2025 Volume 73 Issue 2 Pages 86-93
Prealnumycin (1), a benzoisochromanequinone compound, produces biologically active exfoliamycin or alnumycin through hybridization with D-ribose or oxidation. We report herein a concise and stereoselective synthesis of 1. The anionic annulation of phthalide 5 with enone 6, prepared via a transition metal-catalyzed enantioselective route, afforded tricyclic lactone 4. This intermediate then underwent a highly diastereoselective introduction of an n-propyl group through nucleophilic addition followed by silane reduction. Subsequent regioselective arene oxidation of 18 using cerium(IV) ammonium nitrate (CAN) afforded naphthoquinone 2. Further manipulations, including acidic deprotection and elimination, yielded prealnumycin in 8 steps.
Prealnumycin (1) is a benzoisochromanequinone compound classified as an aromatic polyketide.1) The compound possesses an n-propyl group at the C1 position, and its stereochemistry was assigned as R based on theoretical calculations.2) This compound is known as a platform for C-ribosylation3,4) via Michael addition at the C8 position,5) which leads to the production of biologically active exfoliamycin6) and alnumycin7–9) (Fig. 1). Recent biosynthetic studies of alnumycin have revealed that the unprecedented dioxane moiety is formed through the rearrangement of ribose.10,11) The total synthesis of alnumycin was accomplished by the Tatsuta group in 2011.12) However, this represents the only reported example of an alnumycin-class antibiotic. A direct approach to compound 1, the aglycon of alnumycin, has not been reported, although the synthesis of the structurally related frenolicin B has been reported by several groups.13–15) In this study, we report a concise synthesis of 1 featuring the highly diastereoselective introduction of the n-propyl group and regioselective oxidation of the electron-rich naphthalene moiety.
Following the successful total synthesis of alnumycin reported by Tatsuta,12) we adopted a chiral pool approach to construct the C1 stereogenic center with some modifications. As illustrated in Chart 1, our retrosynthetic analysis of prealnumycin (1) guided us to phthalide 5 and enone 6 as starting materials. In the final stage of the synthesis, we envisioned that compound 1 would be derived from quinone 2 after the removal of the protecting groups and dehydration to form the C3–C4 double bond. Compound 2 would be obtained via regioselective oxidation of the electron-rich naphthol 3, which would be derived from lactone 4 through a diastereoselective propylation. The tricyclic core would be synthesized by an anionic annulation of phthalide 5 with optically active enone 6.16–18) Phthalide 5 can be prepared from commercially available 2,5-dimethoxybenzaldehyde in 2 steps.19,20) Meanwhile, enone 6 can be obtained from optically pure 3,4-di-O-acetyl-L-rhamnal using a previously reported procedure.21,22) In this work, we followed a transition metal-catalyzed enantioselective approach to enone 6, which reduces the number of synthetic steps and improves overall efficiency.23)
Our synthesis commenced with the preparation of enone 6 (Chart 2). 2-Acetylfuran (7) was subjected to an asymmetric transfer hydrogenation reaction using an azeotropic mixture of formic acid (HCO2H) and triethylamine (NEt3) catalyzed by (S,S)-Ts-DENEB to give (S)-alcohol 8 with 97% enantiomeric excess (ee).24,25) The oxidative cleavage of 8 by the Achmatowicz reaction using N-bromosuccinimide (NBS) afforded lactol 9 as a 63 : 37 diastereomeric mixture. Subsequent dynamic kinetic isomerization catalyzed by [Ir(cod)Cl]2 gave γ-hydroxy enone 10 as a single diastereomer.26) The resulting alcohol was protected with a methoxymethyl (MOM) group to yield enone 6, which was prepared on a decagram scale in 1 batch.
