Lipase-Catalyzed Kinetic Resolution Followed by an Intramolecular Diels–Alder Reaction: Enantio- and Diastereoselective Synthesis of an A-Ring Moiety of Furanosteroids

Abstract

Furanosteroids are known to exhibit inhibitory activity against phosphatidylinositol-3-kinase and are expected to serve as a basis for the development of therapeutic drugs for various diseases. In this study, a novel protocol is presented for preparation of the furanosteroid A-ring moiety. More specifically, the lipase-catalyzed kinetic resolution of racemic 1-(3-bromofuran-2-yl)-2-chloroethanol with β-substituted (Z)-acrylates and the subsequent intramolecular Diels–Alder reaction of the generated enantiomerically enriched esters were performed to obtain multi-functionalized fused cyclohexenes in excellent enantiomeric ratios (99 : 1 or 98% enantiomeric excess (ee)) and diastereomeric ratios (≥98 : 2). The obtained products possess the appropriate stereochemical structures and absolute configuration for use in the asymmetric synthesis of the A-ring moieties of naturally occurring furanosteroids, including viridin and viridol.

Introduction

The synthetic efficiency of the lipase-catalyzed acylation of racemic secondary alcohols via kinetic resolution (KR) or dynamic kinetic resolution (DKR) has led to its widespread application in the syntheses of enantiomerically pure compounds.^1–4) To further enhance the usefulness of this method in organic synthesis, our group previously reported the use of acylating agents bearing functionalized acyl moieties through both KR and DKR.^5–15) In this method, the acyl groups introduced to the secondary alcohols were subsequently used to construct more complex molecular skeletons, enabling the enantioselective construction of multi-substituted cyclic molecules in fewer steps. As an application of this method, our group has been engaged in asymmetric synthesis of the naturally occurring furanosteroids, viridin (1)¹⁶⁾ and viridol (2)¹⁷⁾ (Chart 1A). These compounds exhibit inhibitory activity against phosphatidylinositol-3-kinase (PI-3K)^18,19) and represent a group of compounds that are expected to serve as a basis for the development of therapeutic drugs for various diseases, including cancer.

Chart 1. (A) Some Naturally Occurring Furanosteroids, Viridin (1) and Viridol (2); (B) Our Recent Synthesis of the ACDE-Ring Moiety (±)-5, a Potent Synthetic Intermediate of 1 and 2; (C) Synthetic Plan of an Optically Active 4 via Lipase-Catalyzed Kinetic Resolution of (±)-3 Followed by Intramolecular Diels–Alder Reaction of (S)-7

Thus, the aim of our project was to initially synthesize a multi-substituted fused cyclohexane 5 as a potent precursor for the asymmetric synthesis of 1 and 2. This synthetic route commenced with a racemic secondary alcohol (±)-3 and passed through intermediate (±)-4a to generate the desired fused cyclohexane (±)-5, whose synthesis we have recently reported (Chart 1B).²⁰⁾ Subsequently, we have planned the synthesis of optically active 4 through the lipase-catalyzed enantioselective acylation (or KR) of (±)-3 using an acyl donor 6 which bears a heteroatom functional group (FG) at the β-position. Thus, this reaction generates the optically active ester (S)-7, and a subsequent intramolecular Diels–Alder (IMDA) reaction produces 4 (Chart 1C). To accomplish this plan, we investigated the effects of various functional groups (FGs) for the lipase-catalyzed enantioselective esterification and also IMDA reactions and report herein the asymmetric synthesis of 4 with excellent enantiomeric purity (98% enantiomeric excess (ee)).

Results and Discussion

In reference to our previous research for the lipase-catalyzed enantioselective esterification (or KR) of (±)-1-(3-bromofuran-2-yl)propan-1-ol using fumarates as acyl donors,¹⁰⁾ the lipase-catalyzed KR of (±)-3 was investigated using (Z)-acrylates 6a–6f, each bearing a heteroatom FG at the β-position, as acylating reagents to obtain optically active 4 (Table 1).

