A highly efficient synthesis of the DEFG-ring system of rubriflordilactone B

Abstract

A highly efficient synthesis of the DEFG-ring system of rubriflordilactone B via polar-radical-crossover cycloaddition and carbonyl allylation/lactonization is described.

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Introduction

Many species of the Schisandraceae family, which includes the genera Schisandra and Kadsura, have a long history of use as traditional medicines in China. The most famous species are the dried mature fruits of Schisandra chinensis and S. sphenanthera, with the Chinese Pharmacopoeia name of “Wu-Wei-Zi” and “Nan-Wu-Wei-Zi”, respectively, utilized for the treatment of several diseases such as asthenia, insomnia, and enhancing mental and physical working capacity over two thousand years in China.¹ Modern phytochemical and pharmacological studies attract great attention from the medicinal chemistry and drug discovery community owing to the remarkable medicinal functions of the Schisandraceae family. As a milestone in phytochemical investigations of the Schisandraceae species, more than 400 triterpenoids including 200 schinortriterpenoids with 16 frameworks have been isolated from fruits, vines, and stems of the plants.² About ten years ago, the attractive architectures and biologic activities of Schisandraceae triterpenoids attracted attention from synthetic organic chemists.³ The complex oxygenated skeletons of the schinortriterpenoids also represent a formidable synthetic challenge.⁴ The pioneering work was first reported by Yang and co-workers in 2011 for the total synthesis of schindilactone A.⁵ Recently, Schilancitrilactones B and C and propindilactone G were synthesized by Tang and Yang, respectively.⁶

Rubriflordilactones A (1) and B (2) were isolated from the leaves and stems of Schisandra rubriflora in 2006 by Sun.⁷ Their structure and relative configurations were established on the basis of extensive spectroscopic methods and finally confirmed by single-crystal X-ray diffraction (Fig. 1). The structural features of 1 possess a 5/5/7/6/5/6-fused hexacyclic framework containing seven chiral centers and 2 contains a 5/5/7/6/5/5-fused hexacyclic ring system with eight chiral centers. Both their structures have two oxo-quaternary carbon centers in the AB ring system. In addition, an unusual multisubstituted arene motif was embedded in both hexacyclic frameworks, relatively rare in schinortriterpenoids.² Preliminary biological assays indicated that compound 1 showed weak anti-HIV-1 activity, and compound 2 exhibited an EC₅₀ value of 9.75 μg mL⁻¹ (SI = 12.39) against HIV-1 replication with low cytotoxicity. Their bioactivities and intriguing molecular architecture have drawn a great amount of attention from the synthetic community. Studies on the total synthesis of rubriflordilactones A and B have been reported by many groups.⁸ The enantiomer of the original proposed rubriflordilactone A was accomplished by Li's group⁹ in 2014 and Anderson's group¹⁰ in 2015, respectively. Very recently, Li and coworkers completed the total synthesis of rubriflordilactone B, and found the synthetic rubriflordilactone B to be identical to the reported structure of authentic rubriflordilactone B derived from an independent X-ray crystallographic analysis and clearly different from the ¹H and ¹³C NMR spectra of the authentic samples in deuterated chloroform and pyridine. Therefore, with other proofs they speculated that the originally isolated sample of “rubriflordilactone B” was composed of two compounds, named rubriflordilactone B and pseudo-rubriflordilactone B, respectively.¹¹

Fig. 1 Structure of rubriflordilactones A and B.

Recently we have developed a convergent synthetic route for the construction of the C-5-epi ABCDE ring system of rubriflordilactone B by using the stereoselective Mukaiyama–Michael reaction and intramolecular [2 + 2 + 2] cycloaddition of triynes.¹² There are two challenges to complete the total synthesis of B: construction of the remaining ring system and revising the chirality of C-5. Here, we describe our efforts towards the construction of the remaining ring system and the synthesis of the DEFG-ring of rubriflordilactone B.

Results and discussion

Our retrosynthetic analysis is illustrated in Scheme 1. We envisioned that 2 could be made from alkene 3 and alkenol 4 through polar-radical-crossover cycloaddition (PRCC),¹³ which could help us to transfer chiralities form compound 4 to a tetrahydrofuran moiety. Compound 3 with the undesired configuration at the C-5 position has been synthesized in our group. As the secondary challenge, the revision of the chirality of C-5 is underway in our laboratory. Compound 5 was obtained from compound 6via the ring-closing metathesis (RCM) reaction followed by deprotection to afford compound 4. Compound 6 could be formed from the esterification of the allyl alcohol compound 7, which is a known compound.¹⁴

Scheme 1 Retrosynthetic analysis of rubriflordilactone B.

