calcd. for C16H15O2: 237.0916; found, 237.0903.

3.2. Synthesis of Racemic cis-Flavanols 2a–j

A solution of the corresponding flavanone 1a-j (4.5 mmol) in dry THF (15 mL) was cooled to −78 °C under a nitrogen atmosphere. Next, a 1.0 M solution of LiAlH4 in THF was added dropwise (1.25 mL, 1.25 mmol), and the mixture was stirred at –78 °C. Monitorization of the reaction was undertaken by TLC analyses (hexane:EtOAc 3:1), observing the complete consumption of the starting material after 12 h. After this time, the reaction was quenched by pouring it onto icy water and, when warmed, the product was extracted with dichloromethane (3 × 20 mL). The organic phases were combined, washed with brine (3 × 20 mL), dried over Na2SO4, and concentrated under reduced pressure. The reaction crudes were purified by column chromatography on silica gel, using a gradient 5:1 to 3:1 hexane:EtOAc as eluent, obtaining the corresponding cis-flavan-4-ols 2a–j with total control of the selectivity (90–96% isolated yield, Table 2).
(2S*,4S*)-2-Phenylchroman-4-ol (cis-2a). White solid. Rf (hexane:EtOAc 3:1): 0.26. Mp: 98–100 °C. 1H NMR δ (300 MHz, CDCl3): 7.53 (dt, J = 7.7, 1.4 Hz, 1H), 7.50–7.30 (m, 5H), 7.29–7.16 (m, 1H), 7.00 (td, J = 7.5, 1.3 Hz, 1H), 6.91 (dd, J = 8.2, 1.2 Hz, 1H), 5.18 (dd, J = 11.7, 2.0 Hz, 1H), 5.10 (dd, J = 10.5, 6.2 Hz, 1H), 2.52 (ddd, J = 13.1, 6.3, 2.0 Hz, 1H), 2.14 (ddd, J = 13.2, 11.6, 10.4 Hz, 1H), 1.83 (s, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 154.61, 140.62, 129.32, 128.81 (2C), 128.37, 127.11, 126.22 (2C), 125.86, 121.12, 116.88, 76.98, 65.95, 40.18 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H13O: 209.0966; found 209.0959.
(2S*,4S*)-2-(2-Fluorophenyl)chroman-4-ol (cis-2b). Pale brown solid. Mp: 100–102 °C. Rf (hexane:EtOAc 3:1): 0.30. 1H NMR δ (300 MHz, CDCl3): 7.66–7.47 (m, 2H), 7.41–6.77 (m, 6H), 5.59–5.46 (m, 1H), 5.13 (dd, J = 10.5, 6.2 Hz, 1H), 2.56 (ddd, J = 13.1, 6.2, 1.8 Hz, 1H), 2.12 (dt, J = 12.9, 11.1 Hz, 1H), 1.83 (s, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 159.71 (d, J = 246.7 Hz), 154.43, 129.67 (d, J = 8.3 Hz), 129.31, 128.00 (d, J = 12.8 Hz), 127.45 (d, J = 3.7 Hz), 127.15, 125.82, 124.60 (d, J = 3.3 Hz), 121.26, 116.82, 115.59 (d, J = 21.4 Hz), 70.92 (d, J = 3.2 Hz), 65.71, 39.03 ppm. 19F NMR δ (282 MHz, CDCl3): –119.38 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H12FO: 227.0872; found 227.0860.
(2S*,4S*)-2-(3-Fluorophenyl)chroman-4-ol (cis-2c). White solid. Mp: 107–113 °C. Rf (hexane:EtOAc 3:1): 0.31. 1H NMR δ (300 MHz, CDCl3): 7.56 (d, J = 7.7 Hz, 1H), 7.42 (td, J = 8.0, 5.8 Hz, 1H), 7.34–7.19 (m, 3H), 7.15–7.00 (m, 2H), 6.96 (dd, J = 8.2, 1.0 Hz, 1H), 5.22 (dd, J = 11.6, 1.7 Hz, 1H), 5.14 (dd, J = 10.5, 6.2 Hz, 1H), 2.56 (ddd, J = 13.1, 6.2, 2.0 Hz, 1H), 2.13 (ddd, J = 13.1, 11.6, 10.5 Hz, 1H), 1.93 (s, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 163.10 (d, J = 246.2 Hz), 154.26, 143.23 (d, J = 7.3 Hz), 130.34 (d, J = 8.2 Hz), 129.41, 127.10, 125.74, 121.69 (d, J = 2.8 Hz), 121.32, 116.85, 115.16 (d, J = 21.2 Hz), 113.21 (d, J = 22.4 Hz), 76.19, 65.76, 40.18 ppm. 19F NMR δ (282 MHz, CDCl3): –112.41 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H12FO: 227.0872; found 227.0868.
(2S*,4S*)-2-(4-Fluorophenyl)chroman-4-ol (cis-2d). White solid. Mp: 129–131 °C. Rf (hexane:EtOAc 3:1): 0.27. 1H NMR δ (300 MHz, CDCl3): 7.57 (d, J = 7.7 Hz, 1H), 7.52–7.39 (m, 2H), 7.33–7.21 (m, 1H), 7.20–7.09 (m, 2H), 7.04 (td, J = 7.5, 1.1 Hz, 1H), 6.93 (dd, J = 8.2, 1.0 Hz, 1H), 5.20 (dd, J = 11.7, 1.6 Hz, 1H), 5.12 (d, J = 6.5 Hz, 1H), 2.54 (ddd, J = 13.1, 6.3, 1.9 Hz, 1H), 2.15 (ddd, J = 13.1, 11.7, 10.6 Hz, 1H), 1.92 (d, J = 7.2 Hz, 1H) ppm, 1.66 (s, OH). 13C NMR δ (75 MHz, CDCl3): 162.68 (d, J = 246.6 Hz), 154.46, 136.45, 129.38, 128.00 (d, J = 8.2 Hz, 2C), 127.08, 125.77, 121.26, 116.84, 115.69 (d, J = 21.5 Hz, 2C), 76.36, 65.90, 40.26 ppm. 19F NMR δ (282 MHz, CDCl3): –113.86 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H12FO: 227.0872; found 227.0862.