Table 1. Lipase-Catalyzed KR of (±)-3 Using Various Acyl Donors 6a–6e

Entry	6	Lipase	(S)-7, Yield, % eea)	(R)-3, Yield, % eea)	E valueb)
1	6a	CHIRAZYME L-2 C4	(S)-7a 27%, 98% ee	73%, 76% ee	140
2	6a	Lipase CL “Amano” IM	(S)-7a 46%, 97% ee	51%, 80% ee	160
3c)	6b	CHIRAZYME L-2 C4	(S)-7b 29%, 97% ee	70%, 37% ee	94
4c)	6b	Lipase CL “Amano” IM	(S)-7b 10%, 97% ee	79%, 13% ee	75
5	6c	CHIRAZYME L-2 C4	(S)-7c 40%, 98% ee	56%, 71% ee	>200
6	6c	Lipase CL “Amano” IM	(S)-7c 49%, 99% ee	47%, 89% ee	150
7	6d	CHIRAZYME L-2 C4	(S)-7d 40%, 96% ee	58%, 66% ee	98
8	6d	Lipase CL “Amano” IM	(S)-7d 49%, 87% ee	48%, 83% ee	37
9	6e	CHIRAZYME L-2 C4	(S)-7e 41%, 94% ee	57%, 72% ee	70
10	6e	Lipase CL “Amano” IM	(S)-7e 28%, 94% ee	77%, 40% ee	48
11	6f	CHIRAZYME L-2 C4	(S)-7f no reaction	—	—

a) Determined by HPLC using a chiral column (for details, see Supplementary Maerials). b) Determined by the optical purity of (S)-7 and that of (R)-3 using the reported calculation method; E = ln[(1 – conv) × (1 – ee(3)]/ln[(1 – conv) × (1 + ee(3)], conv = ee(3)/[ee(3) + ee(7)].²²⁾ c) The reaction was conducted using lipase (2.0 g/mmol) for 3 d.

Initially, the KR of (±)-3 was examined using 6a, which bears a thiocyanato group, and some commercially available lipases were screened in isooctane at 50 °C for 24 h.²¹⁾ Consequently, it was deduced that Candida antarctica lipase B (CALB) was effective in terms of both its reactivity and enantioselectivity, while lipases, such as Candida antarctica lipase A and Candida rugosa lipase, resulted in poor conversions. The reaction solvent was also investigated to find that low polarity solvents, such as isooctane, heptane, and cyclohexane, are suitable for the KR. The product enantioselectivities (indicated by the E value)²²⁾ obtained using these solvents were not significantly different, although isooctane gave a slightly higher enantioselectivity. In this KR, the use of molecular sieves such as MS4A is essential, because without MS4A, the esterification did not proceed at all. Additionally, upon performing the KR of (±)-3 using two commercial CALBs obtained from different manufacturers (i.e., CHIRAZYME L-2 C4 from Roche and Lipase CL “Amano” IM from Amano Enzyme) afforded ester (S)-7a (97–98% ee) with good conversions (27 and 46%, respectively) and E values of ≥140 (Table 1, entries 1 and 2).

Using another acylating reagent 6b, which contains a methylthio group, the KR proceeded slowly, and the yield of (S)-7b was ≤30% even after 3 d at 50 °C for both CHIRAZYME L-2 C4 and Lipase CL “Amano” IM. However, both lipases yielded (S)-7b in 97% ee (entries 3 and 4). By contrast, using 6c, which contains a trifluoromethylthio group, the two commercial lipases produced ester (S)-7c in high yields (40–49%) and with high E values ( ≥ 150) after 24 h (entries 5 and 6). For other acyl donors, 6d and 6e, bearing chlorine or iodine at the β-position, CHIRAZYME L-2 C4 produced the corresponding esters (S)-7d and (S)-7e in approx. 40% yields after 24 h, and with high E values (98 and 70, respectively; entries 7 and 9), while Lipase CL “Amano” IM gave slightly lower E values (entries 8 and 10). On the other hand, the bulkiness of the β-substituent seems to have a significant impact on the reaction. For example, simply changing 6b, bearing a methylthio group, to 6f, bearing a methylsulfinyl group, completely halted the reaction (entry 11).

These results indicated that acylating reagents 6a–6e bearing different FGs are applicable to the lipase-catalyzed KR protocol, producing the corresponding esters (S)-7a–7e with high enantiomeric purities of 94–99% ee in most cases. In particular, 6a and 6c, each bearing an electron-withdrawing sulfur substituent, were found to accelerate the esterification reaction. However, esters 7a–7e failed to undergo the subsequent IMDA reaction under the KR conditions at 50°C. To further investigate the possibility of IMDA at higher temperature, racemic 7a–7e were heated under reflux in toluene for 24 h; however, the corresponding cyclized products 4a–4e were not detected in any of the reactions.

With the aim of later introduction of an oxygen-containing group at the C3-position of 1 or 2, the IMDA reaction was further investigated for 7 bearing other sulfur FGs. Preliminary studies on the IMDA of (±)-7f (bearing a methylsulfinyl group) showed that the reaction was successful when refluxed in toluene; however, the lipase-catalyzed KR using 6f bearing a methylsulfinyl group did not proceed. Therefore, we concentrated on the transformations of (S)-7a or (S)-7c to obtain optically active 4g and 4c, respectively.