The synthesis of compound 4 was accomplished as illustrated in Scheme 2. The synthesis commenced from the known allyl alcohol compound 7, which was prepared from acrylaldehyde in 3 steps.¹⁴ Esterification of compound 7 with methacryloyl chloride provided compound 6 in 72% yield. The construction of the lactone was accomplished using the RCM reaction. Compound 6 was treated with 10 mol% Grubbs 2nd generation catalyst to produce compound 5, which was treated with CAN to give compound 4 smoothly.¹⁴

Scheme 2 Synthesis of C-5-epi rubriflordilactone B (9).

With compound 4 in hand, the PRCC reaction was investigated. Using compound 8 as an alkene partner, cycloadditions of alkene 8 and alkenol 4 were conducted following the same procedure, which was reported by Nicewicz.¹³ To our disappointment, the alkene partner 8 decomposed very soon under the reaction conditions. All attempts failed to furnish the desired C-5-epi rubriflordilactone B due to the instability of the alkene partner 8.

In order to shed light on the problem of this reaction, we further explored the PRCC reaction (Table 1) and selected indene as the alkene partner, which was used in the synthesis of tetrahydrofurans by catalytic polar-radical-crossover cycloaddition. Originally, we selected some chiral substrates as the alkenol partner in advantage of diastereoselective synthesis (entries 1–3). The results were disappointing. On using compound 4 as the alkenol partner, the obtained reaction product was complex (entry 1). By using compound 7 and compound 10¹⁴ as the alkenol partner, no reaction was observed (entries 2 and 3). To our delight, by using the commercial material penta-1,4-dien-3-ol as the alkenol partner, we obtained the diastereoisomer compounds 13 and 13′ in moderate yields with the dr value 10 : 1. Significant NOE enhancements observed among the marked protons corroborated the above structural assignment. Fortunately, the major compound 13 with the relative configuration of the four stereogenic centers is desired.

Table 1 Polar-radical-crossover cycloaddition of indene 12 and alkenols

Entry	Alkenol	Product	Yield^a (%)	dr^b
a Isolated yields after purification by flash chromatography. See the ESI for further experimental details. b Determined by ^1H NMR spectra. c No reaction. d 13′ = 2-epi-13.
1	4	Complex products
2	7	NR^c
3	10	NR
4	11	13 and 13′^d	66	10 : 1

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With compound 13 in hand, our efforts were focused on the construction of the DEFG-ring system of rubriflordilactone B. As shown in Scheme 3, we first used ozone to cleave the C=C in compound 13. Unfortunately, we could not obtain aldehyde 14 because of the high decomposition of the substrate.¹⁵ When we carried out the same reaction under Li's conditions, we finally achieved compound 14 as expected.¹⁶ We conceived that the construction of ring G via carbonyl allylation/lactonization might be carried out through a one-pot procedure. With this in mind, ethyl 2-(bromomethyl)acrylate in THF was added to the mixture of Zn powder and compound 14 in refluxing THF affording two diastereoisomer compounds 15 and 15′, which were unstable and inseparable by column chromatography.¹⁷ Therefore, we used it without further purification. After filtration through Celite and removal of the solvents under vacuum, the residue was directly treated with the rhodium catalyst in toluene. The disubstituted exocylic double bond of 15 was successfully transferred into the trisubstituted endocyclic form.¹⁸ Fortunately, the desired compound 16 was obtained as a single stereoisomer in 73% yield over two steps. All spectroscopic data of compound 16 were consistent with the previously reported data, which was unambiguously established by X-ray analysis.^8e

Scheme 3 Synthesis of compound 16.

Conclusions

A concise and stereoselective strategy based on polar-radical-crossover cycloaddition has been developed for the efficient assembly of the DEFG-ring system with consecutive five stereogenic centers in rubriflordilactone B from commercial and accessible materials in four steps. Presently, further studies to achieve the total synthesis of rubriflordilactone B using this strategy is still under investigation in our laboratory.