(2S*,4S*)-2-(4-Chlorophenyl)chroman-4-ol (cis-4e). White solid. Mp: 158–160 °C. Rf (hexane:EtOAc 3:1): 0.26. 1H NMR δ (300 MHz, CDCl3): 7.39 (dt, J = 7.8, 1.4 Hz, 1H), 7.26 (s, 3H), 7.18–6.99 (m, 2H), 6.88 (td, J = 7.5, 1.3 Hz, 1H), 6.77 (dd, J = 8.2, 1.2 Hz, 1H), 5.08–4.90 (m, 2H), 2.36 (ddd, J = 13.1, 6.3, 2.0 Hz, 1H), 1.96 (ddd, J = 13.1, 11.7, 10.5 Hz, 1H), 1.79 (d, J = 8.5 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 154.36, 139.17, 134.07, 129.40, 128.96 (2C), 127.59 (2C), 127.10, 125.74, 121.31, 116.84, 76.26, 65.80, 40.17 ppm. ESI-TOF-HRMS: [M + Na]+ calcd. for C15H13ClO2Na: 283.0496; found 283.0496.
(2S*,4S*)-2-(4-Bromophenyl)chroman-4-ol (cis-2f). White solid. Mp: 160–162 °C. Rf (hexane:EtOAc 3:1): 0.34. 1H NMR δ (300 MHz, CDCl3): 7.50–7.38 (m, 3H), 7.24 (d, J = 6.4 Hz, 1H), 7.19–7.09 (m, 1H), 6.92 (td, J = 7.5, 1.3 Hz, 1H), 6.81 (dd, J = 8.2, 1.2 Hz, 1H), 5.11–4.95 (m, 2H), 2.41 (ddd, J = 13.1, 6.3, 2.0 Hz, 1H), 2.00 (ddd, J = 13.2, 11.6, 10.5 Hz, 1H), 1.74 (d, J = 8.3 Hz, 1H) ppm.13C NMR δ (75 MHz, CDCl3): 154.33, 139.70, 131.92 (2C), 129.42, 127.90 (2C), 127.09, 125.73, 122.18, 121.32, 116.85, 76.29, 65.80, 40.17 ppm. ESI-TOF-HRMS: [M + Na]+ calcd. for C15H13BrO2Na: 326.9991; found 326.9988.
(2S*,4S*)-2-(2-Methoxyphenyl)chroman-4-ol (cis-2g). White solid. Mp: 132–137 °C. Rf (hexane:EtOAc 3:1): 0.24. 1H NMR δ (300 MHz, CDCl3): 7.66–7.59 (m, 1H), 7.58–7.44 (m, 1H), 7.43–7.30 (m, 1H), 7.29–7.20 (m, 1H), 7.12–6.86 (m, 4H), 5.63 (dd, J = 11.3, 1.9 Hz, 1H), 5.15 (t, J = 6.0 Hz, 1H), 3.90 (d, J = 4.8 Hz, 3H), 2.63 (ddd, J = 13.0, 6.2, 2.0 Hz, 1H), 2.09 (s, OH), 2.08–2.00 (m, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 155.96, 154.91, 129.21, 129.11, 128.96, 127.15, 126.43, 126.11, 120.96, 120.84, 116.81, 110.55, 71.52, 66.00, 55.50, 38.76 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O2: 239.1072; found 239.1069.
(2S*,4S*)-2-(3-Methoxyphenyl)chroman-4-ol (cis-2h). Pale brown solid. Mp: 146–150 °C. Rf (hexane:EtOAc 3:1): 0.24. 1H NMR δ (300 MHz, CDCl3): 7.54 (d, J = 7.7 Hz, 1H), 7.40–7.19 (m, 2H), 7.08–6.97 (m, 3H), 6.96–6.85 (m, 2H), 5.18 (dd, J = 11.5, 1.7 Hz, 1H), 5.15–5.06 (m, 1H), 3.85 (s, 3H), 2.55 (ddd, J = 13.1, 6.2, 2.0 Hz, 1H), 2.16 (ddd, J = 13.1, 11.5, 10.5 Hz, 1H), 1.80 (d, J = 8.6 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 160.00, 154.47, 142.24, 129.88, 129.35, 127.12, 125.85, 121.15, 118.49, 116.90, 113.78, 111.86, 76.85, 65.95, 55.43, 40.22 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O2: 239.1072; found 239.1066.
(2S*,4S*)-2-(4-Methoxyphenyl)chroman-4-ol (cis-2i). White solid. Mp: 99–100 °C. Rf (hexane:EtOAc 3:1): 0.27. 1H NMR δ (300 MHz, CDCl3): 7.52 (dt, J = 7.7, 1.4 Hz, 1H), 7.43–7.32 (m, 2H), 7.29–7.11 (m, 1H), 7.04–6.82 (m, 4H), 5.18–5.02 (m, 2H), 3.83 (s, 3H), 2.49 (ddd, J = 13.1, 6.3, 1.9 Hz, 1H), 2.15 (ddd, J = 13.1, 11.7, 10.5 Hz, 1H), 1.89 (d, J = 8.0 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 159.69, 154.71, 132.69, 129.28, 127.68 (2C), 127.09, 125.85, 121.04, 116.86, 114.18 (2C), 76.69, 66.04, 55.47, 39.98 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O2: 239.1072; found 239.1065.
(2S*,4S*)-2-(4-Methylphenyl)chroman-4-ol (cis-2j). White solid. Mp: 106–108 °C. Rf (hexane:EtOAc 3:1): 0.42. 1H NMR δ (300 MHz, CDCl3): 7.45 (d, J = 7.7 Hz, 1H), 7.29 (s, 1H), 7.22–7.08 (m, 4H), 6.92 (td, J = 7.5, 1.1 Hz, 1H), 6.82 (dd, J = 8.2, 0.9 Hz, 1H), 5.05–4.96 (m, 1H), 5.02 (d, J = 6.6 Hz, 1H), 2.43 (ddd, J = 13.1, 6.3, 1.9 Hz, 1H), 2.31 (s, 3H), 2.08 (ddd, J = 13.1, 11.6, 10.6 Hz, 1H), 1.77 (d, J = 8.6 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 154.69, 138.18, 137.61, 129.47 (2C), 129.28, 127.10, 126.23 (2C), 125.86, 121.03, 116.89, 76.87, 66.01, 40.08, 21.33 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O: 223.1123; found 223.1114.