Initially, (S)-7a (98% ee) bearing a thiocyanato group was converted to (S)-7b (98% ee) upon reaction with MeI and Cs₂CO₃ in methanol, without loss of the optical purity, and (S)-7b (98% ee) was oxidized to give a 1 : 1 diastereomeric mixture of sulfoxide 7f. The subsequent IMDA reaction of 7f in acetonitrile proceeded under reflux to yield the cyclized product 4f as a 5:1 diastereomeric mixture. Further oxidation of 4f using meta-chloroperoxybenzoic acid (mCPBA, 1.0 equivalent (equiv.)) gave sulfone 4g (98% ee) as a single compound based on ¹H-NMR analysis. Additionally, the oxidation of (S)-7b (98% ee) using mCPBA (2.5 equiv.) at room temperature directly produced 4g (92% yield, 98% ee) as a single diastereomer (¹H-NMR analysis), while the intermediate sulfone (S)-7g immediately underwent the IMDA reaction. A similar oxidation of (S)-7c bearing a trifluoromethyl group proceeded extremely slowly at 50 °C, whereas the subsequent IMDA reaction was rapid to give sulfoxide 4c as a single diastereomer in an approx. 10% yield (Chart 2).

Chart 2. Conversion of 7a–7c into 4g and 4c

Considering the general stereochemical trends resulting from the use of natural lipases, it was expected that all esters 7a–7e would possess an S chiral center. As an example to confirm this, the stereochemical structure and absolute configuration of 4g were determined by the X-ray diffraction analysis as shown in Fig. 1,²³⁾ confirming the S configuration for the chiral centers of 7a and 7b. The corresponding chiral centers of esters 7c–7e were subsequently inferred from the fact that lipases lead to the same KR enantioselectivity when using similar molecules and acylating reagents. Notably, the absolute configuration of 4g renders it suitable for use in the asymmetric syntheses of 1 and 2 (Chart 1).

Fig. 1. Absolute Stereochemical Structure of 4g Determined by X-Ray Crystallography

Conclusion

In this study, a systematic investigation of the lipase-catalyzed enantioselective esterification (or kinetic resolution, KR) of a racemic secondary alcohol 3 with six acyl donors 6a–6f bearing different heteroatom functional groups at the β-position was performed, and the subsequent IMDA reactions of the obtained optically active esters were performed. In the lipase-catalyzed esterification, acyl donors 6a and 6c, bearing electron-withdrawing thiocyanato and trifluoromethylthio groups, respectively, were found to be suitable in terms of their enantioselectivities. However, the subsequent IMDA reactions of the prepared optically active esters 7a and 7c did not proceed, even under harsh conditions. Instead, IMDA cyclization was achieved following derivatization of the thiocyanato group to either a sulfinyl or sulfonyl group, producing the desired tricyclic compound 4g with excellent enantiomeric (99:1, 98% ee) and diastereomeric (>98:2) ratios. Notably, 4g is expected to serve as an important synthetic intermediate in the asymmetric total syntheses of natural furanosteroids, including viridin (1) and viridol (2). To accomplish one-pot continuous KR and IMDA reactions, the next challenge will be the preparation of lipase mutants with larger reaction sites that can accommodate acyl donors bearing the bulkier sulfinyl or sulfonyl group. Further work is also underway in our laboratory to combine the KR with the in situ racemization of the remaining enantiomer (R)-3 to achieve DKR. By taking advantage of the outstanding enantio-discriminating ability of lipases, the application of this chemoenzymatic transformation strategy is being explored for the asymmetric syntheses of complex molecules from readily available racemic alcohols.

Experimental

Analytical Methods

Melting points were determined on a Yanagimoto melting point apparatus and are uncorrected. IR absorption spectra were recorded on a SHIMADZU FTIR-8400S spectrophotometer. ¹H- and ¹³C-NMR spectra were measured on a JEOL JNM-ECA500 (¹H: 500 MHz, ¹³C: 125 MHz, ¹⁹F: 470 MHz) or a JEOL JNM-ECS400 (¹H: 400 MHz, ¹³C: 100 MHz) instrument with chemical shifts reported in δ (ppm) relative to the residual nondeuterated solvent signal for ¹H (CHCl₃: δ 7.26 ppm) and relative to the deuterated solvent signal for ¹³C (CDCl₃: δ 77.0 ppm). The mass spectra (MS) were measured on a JEOL JMS-S3000 (MALDI) spectrometer with a TOF mass analyzer, a JMS-T100LP (ESI) spectrometer, and a JEOL JMS-700EI (CI) spectrometer. HPLC analysis was carried out using a JASCO LC-2000 Plus system (HPLC pump: PU-2080, UV detector: MD-2018) equipped with Daicel CHIRALPAK IG-3, CHIRALPAK IH-3, CHIRALPAK IJ-3, CHIRALCEL OD-3 columns, each with a size of 4.6 × 250 mm. Optical rotations were measured on a JASCO P-1020 polarimeter.