Experimental

General experimental methods

Oxygen- and moisture-sensitive reactions were carried out under an argon atmosphere. Solvents were purified and dried by standard methods prior to use. All commercially available reagents were used without further purification unless otherwise noted. Column chromatography was performed on silica gel (200–300 mesh). NMR spectra were recorded on Bruker 400 MHz and Oxford 600 MHz spectrometers in CDCl₃ or acetone d₆. Chemical shifts are reported as δ values relative to internal chloroform (δ 7.27 for ¹H NMR and 77.00 for ¹³C NMR) and acetone-d₆ (δ 2.05 for ¹H NMR and 29.92 for ¹³C NMR). High resolution mass spectra (HRMS) were obtained on a 4G mass spectrometer by using electrospray ionization (ESI) analyzed by quadrupole time-of-flight (Q-TOF). Optical rotations were measured on a Rudolph Autopol IV polarimeter.

(3S,4S)-4-(4-Methoxyphenoxy)hexa-1,5-dien-3-yl methacrylate (6). To a solution of alcohol 7 (600.0 mg, 2.73 mmol, 1.0 equiv.) were added triethylamine (331.5 mg, 3.28 mmol, 1.2 equiv.) and DMAP (14.0 mg, 0.11 mmol, 0.04 equiv.) in CH₂Cl₂. Then the reaction mixture was cooled to 0 °C, and methacryloyl chloride (313.6 mg, 3.0 mmol, 1.1 equiv.) was added in one portion. After 2 h, the reaction mixture from the ice bath was removed and allowed to warm to room temperature. After two additional hours, TLC analysis showed that a significant amount of the starting material was still present and then an additional portion of methacryloyl chloride (313.6 mg, 3.0 mmol, 1.1 equiv.) was added. TLC analysis after one hour then showed that most of the starting material had been consumed. The reaction was diluted with ether and washed with two portions of HCl (0.1 M) and one portion of brine, dried over anhydrous MgSO₄ and the solvent was removed in vacuo. The resulting mixture was purified by silica gel chromatography (petroleum ether/ethyl acetate = 20 : 1) to afford 6 (538.6 mg, 72% yield) as a colorless oil. [α]^20.3_D −30.0 (c 1.00, CHCl₃); ¹H NMR (400 MHz, CDCl₃) δ 6.92–6.86 (m, 2H), 6.83–6.80 (m, 2H), 6.16 (s, 1H), 6.00–5.84 (m, 2H), 5.59 (dd, J = 3.2, 1.6 Hz, 2H), 5.40–5.31 (m, 4H), 4.66 (t, J = 6.0 Hz, 1H), 3.77 (s, 3H), 1.96 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 166.3, 154.3, 152.2, 136.2, 133.6, 132.3, 125.9, 119.2, 118.7, 117.6, 114.5, 80.8, 75.2, 55.6, 18.3. IR (neat, cm⁻¹): ν 2929, 1720, 1506, 1227, 1162; HRMS (ESIMS) calcd for C₁₇H₂₀O₄ Na⁺ [M + Na]⁺ 311.1254, found 311.1263.

(R)-5-((S)-1-(4-Methoxyphenoxy)allyl)-3-methylfuran-2(5H)-one (5). To a solution of triene 6 (274.0 mg, 1.0 mmol, 1.0 equiv.) was added the Grubbs 2nd generation catalyst (84.8 mg, 0.1 mmol, 0.1 equiv.) in CH₂Cl₂ (75.0 mL), and the resulting solution was heated to reflux under Ar. After 2.5 hours, TLC analysis of the reaction mixture showed that the starting material had been consumed. The reaction mixture was cooled to room temperature, stirred for another 2 hours in air. The residual solid was filtered through Celite. After evaporation of the filtrate the residue was purified by column chromatography (petroleum ether/ethyl acetate = 8 : 1) to afford 5 (175.0 mg, 67%) as a colorless oil. [α]^20.1_D −48.0 (c 1.00, CHCl₃); ¹H NMR (400 MHz, CDCl₃) δ 7.13 (t, J = 1.2 Hz, 1H), 6.86–6.78 (m, 4H), 5.76–5.67 (m, 1H), 5.40–5.33 (m, 2H), 5.10–5.08 (m, 1H), 4.71 (t, J = 5.6 Hz, 1H), 3.74 (s, 3H), 1.94 (t, J = 1.6 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 173.6, 154.5, 151.3, 145.1, 132.2, 131.9, 120.6, 117.7, 117.6, 114.5, 81.1, 79.8, 55.5, 10.6. IR (neat, cm⁻¹): ν 2953, 2928, 1761, 1506, 1224, 1061, 1037, 827; HRMS (ESIMS) calcd for C₁₅H₁₆O₄ Na⁺ [M + Na]⁺ 283.0941, found 283.0948.