With a sufficient amount of optically active enone 6 in hand, the anionic annulation to form the tricyclic compound was examined (Chart 3). Phthalide 5 was treated with lithium hexamethyldisilazide (LHMDS, 3 equivalent (equiv.)), generating the stabilized anion, which underwent a Michael–Dieckmann reaction with enone 6 (1.05 equiv.) to give tricyclic intermediate 11. Further, in situ treatment with methanesulfonyl chloride (MsCl) and NEt3 promoted aromatization via a mesylation–elimination process, affording naphthol 4 in 73% yield. Subsequent diastereoselective introduction of the n-propyl group on the lactone carbonyl group in 4 was performed by treatment with n-PrLi (tetrahydrofuran (THF), –78 °C→0 °C).27) The resultant diastereomeric mixture of diol 12 was treated with Et3SiH and trifluoroacetic acid (TFA) to afford 3 as a single diastereomer. The stereochemical outcome is explained by a hydride attack on the oxocarbenium ion, where the diastereotopic faces are sufficiently discriminated by the substituents at C3 and C4. In this reductive deoxygenation, triethylsilylated product 13 was also partially generated,28) which was liberated upon exposure to 1 M HCl in THF at room temperature without affecting the MOM group at C4. Upon treatment of naphthol 13 with 6 M HCl (THF, 40 °C), the MOM group was cleaved to give diol 14. To suppress undesired silylation, i-Pr3SiH was used in place of Et3SiH as a hydride source. However, the corresponding triisopropyl ether was also obtained (27%) without forming 3, and the starting material was recovered (37%).
The relative stereochemistry of 3 was confirmed by 2-dimensional (2D) NMR studies. The nuclear Overhauser effect spectroscopy (NOESY) between the C1 and C3 protons was observed, as depicted in Fig. 2, confirming the requisite stereochemistry at the C1 position in 3.
Naphthol 3 was subjected to oxidation by cerium(IV) ammonium nitrate (CAN) in aqueous MeCN at 0 °C, providing hydroquinone 15 in 76% yield, which was observed as a single isomer (the stereochemistry at C5 was not determined). The C5-alcohol (δH 5.59) in 15 was observed as the H–D exchangeable proton coupling with the C5 methine proton (δH 3.52). The unexpected product could be isolated by silica gel column chromatography without aromatization. Attempts to further oxide compound 15 using CAN with prolonged reaction times led to decomposition, whereas treatment with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) cleanly yielded naphthazarin 16 in 75% yield. In contrast, the oxidation of 14 by treatment with CAN proceeded rather quickly to give naphthazarin 17 in good yield29) (Chart 4).
It became evident that the protection of naphthol 3 prior to arene oxidation is required. Treatment of naphthol 3 with BnBr and Cs2CO3 gave benzyl ether 18 in 98% yield. Subsequent oxidation with CAN under the same conditions proceeded smoothly to give naphthoquinone 2. Next, we examined the removal of the MOM group under acidic conditions to introduce the C3–C4 double bond. We found that the benzyl group was relatively labile under acidic conditions and that harsh conditions were required to detach the MOM group, resulting in a lower yield of the product. After extensive screening, exposure of 2 to neat TFA at room temperature proceeded via stepwise deprotection through juglone 19 to give 20 in 78% yield.30) Dehydration of 20 via a triflation–elimination process gave prealnumycin (1) in 68% yield31) (Chart 5). Other conditions using MsCl or POCl3 in combination with tertiary amines led to the decomposition of the starting material, whereas Burgess reagent or Martin sulfurane also produced 1, albeit in low yield. The 1H- and 13C-NMR data (CDCl3) for synthetic 1 were in good agreement with those reported for the natural product. In addition, the UV–Vis absorption and electronic circular dichroism (ECD) spectra of 1 measured in methanol (25 μg/mL) as shown in Fig. 3.2) The ECD spectrum of synthetic 1 showed 3 positive Cotton effects at 220, 304, and 486 nm (Δε + 4.83, +5.59, and +1.58, respectively) and a negative Cotton effect at 255 nm (Δε –4.42), which closely matched that of natural 1. Thus, the natural enantiomer of prealnumycin has been successfully synthesized.
We accomplished the enantioselective synthesis of prealnumycin (1) in 8 steps from known phthalide 5. The key feature of the synthesis was the highly diastereoselective introduction of the n-propyl group using the suitable tricyclic compound derived from optically active enone 6. Further investigations toward the synthesis of alnumycin-class natural products are underway in our laboratory.