Reagents

All reagents and solvents were purchased from FUJIFILM Wako Pure Chemical Corporation, Tokyo Chemical Industry, Sigma-Aldrich, Nacalai Tesque, Kishida Chemical, and Combi Blocks and used without further purification. All reactions were monitored by TLC on glass-backed silica gel 60 F₂₅₄, plates (Merck). Flash chromatography was performed on silica gel 60N (particle size 40–50 μm) purchased from Kanto Chemical. Candida antarctica lipase B immobilized on a polyacrylic resin (trade name: Roche CHIRAZYME L-2 C4) was purchased from FUJIFILM Wako Pure Chemical Corporation and Candida antarctica lipase B immobilized on a polyacrylic resin (trade name: Lipase CL “Amano” IM) was gifted from Amano Enzyme. Both were used without further treatment.

Lipase-Catalyzed Kinetic Resolution of (±)-3 with Various Acyl Donors 6a–6f

General procedure

A mixture of (±)-3 (45 mg, 0.20 mmol), 6 (0.40 mmol), immobilized lipase (135 mg), MS4A (135 mg), and isooctane (2.0 mL, 0.10 M) was stirred at 50°C for 24 h. After cooling to RT, the reaction mixture was filtered through a Celite pad, and the Celite pad was washed with EtOAc. The combined organic phase was concentrated under reduced pressure, and the residue was purified by column chromatography (hexane/EtOAc) to afford (S)-7 and (R)-3. The exantiomeric excess of (S)-7 and that of (R)-3 were determined by HPLC using a chiral column.

(S)-1-(3-Bromofuran-2-yl)-2-chloroethyl (Z)-3-thiocyanatoacrylate [(S)-7a] (Table 1, entry 2).

The reaction of (±)-3 (45 mg, 0.20 mmol), 6a (57 mg, 0.40 mmol), and Lipase CL “Amano” IM (135 mg) was conducted according to the general procedure, and a crude product was purified by column chromatography (hexane/EtOAc = 10 : 1 → 4 : 1) to afford (S)-7a (31 mg, 46% yield, 97% ee) and (R)-3 (23 mg, 51% yield, 80% ee).

(S)-7a:

A colorless oil. ${\left[\alpha \right]}_{\text{D}}^{24}$ +167 (c 1.0, CHCl₃); ¹H-NMR (500 MHz, CDCl₃) δ: 7.41 (1H, d, J = 2.0 Hz), 7.23 (1H, d, J = 9.5 Hz), 6.47 (1H, d, J = 1.5 Hz), 6.31 (1H, d, J = 9.5 Hz), 6.15 (1H, dd, J = 8.0, 6.0 Hz), 3.97 (1H, dd, J = 11.5, 8.0 Hz), 3.86 (1H, dd, J = 11.5, 6.0 Hz); ¹³C-NMR (125 MHz, CDCl₃) δ: 164.7, 145.4, 144.1, 140.3, 119.0, 114.7, 111.9, 101.8, 67.7, 42.4; IR (neat) ν: 2167, 1703, 1587 cm⁻¹; HRMS (ESI) m/z: Calcd. for C₁₀H₇NO₃NaSClBr [M + Na]⁺: 357.8911, Found: 367.8921. Its enantiomeric excess was determined by HPLC analysis at 30 °C using a CHIRALPAK IG-3 column [hexane/propan-2-ol (95 : 5), 1 mL/min, UV detection 220 nm, retention time 10.6 min (S), 10.5 min (R)].