(R)-5-((S)-1-Hydroxyallyl)-3-methylfuran-2(5H)-one (4). To a solution of compound 5 (78.0 mg, 0.3 mmol, 1.0 equiv.) in CH₃CN/H₂O = 4/1 (5.0 mL) was added CAN (411.2 mg, 0.75 mmol, 2.5 equiv.). After 5 minutes, TLC analysis indicated that the substrate was consumed completely. After 10 minutes the total reaction mixture was diluted with brine and EtOAc, the layers separated, and the aqueous layer was extracted for two additional times with EtOAc. The combined extracts were dried over anhydrous MgSO₄ and the solvent was removed in vacuo. The resulting mixture was purified by silica gel chromatography (petroleum ether/ethyl acetate = 2 : 1) to afford 4 (38.2 mg, 82%) as a yellow oil. [α]^20.5_D −62.0 (c 1.00, CHCl₃); ¹H NMR (400 MHz, CDCl₃) δ 7.02 (t, J = 2.0 Hz, 1H), 5.81–5.73 (m, 1H), 5.23 (d, J = 17.2 Hz, 1H), 5.22 (d, J = 10.4 Hz, 1H), 4.83 (dd, J = 4.0, 2.0 Hz, 1H), 4.18 (t, J = 5.6 Hz, 1H), 3.62 (br, 1H), 1.85 (d, J = 1.6 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 174.1, 145.9, 134.7, 131.1, 118.4, 83.5, 73.1, 10.4. IR (neat, cm⁻¹): ν 3427, 1751, 1098, 1060; HRMS (ESIMS) calcd for C₈H₁₀O₃ Na⁺ [M + Na]⁺ 177.0522, found 177.0527.

(2R,3S,3aS,8aR)-3-Methyl-2-vinyl-3,3a,8,8a-tetrahydro-2H-indeno [2,1-b]furan (13). Phenyl-malononitrile (20.6 mg, 0.05 mmol, 0.05 equiv.) and the acridinium catalyst 1 (284.3 mg, 2.0 mmol, 1.0 equiv.) were added to a dry 25 mL round bottom flask in the glove box, and then CH₂Cl₂ was added. The flask was transferred from the glove box, stirred at room temperature, and indene 12 (232.3 mg, 2.0 mmol, 1.0 equiv.) and penta-1,4-dien-3-ol 11 (336.3 mg, 4.0 mmol, 2.0 equiv.) were added successively. Then the mixture was placed in front of a blue light flood lamp (450 nm) and stirred for 3 days, and then the volatile liquid was removed under reduced pressure. The resulting mixture was purified by silica gel chromatography (petroleum ether/ethyl acetate = 32 : 1) to afford 13 and 13′ (263 mg, 66%) as a colorless oil. 13¹H NMR (400 MHz, CDCl₃) δ 7.28–7.16 (m, 4H), 5.81–5.72 (m, 1H), 5.20–5.14 (m, 2H), 5.02 (td, J = 6.4, 2.0 Hz, 1H), 3.85 (t, J = 7.2 Hz, 1H), 3.54 (dd, J = 9.6, 7.2 Hz, 1H), 3.20 (dd, J = 17.6, 6.8 Hz, 1H), 3.10 (d, J = 17.6, 1H), 2.26–2.16 (m, 1H), 1.07 (d, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 143.6, 140.4, 137.4, 127.0, 126.3, 125.9, 124.8, 117.4, 85.0, 82.3, 54.0, 43.8, 41.1, 12.0. IR (neat, cm⁻¹): ν 2926, 1643, 1458, 1038, 924, 751; HRMS (ESIMS) calcd for C₁₄H₁₇O⁺ [M + H]⁺ 201.1274, found 201.1276.