All experiments dealing with air- and moisture-sensitive compounds were conducted under an atmosphere of dry argon. THF and dichloromethane (dehydrated; Kanto Chemical Co., Inc., Tokyo, Japan) were used as received. For TLC analysis, Merck (Darmstadt, Germany) pre-coated plates (silica gel 60 F254, Art 5715, 0.25 mm) were used. For column chromatography, silica gel 60 N (spherical, 63–210 μm, Kanto Chemical Co., Inc.) was used. Optical rotations were measured with a JASCO (Tokyo, Japan) P-2300 polarimeter. Melting points were determined on a Büchi B-545 apparatus and were uncorrected. IR spectra were recorded using a JASCO FT/IR-4200 spectrophotometer. Circular dichroism (CD) spectra were recorded on a JASCO J-1100 spectrometer. UV–Vis spectra were measured on a JASCO V-650 spectrophotometer. 1H-NMR and 13C-NMR spectra were measured on a Bruker (Billerica, MA, U.S.A.) AVANCE III HD-500 (500 MHz/125 MHz). High-resolution mass spectra (HRMS) were recorded with a JEOL JMS-700 (EI/CI) or Waters SYNAPT G2-Si HDMS (ESI). HPLC was performed using a Jasco UV-2070 plus for the UV/VIS detector and a PU-980 for the HPLC pump.
Synthesis of 8A mixture of 2-acetylfuran (7) (9.02 g, 81.9 mmol), (S,S)-Ts-DENEB (106 mg, 0.163 mmol), and a HCO2H–triethylamine azeotropic mixture (45 mL) was stirred at 60 °C for 5 h. After cooling to room temperature, the mixture was diluted with CH2Cl2, washed with sat. aq. NaHCO3, and dried over Na2SO4. The volatiles were removed under reduced pressure (< 20 mmHg), and the residue was used in the next step without further purifications. The analytically pure sample was obtained by silica-gel column chromatography (hexane/EtOAc = 8/2) to give 8 as a colorless oil.
Rf 0.57 (hexane/EtOAc = 6/4);
To a solution of crude alcohol 8 (12.0 g) in THF (102 mL) and H2O (34 mL), NaHCO3 (13.8 g, 164 mmol), NaOAc (7.06 g, 86.1 mmol), and NBS (15.3 g, 86.0 mmol) were successively added at 0 °C. After stirring for 10 min at this temperature, the reaction was quenched with sat. aq. NaHCO3. The products were extracted with EtOAc (×5). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/EtOAc = 3/2 → 1/1) to give lactol 9 (9.93 g, dr 63/37, 95% over 2 steps) as a brown oil.
Rf 0.33 (hexane/EtOAc = 6/4); 1H-NMR (CDCl3, 500 MHz) δ: 1.40 (d, 1.89H, J = 6.8 Hz), 1.47 (d, 1.11H, J = 6.8 Hz), 3.26 (d, 0.63 H, J = 4.7 Hz), 3.54 (d, 0.37H, J = 7.2 Hz), 4.24 (dq, 0.37H, J = 1.2, 6.8 Hz), 4.72 (q, 0.63H, J = 6.8 Hz), 5.64 (dd, 0.63H, J = 4.7, 3.4 Hz), 5.68 (br d, 0.37H, J = 7.2 Hz), 6.11 (d, 0.63H, J = 10.3 Hz), 6.16 (dd, 0.37H, J = 10.3, 1.2 Hz), 6.90 (dd, 0.63H, J = 10.3, 3.4 Hz), 6.95 (dd, 0.37H, J = 10.3, 1.4 Hz); 13C-NMR (CDCl3, 125 MHz) δ: 15.3, 16.2, 70.4, 75.2, 87.7, 91.0, 127.3, 128.5, 144.2, 147.9, 196.4, 196.8; IR (ATR) 3392, 2987, 1693, 1373, 1234, 1021, 808 cm–1; HRMS (EI) m/z [M]+ Calcd for C6H8O3: 128.0468; Found: 128.0474.
