Stereoselective Total Synthesis of Natural Decanolides Bellidisin C and Pinolidoxin

Abstract

A divergent total synthesis of bioactive, naturally occurring decanolides, pinolidoxin and bellidisin C, was accomplished by taking advantage of chiral templates L-ribose and L-malic acid. In particular, bellidisin C, which is the first total synthesis so far, was achieved through a cascade reaction of reductive elimination and nucleophilic addition in a one-pot process and a sodium–alkoxide-promoted intramolecular lactonization as the key steps.

1. Introduction

Naturally occurring decanolides (ten-membered lactones, TMLs) represent an interesting and structurally diverse group of secondary metabolites with potent and diverse biological activity [1,2,3]. Among these decanolides are important compounds such as herbarumins [4], microcarpalide [5], and stagonolide [6], which exhibited promising biological activities, including phytotoxic, antibacterial, antitumoral, and insecticidal properties. The broad spectrum of biological activities and diverse architectural features of these medium-ring lactones have attracted immense interest from the synthetic community [7,8,9,10].

Recently, three novel naturally occurring decanolides, named bellidisins B–D (Figure 1), were isolated from the ethyl acetate extraction of the rice fermentation of Phoma bellidis along with known phytotoxic metabolites pinolidoxin (1) [11,12,13,14,15,16] and 2-epi-herbarumin II (5) [17]. The bellidisins exhibited significant cytotoxicity against a panel of tumor cell lines, especially HL-60 and SW480 cell lines. The structures of the bellidisins B–D (2–4) were elucidated using extensive spectroscopic analysis and electronic circular dichroism (ECD) calculations [11]. The bellidisins B–D comprise similar decanolide skeletons, with an extra sorbate side chain connecting at C-2. Architecturally, the backbone and stereochemistry of the bellidisins are very close to the previously isolated pinolidoxin (1). Pinolidoxin was first isolated in 1993 from the fungus Ascochyta pinodes by Evidente et al. [12] and later, from an aggressive strain of Didymella pinodes isolated from infected pea in Spain by same group in 2012 [13]. A combination of extensive spectroscopic analysis and degradation experiments led Evidente [12,13] and Piccialli [14] to propose the structure of pinolidoxin (1), as shown in Figure 1, in which the absolute configuration at C-2 remained unclear due to limited supply. It is noteworthy that decanolide 1 was also isolated from a strain of the fungus Mycosphaerella lethalis by Arnone et al. in 1993 and named lethaloxin [15]. Preliminary biological studies indicated that pinolidoxin appeared to be potent inhibitors of phenylalanine ammonialyase (PAL) activity, a key regulatory enzyme in the phenylpropanoid metabolic pathway activated following the pathogen attack [12,13]. Very recently, Hitora et al. reported that pinolidoxin displayed an inhibitory effect on osteoclast differentiation and did not exhibit cytotoxicity on RAW264 cells [16].

To date, three syntheses of pinolidoxin have been disclosed, and no synthetic efforts have been reported toward the synthesis of bellidisins [18,19,20]. Pioneering synthetic studies of pinolidoxin/lethaloxin was conducted by Fürstner group [18]. In 2002, Fürstner and co-workers reported the first total synthesis and configuration assignment of (+)-pinolidoxin on the basis of dynamic calculations and single-crystal X-ray diffraction analysis. Soon after, Kozmin group reported the total synthesis of unnatural enantiomer (−)-pinolidoxin from meso silacyclic epoxides using their catalytic desymmetrization and a late-stage ring-closing metathesis (RCM) as the key steps [19]. In 2005, Marco and co-workers reported their synthesis of (+)-lethaloxin from (R)-glycidol and D-ribose in nine linear steps with 8.38% overall yield and thereby established that (+)-lethaloxin and (+)-pinolidoxin were identical [15,20]. In the context of our studies on the enantioselective synthesis of bioactive natural products based on a chiron approach [21,22,23,24,25], we herein report a concise and efficient synthetic approach to pinolidoxin and bellidisin C.

2. Results and Discussion

The retrosynthetic strategy for pinolidoxin (1) and bellidisin C (2) was depicted in Scheme 1. Initially, we envisaged that decanolides 1 and 2 could be elaborated from dioxolanones 6a/b by sodium-hydride-promoted one-pot intramolecular lactonization, followed by esterification with sorbic acid and deprotection. The trans double bond at C5-C6 in 6a/b could be installed by coupling triols 7a/b with the olefinic side chain 8 through the olefin cross-metathesis (CM) strategy. The masked triols 7a/b, in which three consecutive stereocenters are in place, could be accessible from the iodide 9 through a cascade reaction of reductive elimination and nucleophilic addition in a one-pot process as previously developed by us [26,27]. The primary iodide 9 would be readily accessible, obtained from commercially available L-ribose, whereas the alcohol 10 could be derived from L-malic acid through iodination and two-carbon homologation. Details of the studies thus undertaken are described below.