3.3. Synthesis of Racemic trans-Flavanols 2a–j

The corresponding cis-flavan-4-ol 2a–j (200–500 mg, 0.88–2.05 mmol) was dissolved in dry THF (15 mL per mmol of cis-flavan-4-ol) under nitrogen atmosphere, successively adding triphenylphosphine (two equivalents) and PTSA (two equivalents, Method A) or chloroacetic acid (two equivalents, 2.0 mmol, Method B). Next, DEAD (two equivalents, Method A) or DIAD (two equivalents, 2.0 mmol, Method B) was introduced in the Schlenk tube, and the mixture was stirred at room temperature. Monitorization of the reaction was undertaken by TLC analyses (hexane:EtOAc 3:1), observing the complete consumption of the starting material after 1 h. After this time, the reaction was stopped by removal of the solvent under reduced pressure. The residue was then hydrolyzed by treatment with a mixture of an aqueous saturated Na2CO3 solution (10 mL per mmol of starting cis-flavan-4-ol), methanol (10 mL per mmol of starting cis-flavan-4-ol), and THF (15 mL per mmol of starting cis-flavan-4-ol) for 1 h at room temperature. The desired trans-flavan-4-ol trans-2a–j was extracted from the mixture with dichloromethane (3 × 30 mL). The organic phases were combined, washed with brine (3 × 30 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The reaction crudes were purified by column chromatography on silica gel using an eluent gradient of hexane:EtOAc 5:1 to 3:1 (74–88% isolated yield, Table 2), obtaining the racemic trans-2a–j as major diastereoisomer.
(2S*,4R*)-2-Phenylchroman-4-ol (trans-2a). White solid. Mp: 116–118 °C. Rf (hexane:EtOAc 3:1): 0.34. 1H NMR δ (300 MHz, CDCl3): 7.56–7.30 (m, 7H), 7.05–6.98 (m, 2H), 5.33 (dd, J = 11.9, 2.2 Hz, 1H), 4.88 (t, J = 3.2 Hz, 1H), 2.32 (dt, J = 14.4, 2.4 Hz, 1H), 2.23–2.13 (m, 2H) ppm. 13C NMR δ (75 MHz, CDCl3): 154.99, 141.05, 130.13, 128.79, 128.71 (2C), 128.17, 126.38 (2C), 123.57, 120.95, 117.61, 73.18, 63.96, 38.36 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H13O: 209.0966; found 209.0958.
(2S*,4R*)-2-(2-Fluorophenyl)chroman-4-ol (trans-2b). White solid. Mp: 86–88 °C. Rf Rf (hexane:EtOAc 3:1): 0.30. 1H NMR (300 MHz, CDCl3): δ 7.66–7.60 (m, 1H), 7.38–7.32 (m, 2H), 7.29–7.22 (m, 2H), 7.17–7.10 (m, 1H), 7.03–6.98 (m, 2H), 5.64 (dd, J = 11.8 Hz, J = 2.1 Hz, 1H), 4.82 (t, J = 2.9 Hz, 1H), 2.73 (brs, 1H), 2.32 (dt, J = 14.3 Hz, J = 2.3 Hz, 1H), 2.22–2.10 (m, 1H). 13C NMR (75 MHz, CDCl3): δ 159.9 (d, J = 274.4 Hz), 154.71, 130.11 (d, J = 20.4Hz), 129.51 (d, J = 8.2 Hz), 128.19 (d, J = 12.6 Hz), 127.84 (d, J = 3.9 Hz), 124.42, 124.38, 123.44, 120.97, 117.38, 115.56 (d, J = 21.4 Hz), 67.47 (d, J = 3.0 Hz, 63.6, 37.1), 37.05. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H12FO: 227.0872; found 227.0866.
(2S*,4R*)-2-(3-Fluorophenyl)chroman-4-ol (trans-2c). Pale yellow solid. Mp: 111–113 °C. Rf (hexane:EtOAc 3:1): 0.32. 1H NMR δ (300 MHz, CDCl3): δ 7.48–7.17 (m, 5H), 7.12–6.91 (m, 3H), 5.39 (dd, J = 12.1 Hz, J = 2.2 Hz, 1H), 4.85 (t, J = 2.8 Hz, 1H), 2.42 (bs, 1H), 2.29 (dt, J = 14.2 Hz, J = 2.3 Hz, 1H), 2.11 (dd, J = 12.0 Hz, J = 3.0 Hz, 1H). 13C NMR (75 MHz, CDCl3): δ 162.99 (d, J = 245.7 Hz), 154.55, 143.61 (d, J = 7.4 Hz), 130.13, 130.07, 123.38 (d, J = 3.0 Hz), 121.04, 117.47, 114.84 (d, J = 21.1 Hz), 72.36, 63.69, 38.24. 19F NMR δ (282 MHz, CDCl3): –112.66 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H12FO: 227.0872; found 227.0868.