Synthesis of 10To a mixture of lactol 9 (9.93 g, 77.5 mmol) and 2,6-dichlorobenzoic acid (7.40 g, 38.7 mmol) in CHCl3 (120 mL) was added [Ir(cod)Cl]2 (1.38 g, 2.05 mmol) at room temperature. After stirring for 1 h, the volatiles were removed under reduced pressure. The residue was purified by silica-gel column chromatography (hexane/EtOAc = 6/4 → 4/6) to give enone 10 (9.22 g, 93%) as a brown oil.
Rf 0.10 (hexane/EtOAc = 6/4);
To a solution of enone 10 (9.22 g, 72.0 mmol) in 1,2-dichloroethane (180 mL) was successively added i-Pr2NEt (24.8 mL, 144 mmol) and MOMCl (8.1 mL, 108 mmol) at room temperature. After stirring at 70 °C for 8 h, the mixture was cooled to room temperature. The reaction was then quenched with H2O and 1 M HCl. The products were extracted with CH2Cl2 (×3). The combined extracts were successively washed with brine, sat. aq. NaHCO3, and brine, and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/acetone = 6/4) to give methoxymethyl ether 6 (10.7 g, 87%) as a colorless solid.
Mp 30.1 °C–31.4 °C; Rf 0.26 (hexane/EtOAc = 6/4);
To a solution of phthalide 5 (972 mg, 5.00 mmol) in THF (50 mL) was added LHMDS (15.0 mL, 1.0 M in THF, 15.0 mmol) at –78 °C. After stirring for 20 min, the solution of enone 6 (914 mg, 5.31 mmol) in THF (12.5 mL) was added at –78 °C, and the mixture was stirred at room temperature for 13 h. After cooling to 0 °C, NEt3 (6.9 mL, 50 mmol) and MsCl (1.2 mL, 15 mmol) were successively added and stirred at room temperature for 6 h. The reaction was quenched with pH 7 phosphate buffer and then acidified with 2 M HCl. The products were extracted with EtOAc (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/acetone = 8/2) to give naphthol 4 (1.28 g, 73%) as a yellow solid.
Mp 155–156 °C; Rf 0.40 (hexane/acetone = 7/3);
To a solution of tert-BuLi (27.3 mL, 1.60 M in pentane, 43.7 mmol) in Et2O (50 mL) was added n-PrI (2.13 mL, 21.8 mmol) at –78 °C. The mixture was stirred for 30 min at this temperature and further stirred for 30 min at room temperature. After cooling to –78 °C, the mixture was diluted with THF (50 mL) and then added naphthol 4 (2.26 g, 6.50 mmol) in one portion. The suspension was stirred for 30 min at –78 °C and further stirred for 4 h at 0 °C. The reaction was quenched with pH 7 phosphate buffer and then acidified with 1 M HCl. The products were extracted with EtOAc (×3). The combined extracts were successively washed with 10% Na2S2O3 aq. and brine, and dried over Na2SO4. After concentration, the residue was azeotropically evaporated with toluene (×2) and used for the next step without further purification.
The crude lactol 12 was dissolved in CH2Cl2 (100 mL), and the solution was cooled to –78 °C. To this solution, Et3SiH (4.56 mL, 28.5 mmol) and TFA (1.12 mL, 14.2 mmol) were successively added at –78 °C. After stirring at room temperature for 10 h, the reaction was quenched with pH 7 phosphate buffer at 0 °C. The products were extracted with CH2Cl2 (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was used for the next step without further purification.
The mixture was dissolved in THF (80 mL), and the solution was cooled to 0 °C. To this solution was added 1 M HCl (20 mL) at this temperature. After stirring at room temperature for 28 h, the reaction was quenched with sat. aq. NaHCO3 at 0 °C. The products were extracted with EtOAc (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/acetone = 85/15) to give naphthol 3 (2.30 g, 94% over 3 steps) as a pale-yellow oil.