Synthesis of the precursors dioxolanone 8 and triols 7a,b were presented in Scheme 2. The known alcohol 10, easily obtained from L-malic acid in 91% of overall yield in two steps according to a literature procedure [28], was subjected to a standard iodination condition to give iodide 11 in 89% yield. Subsequently, further CuI-catalyzed coupling of iodide 11 and vinylmagnesium bromide in the presence of HMPA [29] furnished terminal alkene 8 in a satisfactory yield (78%).

The synthesis of triols 7a and 7b began with conversion of commercially available L-ribose into the corresponding iodide 9 in two steps according to our reported procedure [30] (Scheme 2). The primary iodide 9 was treated sequentially with t-buthyllithium through a cascade reaction of a Bernet–Vasella-type reductive elimination and followed by a nucleophilic addition with n-propyl magnesium bromide to deliver the known alcohol 7a [20] as a separable mixture of diastereomers (anti/syn = 2.7:1, 1H NMR analysis of the crude product) in 80% combined yield. Attempts to improve the ratio of anti/syn selectivity by changing solvents or reacting at lower temperatures were fruitless. Similarly, treatment of the iodide 9 with t-buthyllithium and n-pentyl magnesium bromide also afforded the desired anti-diastereomer 7b as major diastereomer in a 75% combined yield (anti/syn = 2.4: 1) under the optimized conditions. It is noteworthy that this three-step in one-pot sequence involves an initial ring-opening by a lithium–iodide exchange and β-elimination, followed by a stereoselective nucleophilic addition to the intermediate hemiacetal [26,27]. The observed anti-stereoselectivity in the formation of 7a/b would be explained by the steric hindrance of the bulky cyclic acetonide, which prevents the coordination of magnesium ion at the α site.

With both key building blocks in hand, the stage was set for completion of the total synthesis of pinolidoxin (1) and bellidisin C (2), as summarized in Scheme 3. However, the envisaged assembly between triols 7a/b and dioxolanone 8 turned out to be more difficult than we expected. On the basis of literature precedent [31,32,33] and our previous procedure [34,35], a variety of reaction conditions (reagents and solvents) were examined, and some of the representative results are shown in

Table 1. Transesterification of dioxolanone 8 with 7a under various conditions.

Entry	Reagents and Conditions a	Temperature	Solvent	Yieid of 13b (%) b
1	NaH (1.5–6.0 eq)	−78 °C	THF	NR c
2	NaH (2.0 eq)	0 °C	THF	Decomposition c,d
3	NaH (2.0 eq)	0 °C to rt	THF	Decomposition c,d
4	NaH (2.0 eq)	0° to rt	DMF	Decomposition d
5	PTSA (0.1 eq)	rt to reflux	Toluene	Decomposition d,e
6	NaHMDS (1.5 eq)	0 °C	THF	20%
7	NaHMDS (2.0 eq)	rt	THF	66%

^a Unless otherwise noted, the conditions were carried out with 0.2 mmol 7a and 0.1 mmol 8. ^b The ester 13 was unstable and decomposed rapidly on flash column or standing in air. Thus, the ester 13 was converted into the corresponding acetyl ester 13b under standard acetylation conditions. ^c Starting material 7a was recovered with unidentified mixtures. ^d With considerable unidentified products. ^e No desired product was observed.

. Initially, treatment of the dioxolanones 8 with triol 7a under De Brabander’s transesterification conditions (NaH in DMF or THF) [31] resulted in the recovery of starting material triol 7a (entries 1–3) or led to unidentified decomposition products (entry 4). The acid-mediated esterification strategy [36] was also examined, and no desired ring-opening product was observed (entry 5). After screening several conditions, we were pleased to find that the alkoxide-mediated transesterification was most effective in the presence of NaHMDS [37] in anhydrous THF, leading to acetyl ester 13b in a satisfactory yield (66%).

Esterification of the resulting secondary hydroxyl group 13 with commercially available sorbic acid was performed under a Keck esterification condition [38], affording the corresponding sorbate ester 13a in a satisfactory yield of 61% over two steps. Hydrolysis of the isopropylidene group in 13a with a p-toluenesulfonic acid (PTSA) in methanol furnished diol 14 in a high yield of 90%. Ring-closing metathesis (RCM) of 13a using Grubbs second-generation catalyst [39] in refluxing CH₂Cl₂ predominantly afforded undesired Z-decanolide [19] in a 67% yield, with Z/E > 20:1. As a comparison, ring-closing metatheses of less-hindered diol 14 were carried out to deliver a separable mixture of pinolidoxin (1) and Z-isomer (E/Z = 1:2.3) in a 58% yield. Attempts to suppress the undesired Z-isomer by using benzoquinone or phenol as additives or running at higher temperature were fruitless. The low E/Z selectivity of the cross-metathesis reaction probably originated from the strong ring strain at the cycloalkene stage in the transition. Similar observations were also made by Kozmin and co-workers [19].