(2S*,4R*)-2-(4- Fluorophenyl)chroman-4-ol (trans-2d). White solid. Mp: 105–107 °C. Rf (hexane:EtOAc 3:1): 0.30. 1H NMR δ (300 MHz, CDCl3): 7.51–7.38 (m, 2H), 7.34 (dd, J = 7.5, 1.7 Hz, 1H), 7.30–7.23 (m, 1H), 7.14–7.05 (m, 2H), 7.02–6.92 (m, 2H), 5.27 (dd, J = 12.0, 2.3 Hz, 1H), 4.85 (c, J = 3.0 Hz, 1H), 2.26 (dt, J = 14.4, 2.4 Hz, 1H), 2.13–2.01 (m, 2H) ppm. 19F NMR δ (282 MHz, CDCl3): –114.27 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H12FO: 227.0872; found 227.0863.
(2S*,4R*)-2-(4-Chlorophenyl)chroman-4-ol (trans-2e). Pale yellow solid. Mp: 106–108 °C. Rf (hexane:EtOAc 3:1): 0.29. 1H NMR δ (300 MHz, CDCl3): 7.50–7.40 (m, 4H), 7.39–7.28 (m, 2H), 7.10–6.89 (m, 2H), 5.30 (dd, J = 12.0, 2.1 Hz, 1H), 4.87 (t, J = 2.8 Hz, 1H), 2.29 (dt, J = 14.3, 2.4 Hz, 1H), 2.17–2.02 (m, 2H) ppm. 13C NMR δ (75 MHz, CDCl3): 154.74, 139.62, 133.85, 130.20, 130.11, 128.86 (2C), 127.73 (2C), 123.48, 121.11, 117.55, 72.48, 63.80, 38.33 ppm. ESI-TOF-HRMS: [M + Na]+ calcd. for C15H13ClO2Na: 283.0496; found 283.0495.
(2S*,4R*)-2-(4-Bromophenyl)chroman-4-ol (trans-2f). White solid. Mp: 117–119 °C. Rf (hexane:EtOAc 3:1): 0.30. 1H NMR δ (300 MHz, CDCl3): 7.51–7.42 (m, 2H), 7.33–7.02 (m, 4H), 6.95–6.85 (m, 2H), 5.16 (dd, J = 12.0, 2.2 Hz, 1H), 4.75 (dt, J = 4.1, 2.3 Hz, 1H), 2.16 (dt, J = 14.3, 2.4 Hz, 1H), 2.11–2.01 (m, 1H), 2.03–1.86 (m, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 154.72, 140.15, 131.82 (2C), 130.23, 130.10, 128.06 (2C), 123.48, 121.98, 121.14, 117.56, 72.52, 63.80, 38.30 ppm. ESI-TOF-HRMS: [M + Na]+ calcd. for C15H13BrO2Na: 326.9991; found 326.991.
(2S*,4R*)-2-(2-Methoxyphenyl)chroman-4-ol (trans-2g). White solid. Mp: 96–100 °C. Rf (hexane:EtOAc 3:1): 0.16. 1H NMR δ (300 MHz, CDCl3): 7.72–7.43 (m, 2H), 7.36–7.16 (m, 2H), 7.10–6.84 (m, 4H), 5.58 (d, J = 10.9 Hz, 1H), 5.10 (dd, J = 9.9, 6.3 Hz, 1H), 3.86 (s, 3H), 2.64–2.52 (m, 1H), 2.04–1.91 (m, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 156.23, 155.30, 130.19, 129.95, 129.48, 128.91, 126.90, 123.89, 120.90, 120.75, 117.58, 110.63, 67.93, 64.24, 55.60, 37.05 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O2: 239.1072; found 239.1069.
(2S*,4R*)-2-(3-Methoxyphenyl)chroman-4-ol (trans-2h). White solid. Mp: 105–109 °C. Rf (hexane:EtOAc 3:1): 0.30. 1H NMR δ (300 MHz, CDCl3): 7.47–7.20 (m, 3H), 7.16–6.98 (m, 4H), 6.94 (dd, J = 7.2, 2.2 Hz, 1H), 5.38–5.26 (m, 1H), 4.89 (t, J = 3.0 Hz, 1H), 3.89 (s, 3H), 2.56 (dd, J = 10.9, 6.1 Hz, 1H), 2.33 (dt, J = 14.3, 2.1 Hz, 1H) ppm. 13C NMR δ (75 MHz, CDCl3): 159.87, 154.88, 130.15, 130.02, 129.72, 129.16, 127.10, 123.61, 120.88, 118.61, 117.53, 111.89, 70.15, 63.82, 55.36, 38.42 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O2: 239.1072; found 239.1061.
(2S*,4R*)-2-(4-Methoxyphenyl)chroman-4-ol (trans-2i). White solid. Mp: 128–130 °C. Rf (hexane:EtOAc 3:1): 0.21. 1H NMR δ (300 MHz, CDCl3): 7.49–7.41 (m, 2H), 7.36 (dd, J = 7.9, 1.7 Hz, 1H), 7.35–7.26 (m, 1H), 7.07–6.94 (m, 4H), 5.26 (dd, J = 11.7, 2.6 Hz, 1H), 4.91–4.82 (m, 1H), 3.86 (s, 3H), 2.27 (dt, J = 14.3, 2.6 Hz, 1H), 2.22–2.14 (m, 2H) ppm. 13C NMR δ (75 MHz, CDCl3): 159.58, 155.12, 133.12, 130.15, 130.08, 127.82 (2C), 123.57, 120.86, 117.60, 114.12 (2C), 72.86, 64.07, 55.45, 38.09 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O2: 239.1072; found 239.1062.
(2S*,4R*)-2-(4-Methylphenyl)chroman-4-ol (trans-2j). White solid. Mp: 103–106 °C. Rf (hexane:EtOAc 3:1): 0.27. 1H NMR δ (300 MHz, CDCl3): 7.50–7.40 (m, 3H), 7.32 (t, J = 6.8 Hz, 3H), 7.08–7.02 (m, 2H), 5.33 (dd, J = 11.8, 2.3 Hz, 1H), 4.92 (t, J = 3.2 Hz, 1H), 2.46 (s, 3H), 2.34 (dt, J = 14.4, 2.5 Hz, 1H), 2.26–2.14 (m, 2H) ppm. 13C NMR δ (75 MHz, CDCl3): 155.10, 138.05, 137.95, 130.10, 129.39 (2C), 126.40 (2C), 126.23, 123.59, 120.87, 117.62, 73.07, 64.03, 38.23, 21.31 ppm. ESI-TOF-HRMS: [M-OH]+ calcd. for C15H15O: 223.1123; found 223.1114.