(R)-1-(3-Bromofuran-2-yl)-2-chloroethanol [(R)-3]:

A colorless oil. ${\left[\alpha \right]}_{\text{D}}^{29}$ –7.8 (c 1.4, CHCl₃ for 71% ee); ¹H-NMR (500 MHz, CDCl₃) δ: 7.40 (1H, d, J = 2.0 Hz), 6.45 (1H, d, J = 2.0 Hz), 5.04 (1H, dd, J = 7.5, 5.0 Hz), 3.90 (1H, dd, J = 11.0, 7.5 Hz), 3.80 (1H, dd, J = 11.0, 5.0 Hz). ¹³C-NMR (100 MHz, CDCl₃) δ: 148.7, 144.0, 114.3, 99.2, 66.3, 46.7. Its enantiomeric excess was determined by HPLC analysis at 30 °C using an OD-3 column [hexane/propan-2-ol (95 : 5), 1 mL/min, UV detection 220 nm, retention time 12.2 min (S), 14.2 min (R)].

(S)-1-(3-Bromofuran-2-yl)-2-chloroethyl (Z)-3-(methylthio)acrylate [(S)-7b] (entry 3).

The reaction of (±)-3 (45 mg, 0.20 mmol), 6b (53 mg, 0.40 mmol), and CHIRAZYME L-2 C4 (0.41 g) was conducted at 50 °C for 3 d according to the general procedure. A crude product was purified by column chromatography (hexane/EtOAc = 20 : 1 → 10 : 1 → 4 : 1) to afford (S)-7b (18.9 mg, 29% yield, 97% ee) and (R)-3 (32 mg, 70% yield, 37% ee).

(S)-7b:

A colorless oil. ${\left[\alpha \right]}_{\text{D}}^{25}$ +141 (c 1.0, CHCl₃); ¹H-NMR (500 MHz, CDCl₃) δ: 7.39 (1H, d, J = 2.0 Hz), 7.17 (1H, d, J = 10.0 Hz), 6.44 (1H, d, J = 2.0 Hz), 6.16 (1H, t, J = 7.5 Hz), 5.88 (1H, d, J = 10.0 Hz), 3.94 (1H, dd, J = 11.5, 7.5 Hz), 3.86 (1H, dd, J = 11.5, 7.5 Hz), 2.42 (3H, s); ¹³C-NMR (125 MHz, CDCl₃) δ: 164.9, 154.4, 146.4, 143.6, 114.4, 111.6, 101.1, 66.2, 42.6, 19.4; IR (neat) ν: 1702, 1565 cm⁻¹; HRMS (CI) m/z: calcd. for C₁₀H₁₁BrClO₃S [M + H]⁺: 324.9301, found: 324.9298. Its enantiomeric excess was determined by HPLC analysis at 30°C using a CHIRALPAK IG-3 column [hexane/propan-2-ol (95 : 5), 1 mL/min, UV detection 220 nm, retention time 10.6 min (S), 12.6 min (R)].

(S)-1-(3-Bromofuran-2-yl)-2-chloroethyl (Z)-3-((trifluoromethyl)thio)acrylate [(S)-7c] (entry 6).

The reaction of (±)-3 (45 mg, 0.20 mmol), 6c (74 mg, 0.40 mmol), and Lipase CL “Amano” IM (135 mg) was conducted at 50°C for 24 h according to the general procedure. A crude product was purified by column chromatography (hexane → hexane/EtOAc = 4 : 1) to afford (S)-7c (37 mg, 49% yield, 99% ee) and (R)-3 (21 mg, 47% yield, 89% ee).

(S)-7c:

A colorless oil. ${\left[\alpha \right]}_{\text{D}}^{24}$ +143 (c 1.0, CHCl₃); ¹H-NMR (500 MHz, CDCl₃) δ: 7.41 (1H, d, J = 2.0 Hz), 7.28 (1H, d, J = 10.0 Hz), 6.46 (1H, d, J = 2.0 Hz), 6.19 (1H, d, J = 10.0 Hz), 6.17 (1H, dd, J = 7.5, 6.5 Hz), 3.96 (1H, dd, J = 11.5, 7.5 Hz), 3.86 (1H, dd, J = 11.5, 6.5 Hz); ¹³C-NMR (125 MHz, CDCl₃) δ: 164.8, 145.8, 143.9, 138.5, 138.5, 129.3 (q, J = 308 Hz), 115.9, 114.6, 101.6, 67.2, 42.5; ¹⁹F-NMR (470 MHz, CDCl₃) δ: –44.6; IR (neat) ν: 1705, 1586 cm⁻¹; HRMS (ESI) m/z: calcd. for C₁₀H₇O₃F₃NaSClBr [M + Na]⁺: 400.8832, found: 400.8841. Its enantiomeric excess was determined by HPLC analysis at 30°C using a CHIRALPAK IH-3 column [hexane/propan-2-ol (95 : 5), 1 mL/min, UV detection 220 nm, retention time 4.8 min (S), 5.2 min (R)].