Rf 0.34 (hexane/acetone = 8/2);
Colorless oil; Rf 0.50 (hexane/acetone = 8/2);
Pale yellow oil; Rf 0.31 (hexane/acetone = 8/2);
To a solution of naphthol, 3 (20.7 mg, 0.0550 mmol) in MeCN (1.8 mL) and H2O (0.2 mL) was added CAN (65.8 mg, 0.120 mmol) at 0 °C. After stirring at this temperature for 10 min, the reaction was quenched with sat aq. NaHCO3 and the products were extracted with EtOAc (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/acetone = 7/3) to give quinol 15 (16.5 mg, 76%) as a yellow oil.
Rf 0.39 (hexane/acetone = 6/4);
To a mixture of alcohol, 15 (19.4 mg, 0.0494 mmol) in CH2Cl2 (1.8 mL) and pH 7 phosphate buffer (0.2 mL) was added DDQ (28.7 mg, 0.126 mmol) at 0 °C. After stirring at room temperature for 3 h, the mixture was diluted with CH2Cl2 and sat aq. NaHCO3 at 0 °C. The products were extracted with CH2Cl2 (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/acetone = 75/25) to give naphthazarin 16 (14.5 mg, 75%) as a yellow solid.
Mp 166–168 °C; Rf 0.47 (hexane/acetone = 6/4);
To a solution of naphthol, 14 (26.2 mg, 0.0788 mmol) in MeCN (1.8 mL) and H2O (0.2 mL) was added CAN (108 mg, 0.197 mmol) at 0 °C. After stirring at this temperature for 90 min, the reaction was quenched with sat aq. NaHCO3 and the products were extracted with EtOAc (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/acetone = 7/3) to give naphthazarin 17 (20.0 mg, 73%) as an orange solid.
Mp 147–149 °C; Rf 0.37 (hexane/acetone = 6/4);
To a solution of naphthol 3 (1.06 g, 2.81 mmol) in acetone (45 mL) was successively added Cs2CO3 (3.76 g, 11.5 mmol) and BnBr (835 μL, 7.03 mmol) at room temperature. After stirring for 21 h at 40 °C, the reaction was quenched with 1 M HCl at 0 °C. The products were extracted with EtOAc (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/acetone = 95/5) to give benzyl ether 18 (1.29 g, 98%) as a colorless oil.
Rf 0.45 (hexane/acetone = 8/2);
To a mixture of benzyl ether 18 (507 mg, 1.09 mmol) in MeCN (13.5 mL) and H2O (1.5 mL) was added CAN (1.21 g, 2.21 mmol) at 0 °C. After stirring for 40 min at this temperature, the reaction was quenched with sat. aq. NaHCO3. The products were extracted with EtOAc (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/EtOAc = 85/15) to give quinone 2 (410 mg, 87%) as a yellow oil.
Rf 0.23 (hexane/EtOAc = 8/2);
To a solid of quinone 2 (711 mg, 1.63 mmol), TFA (5.4 mL) was slowly added at room temperature. After stirring for 2 h at this temperature, the mixture was diluted with EtOAc and successively washed with H2O, sat. aq. NaHCO3, brine, and then dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/EtOAc = 9/1) to give quinone 20 (383 mg, 78%) as a yellow oil.
Rf 0.41 (hexane/EtOAc = 7/3);
Orange oil; Rf 0.51 (hexane/EtOAc = 7/3);
To a solution of quinone, 20 (43.1 mg, 0.143 mmol) in CH2Cl2 (6 mL) was successively added pyridine (56 μL, 0.69 mmol) and Tf2O (47 μL, 0.29 mmol) at 0 °C. After stirring for 1.5 h at this temperature, the reaction was quenched with sat. aq. NaHCO3 and then acidified by the addition of 2 M HCl. The products were extracted with CH2Cl2 (×3). The combined extracts were washed with brine and dried over Na2SO4. After concentration, the residue was purified by silica-gel column chromatography (hexane/EtOAc = 95/5) to give prealnumycin (1) (27.4 mg, 68%) as a red powder.
Mp 90 °C (dec); Rf 0.42 (hexane/EtOAc = 9/1);
This research was supported by a Grant-in-Aid for Scientific Research (C) (20K06955) from JSPS and a SUNBOR Grant from the Suntory Foundation for Life Sciences.
The authors declare no conflict of interest.
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