3.4. Synthesis of trans-Flavanol Acetate 3a

Acetic anhydride (80 µL, 0.72 mmol), triethylamine (100 µL, 0.71 mmol), and catalytic DMAP (10 mg, 0.08 mmol) were added to a solution of racemic trans-flavan-4-ol 2a (50 mg, 0.24 mmol) in dichloromethane (6 mL). After stirring for 2 h at room temperature, the reaction mixture was quenched by pouring it on ice-cooled water (10 mL) and extracted with dichloromethane (3 × 10 mL). The combined organic layers were washed with a saturated aqueous NaHCO3 solution (20 mL) and brine (20 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The resulting reaction crude was purified via column chromatography on silica gel using hexane:EtOAc 5:1 as eluent, obtaining racemic trans-flavan-4-ol acetate 3a (51.6 mg, 87%). Chemical acetylations of racemic trans-flavan-4-ols 2b–j were performed in a similar manner using fractions of the Mitsunobu–deprotection column chromatographies and used only for the development of chiral HPLC methods of the so-obtained trans-flavanol acetates 3b–j.
(2S*,4R*)-2-Phenylchroman-4-ol acetate (trans-3a). White solid. Mp: 84–87 °C. Rf (hexane:EtOAc 5:1): 0.50. 1H NMR δ (300 MHz, CDCl3): 7.55–7.35 (m, 7H), 7.04–6.93 (m, 2H), 6.07 (t, J = 2.9 Hz, 1H), 5.27 (dd, J = 10.5, 3.9 Hz, 1H), 2.34–2.24 (m, 2H), 2.17 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.48, 155.60, 140.48, 131.11, 130.56, 128.75 (2C), 128.38, 126.39 (2C), 120.92, 119.77, 117.47, 73.66, 65.89, 35.98, 21.61 ppm. ESI-TOF-HRMS: [M-AcO]+ calcd. for C15H13O: 209.0966; found 209.0956.
(2S*,4R*)-2-(2-Fluorophenyl)chroman-4-ol acetate (trans-3b). White solid. Rf (hexane:EtOAc 5:1): 0.61. 1H NMR δ (300 MHz, CDCl3): 7.51–7.35 (m, 3H), 7.35–7.21 (m, 1H), 7.17–7.03 (m, 2H), 7.03–6.88 (m, 2H), 6.05 (t, J = 2.9 Hz, 1H), 5.23 (dd, J = 10.6, 3.8 Hz, 1H), 2.35–2.21 (m, 2H), 2.13 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.50, 162.73 (d, J = 246.5 Hz), 155.47, 136.33 (d, J = 3.0 Hz), 131.15, 130.64, 128.20 (d, J = 8.2 Hz. 2C), 121.08, 119.71, 117.43, 115.66 (d, J = 21.5 Hz, 2C), 73.06, 65.80, 36.03, 21.60 ppm. 19F NMR δ (282 MHz, CDCl3): –113.81 ppm.
(2S*,4R*)-2-(3-Fluorophenyl)chroman-4-ol acetate (trans-3c). White solid. Rf (hexane:EtOAc 5:1): 0.54. 1H NMR δ (300 MHz, CDCl3) 7.35 (dt, J = 8.0, 2.3 Hz, 2H), 7.34–7.21 (m, 1H), 7.19 (td, J = 9.0, 2.1 Hz, 3H), 7.00–6.90 (m, 2H), 6.01 (t, J = 2.9 Hz, 1H), 5.21 (dd, J = 11.7, 2.5 Hz, 1H), 2.34–2.15 (m, 2H), 2.10 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.49, 163.12 (d, J = 246.1 Hz), 155.29, 143.16 (d, J = 7.2 Hz), 131.13, 130.67, 130.33 (d, J = 8.2 Hz), 121.86 (d, J = 2.8 Hz), 121.16, 119.72, 117.44, 115.20 (d, J = 21.2 Hz), 113.41 (d, J = 22.3 Hz), 72.97, 65.67, 36.10, 21.59 ppm. 19F NMR δ (282 MHz, CDCl3): –112.47 ppm.
(2S*,4R*)-2-(4-Fluorophenyl)chroman-4-ol acetate (trans-3d). White solid. Rf (hexane:EtOAc 5:1): 0.61. 1H NMR δ (300 MHz, CDCl3): 7.51–7.35 (m, 3H), 7.35–7.21 (m, 1H), 7.17–7.03 (m, 2H), 7.03–6.88 (m, 2H), 6.05 (t, J = 2.9 Hz, 1H), 5.23 (dd, J = 10.6, 3.8 Hz, 1H), 2.35–2.21 (m, 2H), 2.13 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.50, 162.73 (d, J = 246.5 Hz), 155.47, 136.33 (d, J = 3.0 Hz), 131.15, 130.64, 128.20 (d, J = 8.2 Hz. 2C), 121.08, 119.71, 117.43, 115.66 (d, J = 21.5 Hz, 2C), 73.06, 65.80, 36.03, 21.60 ppm. 19F NMR δ (282 MHz, CDCl3): –113.81 ppm.
(2S*,4R*)-2-(4-Chlorophenyl)chroman-4-ol acetate (trans-3e). White solid. Rf (hexane:EtOAc 5:1): 0.51. 1H NMR δ (300 MHz, CDCl3): 7.41 (dd, J = 8.9, 1.8 Hz, 5H), 7.34–7.21 (m, 1H), 7.04–6.90 (m, 2H), 6.05 (t, J = 2.9 Hz, 1H), 5.23 (dt, J = 11.4, 3.3 Hz, 1H), 2.36–2.20 (m, 2H), 2.19 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.49, 155.38, 139.09, 134.15, 131.15, 130.67, 128.95 (2C), 127.77 (2C), 121.14, 119.72, 117.44, 73.00, 65.72, 36.06, 21.62 ppm.
(2S*,4R*)-2-(4-Bromophenyl)chroman-4-ol acetate (trans-3f). White solid. Rf (hexane:EtOAc 5:1): 0.51. 1H NMR δ (300 MHz, CDCl3): 7.49–7.37 (m, 2H), 7.32–7.08 (m, 4H), 6.93–6.77 (m, 2H), 5.93 (t, J = 3.0 Hz, 1H), 5.10 (dd, J = 11.4, 3.0 Hz, 1H), 2.23–2.12 (m, 1H), 2.14–2.07 (m, 1H), 2.02 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.49, 155.35, 139.63, 131.91 (2C), 131.15, 130.67, 128.08 (2C), 122.27, 121.15, 119.71, 117.44, 73.03, 65.69, 36.04, 21.62 ppm.
(2S*,4R*)-2-(3-Methoxyphenyl)chroman-4-ol acetate (trans-3g). White solid. Rf (hexane:EtOAc 5:1): 0.52. 1H NMR δ (300 MHz, CDCl3): 7.63–7.52 (m, 1H), 7.45–7.17 (m, 3H), 7.14–6.88 (m, 4H), 6.02 (t, J = 2.6 Hz, 1H), 5.72–5.63 (m, 1H), 3.87 (s, 3H), 2.43 (dt, J = 14.7, 2.0 Hz, 1H), 2.14 (d, J = 13.3 Hz, 4H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.62, 156.19, 156.08, 131.07, 130.45, 129.04, 126.66, 120.98, 120.72, 119.99, 117.55, 110.64, 68.58, 66.53, 55.63, 34.56, 21.60 ppm.
(2S*,4R*)-2-(3-Methoxyphenyl)chroman-4-ol acetate (trans-3h). White solid. Rf (hexane:EtOAc 5:1): 0.43. 1H NMR δ (300 MHz, CDCl3): 7.03 (s, 4H), 7.01–6.85 (m, 4H), 6.03 (s, 1H), 5.26–5.17 (m, 1H), 2.26 (d, J = 9.6 Hz, 1H), 2.17 (s, 1H), 2.12 (s, 3H) ppm.
(2S*,4R*)-2-(4-Methoxyphenyl)chroman-4-ol acetate (trans-3i). White solid. Rf (hexane:EtOAc 5:1): 0.60. 1H NMR δ (300 MHz, CDCl3): 7.49–7.38 (m, 3H), 7.36–7.25 (m, 1H), 7.04–6.92 (m, 4H), 6.08 (t, J = 3.0 Hz, 1H), 5.31–5.15 (m, 1H), 3.87 (s, 3H), 2.32 (dd, J = 3.0, 1.8 Hz, 1H), 2.30 (d, J = 3.0 Hz, 1H), 2.17 (d, J = 5.1 Hz, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.54, 159.78, 155.77, 132.56, 131.12, 130.54, 127.87, 120.87 (2C), 119.76, 117.49, 114.18 (2C), 72.44, 66.07, 55.48, 35.73, 21.78 ppm.
(2S*,4R*)-2-(4-Methylphenyl)chroman-4-ol acetate (trans-3j). White solid. Rf (hexane:EtOAc 5:1): 0.55. 1H NMR δ (300 MHz, CDCl3): 7.45–7.17 (m, 6H), 7.01–6.89 (m, 2H), 6.05 (t, J = 2.8 Hz, 1H), 5.22 (dd, J = 8.4, 5.9 Hz, 1H), 2.38 (d, J = 3.6 Hz, 3H), 2.32–2.22 (m, 2H), 2.13 (s, 3H) ppm. 13C NMR δ (75 MHz, CDCl3): 170.54, 155.73, 138.22, 137.51, 131.11, 130.55, 129.44 (2C), 126.42 (2C), 120.86, 119.78, 117.50, 73.56, 66.00, 35.86, 21.63, 21.31 ppm.