(S)-1-(3-Bromofuran-2-yl)-2-chloroethyl (Z)-3-chloroacrylate [(S)-7d] (entry 7).

The reaction of (±)-3 (45 mg, 0.20 mmol), 6d (48 mg, 0.40 mmol), and CHIRAZYME L-2 C4 (135 mg) was conducted at 50°C for 24 h according to the general procedure. A crude product was purified by column chromatography (hexane/EtOAc = 10 : 1 → 4 : 1) to afford (S)-7d (25 mg, 40% yield, 96% ee) and (R)-3 (26 mg, 58% yield, 66% ee).

(S)-7d:

A colorless oil. ${\left[\alpha \right]}_{\text{D}}^{24}$ +85 (c 1.0, CHCl₃); ¹H-NMR (500 MHz, CDCl₃) δ: 7.61 (1H, d, J = 9.0 Hz), 7.41 (1H, d, J = 2.0 Hz), 6.96 (1H, d, J = 8.0 Hz), 6.46 (1H, d, J = 2.0 Hz), 6.21 (1H, t, J = 7.5 Hz), 3.97 (1H, dd, J = 11.5, 7.5 Hz), 3.88 (1H, dd, J = 11.5, 7.5 Hz); ¹³C-NMR (125 MHz, CDCl₃) δ: 161.8, 146.0, 143.8, 134.9, 120.3, 114.6, 101.5, 66.7, 42.5; IR (neat) ν: 1730, 1611 cm⁻¹; HRMS (ESI) m/z: calcd. for C₉H₇O₃NaCl₂Br [M + Na]⁺: 334.8848, found: 334.8857. Its enantiomeric excess was determined by HPLC analysis at 30°C using a CHIRALPAK IJ-3 column [hexane/propan-2-ol (95 : 5), 1 mL/min, UV detection 220 nm, retention time 15.1 min (S), 21.8 min (R)].

(S)-1-(3-Bromofuran-2-yl)-2-chloroethyl (Z)-3-iodoacrylate [(S)-7e] (entry 10).

The reaction of (±)-3 (45 mg, 0.20 mmol), 6e (85 mg, 0.40 mmol), and Lipase CL “Amano” IM (135 mg) was conducted at 50°C for 24 h according to the general procedure. A crude product was purified by column chromatography (hexane/EtOAc = 10 : 1 → 4 : 1) to afford (S)-7e (23 mg, 28% yield, 94% ee) and (R)-3 (32 mg, 77% yield, 40% ee).

(S)-7e:

A colorless oil. ${\left[\alpha \right]}_{\text{D}}^{22}$ +279 (c 0.9, CHCl₃); ¹H-NMR (500 MHz, CDCl₃) δ: 7.61 (1H, d, J = 9.0 Hz), 7.41 (1H, d, J = 2.0 Hz), 6.96 (1H, d, J = 9.0 Hz), 6.46 (1H, d, J = 2.0 Hz), 6.21 (1H, t, J = 7.0 Hz), 3.97 (1H, dd, J = 11.0, 7.0 Hz), 3.88 (1H, dd, J = 11.0, 7.0 Hz); ¹³C-NMR (125 MHz, CDCl₃) δ: 163.0, 145.9, 143.8, 128.9, 114.6, 101.5, 97.5, 77.4, 77.2, 76.9, 66.8, 42.4; IR (neat) ν: 1733, 1597 cm⁻¹; HRMS (ESI) m/z: calcd. for C₉H₇O₃NaClBrI [M + Na]⁺: 426.8204, found: 426.8214. Its enantiomeric excess was determined by HPLC analysis at 30 °C using a CHIRALPAK IG-3 column [hexane/propan-2-ol (95 : 5), 1 mL/min, UV detection 220 nm, retention time 6.6 min (S), 7.2 min (R)].

Conversion of (S)-7a to (S)-7b.

A solution of (S)-7a (20 mg, 0.059 mmol) in anhydrous MeOH (0.6 mL) was stirred at 0°C for 30 min, and Cs₂CO₃ (19 mg, 0.059 mmol) and MeI (4 μL, 0.06 mmol) were added. The reaction mixture was stirred at 0°C for 1 h, and a saturated aqueous NaHCO₃ solution was added. The mixture was extracted three times with EtOAc. The combined organic layer was washed with saturated brine, dried over Na₂SO₄, and concentrated under reduced pressure. The residue was purified by column chromatography (hexane/EtOAc = 10 : 1 → 5 : 1) to give (S)-7b (9.9 mg, 52% yield, 98% ee). Its NMR spectra are in good agreement with those for (S)-7b obtained by KR (Table 1, entry 3).