(2S,3S,3aS,8aR)-3,3a,8,8a-Tetrahydro-3-methyl-2H-indeno[2,1-b]furan-2-carbaldehyde (14). To a stirred solution of 13 (105.0 mg, 52.5 mmol, 1.0 equiv.) in acetone/water (20 mL, 3 : 1) were added 2,6-lutidine (56.3 mg, 0.06 mL, 52.5 mmol, 1.0 equiv.), NaIO₄ (168.5 mg, 78.8 mmol, 1.5 equiv.) and K₂OsO₄·2H₂O (9.8 mg, 2.6 mmol, 0.05 equiv.) successively at 0 °C. The mixture was stirred for 3 h at room temperature. The reaction was quenched by the addition of saturated aqueous Na₂S₂O₃. The mixture was extracted with EtOAc, and the organic layer was washed with brine and dried over anhydrous Na₂SO₄. After filtration, the solvent was removed under vacuum, and the resulting mixture was purified by silica gel chromatography (petroleum ether/ethyl acetate = 8 : 1) to give aldehyde 17 (87.1 mg, 82%) as a colorless liquid. ¹H NMR (400 MHz, CDCl₃) δ 9.64 (d, J = 2.5 Hz, 1H), 7.29–7.16 (m, 4H), 5.09 (td, J = 5.9, 1.7 Hz, 1H), 3.89–3.85 (t, J = 8.0 Hz, 1H), 3.54 (dd, J = 10.1, 2.4 Hz, 1H), 3.23–3.10 (m, 2H), 2.64–2.57 (m, 1H), 1.22 (d, J = 7.0 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 201.8, 143.1, 139.1, 127.5, 126.4, 126.2, 125.1, 87.4, 84.5, 53.8, 40.4, 40.0, 12.4. IR (neat, cm⁻¹): ν 3407, 2928, 1730, 1458, 1049, 1021, 750; HRMS (ESIMS) calcd for C₁₃H₁₄O₂Na⁺ [M + Na]⁺ 225.0886, found 225.0883.

(S)-Dihydro-5-((2S,3S,3aS,8aR)-3,3a,8,8a-tetrahydro-3-methyl-2H-indeno[2,1-b]furan-2-yl)-3-methylenefuran-2(3H)-one (15). To a solution of aldehyde 14 (50.0 mg, 0.25 mmol, 1.0 equiv.) in anhydrous THF (10 mL) was added zinc powder (80.9 mg, 1.24 mmol, 5.0 equiv.) under argon. The mixture was heated to 85 °C, and a solution of ethyl 2-(bromomethyl)acrylate (95.6 mg, 0.50 mmol, 2.0 equiv.) in anhydrous THF (10 mL) was added dropwise via syringe. The reaction mixture was stirred at this temperature for 2.5 h and then the resultant mixture was cooled to room temperature, the crude product was filtered, and the solvent was evaporated. Product 15 was used immediately without further purification.

(S)-5-((2S,3S,3aS,8aR)-3,3a,8,8a-Tetrahydro-3-methyl-2H-indeno[2,1-b]furan-2-yl)-3-methylfuran-2(5H)-one (16). Catalyst [RhH(Ph₃P)₄] (0.025 mmol, 0.1 equiv.) was added to a stirred solution of 15 under argon. The mixture was heated to reflux for 5 h, after the reaction was complete, the resultant mixture was cooled to room temperature, and then filtered off and the residue was washed with CH₂Cl₂. The resulting mixture was purified by silica gel chromatography (petroleum ether/ethyl acetate = 32 : 1) to afford 16 (48.6 mg, 73% in two steps) as a white solid. ¹H NMR (400 MHz, CDCl₃) δ 7.31 (d, J = 6.8 Hz, 1H), 7.23–7.18 (m, 3H), 6.98 (m, J = 1.6 Hz, 1H), 4.88 (ddd, J = 7.3, 4.9, 1.8 Hz, 2H), 3.86–3.82 (m, 1H), 3.50 (dd, J = 9.6, 2.4 Hz, 1H), 3.08 (dt, J = 31.5, 11.4 Hz, 2H), 2.83–2.74 (m, 1H), 1.95 (t, J = 2.0 Hz, 3H), 1.58 (d, J = 7.1 Hz, 3H), 1.25 (d, J = 7.2 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 174.3, 146.3, 143.4, 140.1, 130.6, 127.3, 126.4, 126.1, 125.2, 83.6, 82.6, 79.8, 53.8, 40.4, 39.2, 13.3, 10.8. IR (neat, cm⁻¹): ν 2959, 2926, 1759, 1455, 1069, 743; HRMS (ESIMS) calcd for C₁₇H₂₂O₃N⁺ [M + NH₄]⁺ 288.1594, found 288.1597.

Acknowledgements

We gratefully acknowledge financial support from the NSFC (Grant No. 21272104), the “111” Program of MOE (111-2-17), the PCSIRT (IRT_15R28), the NCET (NCET-12-0247), and the FRFCU (lzujbky-2013-ct02 and lzujbky-2012-56).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra of all new compounds. See DOI: 10.1039/c6qo00241b

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