3.5. Screening of Biocatalysts for the Kinetic Resolution of trans-Flavanol 2a through an Acetylation Reaction

Vinyl acetate (VinOAc, 24 µL, 0.26 mmol) was added to a mixture of the corresponding trans-flavan-4-ol trans-2a–j (20.0 mg, 0.0867 mmol), hydrolase (20 mg, 1:1 w/w enzyme:2a), and THF (884 µL) under a nitrogen atmosphere. The mixture was stirred under orbital shaking at 250 rpm at 30 °C for 24 h. After this time, the enzyme was filtered and washed with dichloromethane (3 × 2 mL). The filtrate was concentrated under reduced pressure, and the reaction was crude analyzed by a 1H NMR experiment to corroborate the conversion obtained by chiral HPLC analyses (Table 3). After column chromatography purification using an eluent gradient 5:1 to 3:1 hexane:EtOAc, the pure products were injected on the HPLC to measure the enantiomeric excess of both trans-flavanol 2a and the trans-flavanol acetate 3a. For experiments detailed in Table 4, a similar procedure was developed, selecting lipase AK from Pseudomonas fluorescens as the best enzyme and modifying the temperature, acyl donor excess, and loading of catalyst. See HPLC conditions and retention times in Table 6.

3.6. Lipase-Catalyzed Kinetic Resolution of trans-Flavanols 2a–j through an Acetylation Reaction

To a mixture of the corresponding trans-flavan-4-ol trans-2a–j (20.0–30.0 mg, 0.0867–0.121 mmol) and lipase AK from Pseudomonas fluorescens (1:1 w/w enzyme:2a–j), VinOAc (1 mL per 0.1 mmol of 2a–j, 109 equivalents) was added. The mixture was stirred under orbital shaking at 250 rpm at 30 °C for three days, taking aliquots after 6, 24, 48, and 72 h. After this time, the enzyme was filtered and washed with dichloromethane (3 × 1 mL). The filtrate was concentrated under reduced pressure, and the reaction crude was analyzed by 1H NMR experiment to corroborate the conversion obtained by chiral HPLC analyses (Table 5). After column chromatography purification using an eluent gradient 5:1 to 3:1 hexane:EtOAc, the pure products were injected on the HPLC to measure the enantiomeric excess of both trans-flavanols 2a–j and trans-flavanol acetates 3a–j. See HPLC conditions and retention times in Table 6.

3.7. Semi-Preparative Lipase-Catalyzed Kinetic Resolution of trans-Flavanols 2a

VinOAc (11.05 mL) was added to a mixture of the trans-flavan-4-ol trans-2a (250 mg, 1.10 mmol) and lipase AK from Pseudomonas fluorescens (250 mg). The mixture was stirred for 48 h at 250 rpm at 30 °C, and after this time, the enzyme was filtered and washed with dichloromethane (3 × 10 mL). The filtrate was concentrated under reduced pressure, and the reaction crude purified by column chromatography purification using an eluent gradient 5:1 to 3:1 hexane:EtOAc, isolating acetate (2S,4R)-3a (142 mg, 48% isolated yield) and alcohol (2R,4S)-2a (115 mg, 46% isolated yield), both in enantiopure form.

4. Conclusions

A general methodology has been proposed for the synthesis of racemic trans-flavan-4-ols bearing different substitution patterns in the aromatic ring at the C-2 position. The five-step chemical route involved three main steps or reaction sequences: (i) the aldol condensation between 2′-hydroxyacetophenone and different benzaldehydes bearing either electron-withdrawing or electron-donor atoms or groups, followed by an intramolecular Michael-type addition in an acid medium; (ii) the chemical reduction in the flavanones, which led to the corresponding racemic cis-flavan-4-ols with complete selectivity and very high yields; and finally, (iii) the configuration inversion of the C-4 position by a Mitsunobu inversion reaction and subsequent chemical hydrolysis in basic conditions allows the preferential formation of the alcohol trans-isomers after optimization of the reaction conditions of the Mitsunobu reaction in terms of employed nucleophile and alkyl diazocarboxylate.
With a series of ten racemic trans-flavan-4-ols on hand, their classical kinetic resolutions were performed, selecting trans-flavan-4-ol as a benchmark substrate for enzyme screening and reaction conditions optimization, finding lipase AK from Pseudomonas fluorescens as the more selective biocatalyst, and vinyl acetate acting as both an adequate acyl donor and solvent. The (2R,4S)-alcohols and (2S,4R)-acetates were obtained, in all cases, with excellent selectivities (E > 200), finding different conversion values depending on the type and location of the substitution at the phenyl ring located at the C-2 position, the para-substituted derivatives being transformed rapidly by AK lipase from Pseudomonas fluorescens (49–50% conversion) in comparison with the meta- (47–50%) and, especially, the ortho-substituted ones (33–36%).