Conversion of (S)-7b to 7f.

mCPBA (70% purity, 30 mg, 0.12 mmol) was added to a solution of (S)-7b (40 mg, 0.12 mmol) in CH₂Cl₂ (1.2 mL), and the reaction mixture was stirred at RT for 24 h. A saturated aqueous Na₂S₂O₃ solution and a saturated aqueous NaHCO₃ solution were added, and the mixture was extracted three times with CH₂Cl₂. The combined organic layer was washed with saturated brine, dried over Na₂SO₄, and concentrated under reduced pressure. The residue was purified by column chromatography (EtOAc) to give (S)-7f (39 mg, 92% yield) as an approximately 1 : 1 mixture of two diastereomers.

(S)-7f:

A colorless oil. ¹H-NMR (500 MHz, CDCl₃) δ: 7.424 (1/2H, d, J = 2.0 Hz), 7.418 (1/2H, d, J = 2.0 Hz), 7.08 (1/2H, d, J = 2.0 Hz), 7.07 (1/2H, d, J = 2.0 Hz), 6.48-6.47 (1H, m), 6.31 (1/2H, d, J = 6.0 Hz), 6.29 (1/2H, d, J = 6.0 Hz), 6.16 (1/2H, t, J = 10.0 Hz), 6.14 (1/2H, t, J = 10.0 Hz), 3.98 (1/2H, dd, J = 14.5, 10.0 Hz), 3.97 (1/2H, dd, J = 14.0, 9.0 Hz), 3.86 (1/2H, dd, J = 14.0, 8.0 Hz), 3.85 (1/2H, dd, J = 14.5, 7.5 Hz), 2.83 (3H, s); ¹³C-NMR (125 MHz, CDCl₃) δ: 162.7, 161.1, 145.3, 144.1, 123.2, 114.7, 101.7, 67.8, 42.6. 40.7; IR (neat) ν: 1721, 1609 cm⁻¹; HRMS (MALDI) m/z: Calcd. for C₁₀H₁₀O₄NaSClBr [M + Na]⁺: 362.9064, Found: 362.9057.

IMDA reaction of 7f to give 4f.

A solution of (S)-7f (38 mg, 0.11 mmol) in MeCN (2.2 mL) was heated at refluxing temperature for 40 h and concentrated under reduced pressure. The residue was purified by column chromatography (hexane/EtOAc = 1 : 5) to give 4f (23 mg, 61% yield) as a 5:1 mixture of two diastereomers. A colorless oil. ¹H-NMR (500 MHz, CDCl₃) data for the major isomer extracted from the NMR data of the 5:1 mixture δ: 6.59 (1H, d, J = 2.0 Hz), 5.70 (1H, d, J = 2.0 Hz), 5.16 (1H, dd, J = 8.0, 6.5 Hz), 3.84 (1H, dd, J = 11.5, 6.5 Hz), 3.78 (1H, dd, J = 11.5, 8.0 Hz), 3.13 (2H, s), 2.99 (3H, s). Some characteristic ¹H-NMR (500 MHz, CDCl₃) data for the minor isomer extracted from the NMR data of the 5:1 mixture δ: 6.63 (1H, d, J = 2.0 Hz), 5.74 (1H, d, J = 2.0 Hz), 5.19 (1H, d, H, dd, J = 8.0, 6.5 Hz), 3.31 (1H, d, J = 9.0 Hz), 3.29 (1H, d, J = 9.0 Hz), 2.57 (3H, s). ¹³C-NMR (100 MHz, CDCl₃) data for the 5:1 mixture δ: 169.5, 168.8, 135.2, 134.7, 124.7, 124.3, 95.9, 94.8, 81.6, 78.9, 76.7, 66.0, 61.2, 48.0, 41.2, 38.8, 38.3, 34.9; IR (neat) data for the mixture ν: 1783, 1572 cm^–1; HRMS (MALDI) m/z: Calcd. for C₁₀H₁₀O₄NaSClBr [M + Na]⁺: 362.9064, Found: 362.9059.