Although the synthesis of pinolidoxin (1) was short (eight longest linear steps) and efficient, it suffered from low E/Z selectivity and yield in the RCM process [40,41]. Moreover, in view of the similar structural skeleton of pinolidoxin (1) and bellidisin C (2), it is therefore highly desirable to develop a more efficient synthetic strategy for the total synthesis of both natural products. Accordingly, we revised our synthetic expedition towards pinolidoxin (1) and bellidisin C (2). Exposure of the dioxolanones 8 with triol 7a in the presence of a Grubbs second-generation catalyst delivered trans-olefin 6a in an excellent yield of 89% without any detectable cis-isomer product (Scheme 3). Subsequently, sodium–alkoxide-promoted intramolecular lactonization of 6a, followed by esterification with sorbic acid, was carried out following a similar synthetic sequence to furnish the desired ten-membered lactone 15 with a satisfactory result over two steps. Interestingly, the NMR spectra of lactone 15 in CDCl₃ solution at room temperature revealed the presence of two slowly interconverting conformers [42,43] in a ratio of about 4.3:1, which give rise to two sets of partly overlapped signals (see the Supplementary Materials). Similar observations for conformational equilibrium were also made by Fürstner [18] and Marco [20] in their synthesis of 1, respectively. Finally, following the same acetonide deprotection, the synthesis of pinolidoxin (1) was accomplished in 91% yield.

Having developed an efficient route to the decanolide backbone, our focus was then shifted to the synthesis of bellidisin C (2). Accordingly, following the similar synthetic strategy as described above, assembly between triol 7b and dioxolanones 8 was subjected to the cross-metathesis reaction and alkoxide-mediated intramolecular transesterification, followed by esterification with sorbic acid, successfully affording ten-membered lactone 16 in an overall yield of 37% for three steps. Finally, hydrolysis removal of the acetonide group in 16 was achieved with p-toluenesulfonic acid (PTSA) at room temperature, delivering bellidisin C (2) in an 89% yield. The spectroscopic data (¹H and ¹³C NMR, HRMS)and optical rotation of synthetic pinolidoxin (1) and bellidisin C (2) were in excellent agreement with the literature reports [11,18,19].

3. Materials and Methods

3.1. General Experimental Procedures

All reagents were purchased from commercial corporations. All commercially available solvents were dried according to the standard procedures before use, and all reaction process was monitored by TLC (thin layer chromatography). Unless noted otherwise, commercially available materials were used without further purification. The crude products were purified by FC (flash chromatography) using 100–200 mesh silica gel. Infrared spectra were recorded using a FT-IR spectrophotometer with wave numbers expressed in cm⁻¹. The optical rotation data were measured using a polarimeter at 25 °C. HRMS (high-resolution mass spectrometry) were recorded by FT-ICR-MS (Fourier-transform mass spectrometry), and the solvent was chromatographically pure CHCl₃ or methanol. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) were measured on Bruker topspin spectrometers (CDCl₃ with TMS as an internal standard and coupling constants (J) in Hz.). Chemical shifts (δ) are given in ppm relative to residual solvent (usually CDCl₃ or CD₃OD, δ 7.26 (¹H NMR) or 77.3 (¹³C NMR) for CDCl₃; δ 3.31 (¹H NMR) or 49.0 (¹³C NMR) for CD₃OD for proton decoupled; Peak s means singlet state; d means doublet state; t means triplet state; q means quartet state; m means multiple state.

3.2. Experimental Procedures

3.2.1. Synthesis of Compound 11

The known compound 10 can be readily prepared from L-malic acid in 87% of the overall yield in two steps using the procedure reported by Lee [28]. To a solution of compound 10 (3.2 g, 20 mmol) in THF (80 mL) at 0 °C, imidazole (2.72 g, 40 mmol,) and PPh₃ (7.8 g, 30 mmol) were added. After stirring at the same temperature for 10 min, solid I₂ (7.6 g, 30 mmol) was added portion-wise. The mixture was heated to reflux for further 1.5 h. After completion of the reaction, the solution was cooled to room temperature and quenched with 20 mL saturated solution of Na₂S₂O₃. The mixture was extracted with ethyl acetate (2 × 100 mL). The combined organic phase was dried over anhydrous Na₂SO₄ and concentrated in vacuum. Purification of the crude product by flash column chromatography (petroleum ether/EtOAc 8:1) gave compound 11 (4.81 g, 89%) as a colorless oil. ${\left[\alpha \right]}_D^{25}=$ +23.4 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 4.47 (dd, J = 8.0, 4.4 Hz, 1H), 3.34–3.20 (m, 2H), 2.38 (ddt, J = 15.2, 4.4, 7.6 Hz, 1H), 2.17 (ddt, J = 15.2, 5.4, 7.6 Hz, 1H), 1.60 (s, 3H), 1.55 (s, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 172.9, 111.6, 74.42, 74.40, 36.1, 27.8, 26.4. HRMS (ESI): calcd. For C₇H₁₁IO₃Na⁺[M + Na]⁺, 292.9645; found 292.9645.