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11111296/s1. Table S1. HPLC conditions for the chiral analyses of trans-flavan-4-ols 2a-j and their corresponding acetates 3a-j. Next, the chromatograms of trans-flavan-4-ols 2a-j and trans-flavan-4-ol acetates 3a-j in racemic and optically active form can be found.

Author Contributions

Conceptualization, H.R.-S. and V.G.-F.; investigation (experimental design and development of analytical methods), M.S and I.S.-M.; writing—original draft preparation, V.G.-F.; writing—review and editing, M.S., I.S.-M., H.R.-S. and V.G.-F. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support from the Spanish Ministry of Science, Innovation and Universities (PID2019-109253RB-I00) is gratefully acknowledged.

Data Availability Statement

The data presented in this study are openly available in the Repository of the University of Oviedo (https://digibuo.uniovi.es/dspace/).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 11 01296 g001 550
Figure 1. General structure of flavones, flavanones and flavan-4-ols. Above appear the structures of naturally occurring flavan-4-ols including examples of all four different absolute configurations at the C-2 and C-4 positions.
Figure 1. General structure of flavones, flavanones and flavan-4-ols. Above appear the structures of naturally occurring flavan-4-ols including examples of all four different absolute configurations at the C-2 and C-4 positions.
Catalysts 11 01296 g001
Catalysts 11 01296 sch001 550
Scheme 1. Enzymatic kinetic resolution of trans-flavan-4-ols through stereoselective acetylation reaction for the production of the corresponding optically active esters and alcohols.
Scheme 1. Enzymatic kinetic resolution of trans-flavan-4-ols through stereoselective acetylation reaction for the production of the corresponding optically active esters and alcohols.
Catalysts 11 01296 sch001
Catalysts 11 01296 sch002 550
Scheme 2. Synthetic approach to convert flavanone (1a) into trans-flavan-4-ol (trans-2a) by a reduction–Mitsunobu inversion–deprotection sequence.
Scheme 2. Synthetic approach to convert flavanone (1a) into trans-flavan-4-ol (trans-2a) by a reduction–Mitsunobu inversion–deprotection sequence.
Catalysts 11 01296 sch002
Catalysts 11 01296 sch003 550
Scheme 3. Synthesis of flavanones 1a–j followed by chemical reduction and final Mitsunobu inversion–deprotection sequence to produce racemic flavanols trans-2a-j.
Scheme 3. Synthesis of flavanones 1a–j followed by chemical reduction and final Mitsunobu inversion–deprotection sequence to produce racemic flavanols trans-2a-j.
Catalysts 11 01296 sch003
Table
Table 1. Investigation of the optimal conditions for the Mitsunobu protocol for the C-4 configuration inversion of racemic cis-2a to produce the corresponding trans-2a.
Table 1. Investigation of the optimal conditions for the Mitsunobu protocol for the C-4 configuration inversion of racemic cis-2a to produce the corresponding trans-2a.
Catalysts 11 01296 i001
EntryAzodicarboxylateAcidpKadr (trans/cis) aYield (%) b
1DEADAcOH4.76--
2DIADAcOH3.41--
3DEADp-NO2-C6H4-COOH3.4190/1075
4DIADp-NO2-C6H4-COOH3.4191/974
5DEADCl-CH2COOH2.8791/986
6DIADCl-CH2COOH2.8788/1287
7DEADPTSA−2.192/888
8DIADPTSA−2.191/986
a Diastereomeric ratio was determined by 1H-NMR analysis of the crude Mitsunobu–hydrolysis sequence; b Isolated yield of trans-flavan-4-ol (2a) after flash column chromatography.
Table
Table 2. Non stereoselective chemical synthesis of flavanones 1a–j, cis- and trans-alcohols 2a–j.
Table 2. Non stereoselective chemical synthesis of flavanones 1a–j, cis- and trans-alcohols 2a–j.
EntryR1a–j (%) acis-2a–j (%) a,btrans-2a–j (%) a,b
Method AMethod B
1H (a)369588 (92/8)87 (88/12)
22-F (b)419077 (88/12)88 (87/13)
33-F (c)399276 (89/11)84 (93/7)
44-F (d)429674 (90/10)59 (91/9)
54-Cl (e)479280 (90/10)79 (88/12)
64-Br (f)409579 (95/5)68 (93/7)
72-OMe (g)459378 (91/9)51 (90/10)
83-OMe (h)379576 (89/11)44 (84/16)
94-OMe (i)419581 (88/12)78 (85/15)
104-Me (j)429178 (89/11)86 (90/10)
a Isolated yields after column chromatography purification; b Diastereomeric ratio was determined by 1H-NMR analysis and appear in brackets. Alcohols cis-2b–j were obtained after chemical reduction with >98/2 dr in all cases.
Table
Table 3. Screening of hydrolases for the kinetic resolution of trans-flavan-4-ol (2a, 100 mM) using 3 equivalents of vinyl acetate in dry THF at 30 °C and 250 rpm.
Table 3. Screening of hydrolases for the kinetic resolution of trans-flavan-4-ol (2a, 100 mM) using 3 equivalents of vinyl acetate in dry THF at 30 °C and 250 rpm.
Catalysts 11 01296 i002
EntryHydrolasec (%) aeeP (%) beeS (%) bEc
1CAL-A898989
2CAL-B155093
3PSL22921428
4RML183082
5AK19>9923>200
6Acylase from A. melleus48981
a Conversion values were calculated as c = eeS/(eeS + eeP); b Enantiomeric excess values of product 3a (eeP) and substrate 2a (eeS) were determined by HPLC analysis; c Enantiomeric ratio values were calculated: E = ln [(1 − c) × (1 − eeP)]/ln [(1 − c) × (1 + eeP)].
Table