Oxidation of 4f to 4g.

mCPBA (70% purity, 14 mg, 0.059 mmol) was added to a solution of 4f (20 mg, 0.059 mmol, 5:1 dr) in CH₂Cl₂ (0.6 mL), and the reaction mixture was stirred at RT for 24 h. A similar work-up as described for the preparation of 7f gave a crude product, which was purified by column chromatography (EtOAc) to give 4g (20 mg, 96% yield, 98% ee) as a single diastereomer (>99:1 dr by ¹H-NMR analysis). A white solid, mp 158–159 °C. ${\left[\alpha \right]}_{\text{D}}^{24}$ +94 (c 1.0, CHCl₃). ¹H-NMR (500 MHz, CDCl₃) δ: 6.61 (1H, s), 5.82 (1H, s), 5.15 (1H, t, J = 7.0 Hz), 3.86 (1H, dd, J = 11.0, 6.5 Hz), 3.81 (1H, dd, J = 11.0, 7.0 Hz), 3.49 (1H, d, J = 8.5 Hz), 3.30 (3H, s), 3.20 (1H, d, J = 8.5 Hz); ¹³C-NMR (125 MHz, CDCl₃) δ: 168.3, 134.7, 126.6, 95.4, 81.7, 76.0, 65.4, 48.1, 43.1, 38.8; IR (neat) ν: 1789 cm⁻¹; HRMS (MALDI) m/z: calcd. for C₁₀H₁₀BrClO₅SNa [M + Na]⁺: 378.9013, found: 378.9008. Its enantiomeric excess was determined by HPLC analysis at 30°C using a CHIRALPAK IG-3 [hexane/iPrOH (50: 50), 1.0 mL/min, UV 220 nm, retention time 15.1 min (minor), 19.3 min (major)].

Recrystallization of the above product from hexane/acetone (5:1) produced colorless crystals, mp 163–166 °C, which were subject to single crystal X-ray analysis.

Conversion of (S)-7b to 4g.

mCPBA (70% purity, 95 mg, 0.39 mmol) was added to a solution of (±)-7b (50 mg, 0.154 mmol, 98% ee) in CH₂Cl₂ (2.8 mL), and the mixture was stirred at RT for 24 h and worked up similarly to the preparation of 7f. The crude product was purified by column chromatography (hexane/EtOAc = 1:1) to afford 4g (50 mg, 92% yield, 98% ee) (>99:1 dr by ¹H-NMR analysis). Its NMR spectra are in good agreement with those of 4g described above.

Conversion of (S)-7c to 4c.

mCPBA (70% purity, 79 mg, 0.32 mmol) was added to a solution of (S)-7c (120 mg, 0.32 mmol, 98% ee) in ClCH₂CH₂Cl (6.2 mL), and the reaction mixture was stirred at 50 °C for 40 h. A similar work-up as described for the preparation of 7f gave a crude product, which was purified by column chromatography (hexane/EtOAc = 5:1) to give 4c (11.7 mg, 9% yield, 96% ee) (>98: 2 dr by ¹H-NMR analysis). A colorless oil. ${\left[\alpha \right]}_{\text{D}}^{22}$ +84 (c 0.5, CHCl₃); ¹H-NMR (500 MHz, CDCl₃) δ: 6.64 (1H, d, J = 2.0 Hz), 5.74 (1H, d, J = 2.0 Hz), 5.22 (1H, dd, J = 8.0, 6.0 Hz), 3.86 (1H, dd, J = 11.5, 6.0 Hz), 3.78 (1H, dd, J = 11.5, 8.0 Hz), 3.59 (1H, d, J = 8.5 Hz), 3.28 (1H, d, J = 8.5 Hz); ¹³C-NMR (100 MHz, CDCl₃) δ: 167.9, 134.1, 125.8 (q, J = 336 Hz), 125.3, 95.8, 80.5, 76.8, 59.3 (d, J = 2.9 Hz), 48.9, 38.7; ¹⁹F-NMR (470 MHz, CDCl₃) δ: –71.0; IR (neat) ν: 1786, 1572 cm⁻¹; HRMS (MALDI) m/z: Calcd. for C₁₀H₇O₄F₃NaSClBr [M + Na]⁺: 416.8781, Found: 416.8775. Its enantiomeric excess was determined by HPLC analysis at 30 °C using a CHIRALPAK IG-3 [hexane/iPrOH (80 : 20), 1.0 mL/min, UV 220 nm, retention time 11.9 min (minor), 17.0 min (major)].

Acknowledgments

This work was financially supported by the JSPS KAKENHI [Grant Nos: 22KK0073 and 24K02148] and Platform Project for Supporting Drug Discovery and Life Science Research (Basis for Supporting Innovative Drug Discovery and Life Science Research (BINDS)) from AMED (Grant No.: 24ama121054), We acknowledge Amano Enzyme Inc. for the kind gift of lipases. We also thank Mr. H. Yashiro, Dr. T. Matsumoto, and Dr. A. Yamano of Rigaku Corporation for their single crystal X-ray data measurements with XtaLAB Synergy-S.

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

This article contains supplementary materials.

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