Table 4. Optimization of the AK lipase-catalyzed kinetic resolution of trans-2a by acetylation using vinyl acetate (VinOAc).
Table 4. Optimization of the AK lipase-catalyzed kinetic resolution of trans-2a by acetylation using vinyl acetate (VinOAc).
Catalysts 11 01296 i003
EntryVinOAc (equiv)T (°C)AK:2a (w:w)c (%) aeeP (%) beeS (%) bEc
13451:127>9937>200
23302:141>9968>200
36301:144>9980>200
4109 d301:150>99>99>200
a Conversion values were calculated as c = eeS/(eeS + eeP); b. Enantiomeric excess values of product 3a (eeP) and substrate 2a (eeS) were determined by HPLC analysis; c. Enantiomeric ratio values were calculated: E = ln [(1 − c) × (1 − eeP)]/ln [(1 − c) × (1 + eeP)]; d This is a 100 mM solution in vinyl acetate as solvent in the absence of THF.
Table
Table 5. Lipase-catalyzed kinetic resolution of trans-2a-j (100 mM) by acetylation using AK lipase (1:1 w/w substrate:enzyme) and vinyl acetate (VinOAc) as both acyl donor and solvent at 30 °C and 250 rpm.
Table 5. Lipase-catalyzed kinetic resolution of trans-2a-j (100 mM) by acetylation using AK lipase (1:1 w/w substrate:enzyme) and vinyl acetate (VinOAc) as both acyl donor and solvent at 30 °C and 250 rpm.
EntryRt (h)c (%) aeeP (%) beeS (%) bEc
1H (a)4850>99 (48)>99 (46)>200
22-F (b)7236>9957>200
33-F (c)4850>9999>200
44-F (d)7249>9997>200
54-Cl (e)4850>99>99>200
64-Br (f)4850>99>99>200
72-OMe (g)7233>9950>200
83-OMe (h)7247>9990>200
94-OMe (i)4850>99>99>200
104-Me (j)4850>9999>200
a Conversion values were calculated as c = eeS/(eeS + eeP); b Enantiomeric excess values were determined by HPLC analysis. Isolated yields appear in parentheses for the semi-preparative reaction carried out with 250 mg of substrate trans-2a; c Enantiomeric ratio values were calculated: E = ln [(1 − c) × (1 − eeP)]/ln [(1 − c) × (1 + eeP)].
Table
Table 6. HPLC conditions and specific optical rotation values of optically active compounds a.
Table 6. HPLC conditions and specific optical rotation values of optically active compounds a.
Entry(±)-CompoundColumnHexane-2-PrOH (v/v)Retention Times (min) b[α]D20 c
1trans-2a (H)IA92:810.3 (2S,4R) and 10.9 (2R,4S)−77.0 (c 0.40, EtOH) d,e
2trans-3a (H)OJ-H92:816.6 (2S,4R) and 28.1 (2R,4S)+6.0 (c 0.37, EtOH)
3trans-2b (2-F)OJ-H92:815.3 (2S,4R) and 19.0 (2R,4S)−88.6 (c 0.59, EtOH)
4trans-3b (2-F)AS92:85.3 (2R,4S) and 5.6 (2S,4R)+3.2 (c 0.87, EtOH)
5trans-2c (3-F)IA92:89.9 (2S,4R) and 10.8 (2R,4S)−89.5 (c 0.61, EtOH)
6trans-2c (3-F)OJ-H92:89.9 (2S,4R) and 12.7 (2R,4S)−2.5 (c 0.41, EtOH)
7trans-2d (4-F)IA92:810.5 (2S,4R) and 12.3 (2R,4S)−130.0 (c 0.32, EtOH)
8trans-3d (4-F)OJ-H92:813.5 (2S,4R) and 16.0 (2R,4S)+8.5 (c 0.35, EtOH)
9trans-2e (4-Cl)IA92:811.1 (2S,4R) and 13.1 (2R,4S)−79.6 (c 0.42, EtOH)
10trans-3e (4-Cl)AS92:85.7 (2R,4S) and 6.2 (2S,4R)+5.5 (c 0.44, EtOH)
11trans-2f (4-Br)IA92:811.6 (2S,4R) and 13.8 (2R,4S)−111.0 (c 0.35, EtOH)
12trans-3f (4-Br)OD98:28.8 (2R,4S) and 9.6 (2S,4R)−1.0 (c 0.36, EtOH)
13trans-2g (2-OMe)OJ-H92:817.7 (2S,4R) and 20.0 (2R,4S)−12.8 (c 0.18, EtOH)
14trans-3g (2-OMe)OJ-H92:813.4 (2R,4S) and 14.6 (2S,4R)−3.0 (c 0.54, EtOH)
15trans-2h (3-OMe)OJ-H92:826.0 (2S,4R) and 28.8 (2R,4S)−71.3 (c 0.47, EtOH)
16trans-3h (3-OMe)OJ-H92:816.6 (2S,4R) and 23.2 (2R,4S)+2.13 (c 0.38, EtOH)
17trans-2i (4-OMe)IA92:815.0 (2S,4R) and 16.6 (2R,4S)−132.0 (c 0.38, EtOH)
18trans-3i (4-OMe)IB92:86.5 (2S,4R) and 6.8 (2R,4S)+18.7 (c 0.52, EtOH)
19trans-2j (4-Me)IA92:810.4 (2S,4R) and 11.4 (2R,4S)−232.0 (c 0.46, EtOH)
20trans-3j (4-Me)OD98:27.1 (2S,4R) and 7.9 (2R,4S)+8.9 (c 0.64, EtOH)
a The temperature column was 30 °C and the flow 0.8 mL/min. Analyses were performed at 210 and 214 nm wavelengths; b Major enantiomer of the enzymatic reactions appear in bold font; c The specific optical rotation value of the major enantiomer in the lipase-catalyzed kinetic resolution is given; d Specific optical rotation values were calculated: [α]D20 = α/(l × c). Concentration values, c, are expressed in g/100mL; e The assignment of the absolute configuration for (2R,4S)-2a–j and (2S,4R)-3a–j has been performed by comparison of the specific rotation signs of compounds (2S,4R)-2a [34,35] and (2S,4R)-3a [35], as already described in the literature.
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Soto, M.; Sanz-Machín, I.; Rodríguez-Solla, H.; Gotor-Fernández, V. Chemoenzymatic Stereoselective Synthesis of trans-Flavan-4-ols via Lipase-Catalyzed Kinetic Resolutions. Catalysts 2021, 11, 1296. https://doi.org/10.3390/catal11111296

AMA Style

Soto M, Sanz-Machín I, Rodríguez-Solla H, Gotor-Fernández V. Chemoenzymatic Stereoselective Synthesis of trans-Flavan-4-ols via Lipase-Catalyzed Kinetic Resolutions. Catalysts. 2021; 11(11):1296. https://doi.org/10.3390/catal11111296

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Soto, Martín, Irene Sanz-Machín, Humberto Rodríguez-Solla, and Vicente Gotor-Fernández. 2021. "Chemoenzymatic Stereoselective Synthesis of trans-Flavan-4-ols via Lipase-Catalyzed Kinetic Resolutions" Catalysts 11, no. 11: 1296. https://doi.org/10.3390/catal11111296

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