3.2.2. Synthesis of Compound 8

To a solution of compound 11 (1.074g, 3.98 mmol) in anhydrous THF (30 mL) were added CuI (75.8 mg, 0.4 mmol), vinylmagnesium bromide (11.18 mL, 1.0 M in THF, 11.18 mmol), and hexamethylphosphoric triamide (HMPA, 4.09 mL, 24 mmol) at −78 °C under a nitrogen atmosphere. The reaction was warmed to room temperature within 2 h. The mixture was stirred at the same temperature for 3 h. After completion of the reaction, the mixture was quenched with saturated aqueous NH₄Cl (10 mL) at room temperature. The mixture was extracted with EtOAc (60 mL). The organic phase was dried over anhydrous Na₂SO₄ and concentrated in vacuum. Purification of the crude product by flash column chromatography (petroleum ether/EtOAc 10:1) gave the compound 8 (528 mg, 78%) as a colorless oil. ${\left[\alpha \right]}_D^{25}$ = +122.2 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 5.78 (ddt, J = 16.8, 10.2, 6.6 Hz, 1H), 5.27–4.80 (m, 2H), 4.38 (dd, J = 7.2, 4.4 Hz, 1H), 2.22–2.17 (m, 2H), 2.00–1.87 (m, 1H), 1.86–1.72 (m, 1H), 1.58 (s, 3H), 1.51 (s, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 173.2, 136.8, 115.8, 110.4, 73.3, 30.7, 28.9, 27.1, 25.7. HRMS (ESI): calcd. For C₉H₁₄O₃Na⁺[M + Na]⁺, 193.0835; found 193.0836.

3.2.3. Synthesis of Compound 7a

The known iodide 9 was easily obtained from commercially available L-ribose in two steps according to our reported procedure [30]. To a solution of compound 9 (2.07 g, 6.6 mmol) in anhydrous THF (50 mL) was added t-BuLi (12.6 mL, 1.3 M in Pentane, 16.4 mmol) at −78 °C under a nitrogen atmosphere. The mixture was stirred at same temperature for 30 min, and n-PrMgBr was added (13.2 mL, 2 M in THF, 26.4 mmol). The mixture was stirred at −78 °C for 2 h. After completion of the reaction, the solution was warmed to room temperature and quenched with saturated 40 mL solution of NH₄Cl. The mixture was extracted with ethyl acetate (2 × 50 mL). The organic phase was dried over anhydrous Na₂SO₄ and concentrated in vacuum. Purification of the crude product by flash column chromatography (petroleum ether/EtOAc 10:1) gave known compound 7a (765 mg, 58%) as colorless oil and minor diastereomer cis-7a (283 mg, 22%) as colorless oil. For compound 7a: ${\left[\alpha \right]}_D^{25}=$ +40.4 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 6.04 (ddd, J = 17.6, 10.2, 7.6 Hz, 1H), 5.42 (dd, J = 17.2, 1.2 Hz, 1H), 5.31 (dd, J = 10.2, 1.2 Hz, 1H), 4.68–4.60 (m, 1H), 3.97 (dd, J = 8.2, 6.4 Hz, 1H), 3.67 (td, J = 8.4, 2.8 Hz, 1H), 1.68 (tq, J = 7.2, 2.8 Hz, 2H), 1.63–1.49 (m, 1H), 1.48 (s, 3H), 1.46–1.40 (m, 2H), 1.37 (s, 3H), 0.94 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 134.8, 118.6, 108.7, 80.8, 79.0, 69.8, 35.9, 27.8, 25.3, 18.4, 14.1. IR (film): 3449.1, 2963.6, 2932.4, 2871.7, 1455.5, 1379.6, 1370.8, 1214.6, 1167.6, 1099.2, 1015.8, 924.9, 872.1, 512.0 cm⁻¹. HRMS (ESI): calcd. For C₁₁H₂₀O₃Na⁺[M + Na]⁺, 223.1305; found 223.1304. The spectra data of 7a were identical to those reported [20].

For cis-7a: ${\left[\alpha \right]}_D^{25}=$−121.3 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 6.05 (ddd, J = 17.2, 10.3, 8.2 Hz, 1H), 5.37 (dt, J = 17.2, 1.4 Hz, 1H), 5.29 (dd, J = 10.3, 1.8 Hz, 1H), 4.57 (dd, J = 8.2, 6.0 Hz, 1H), 4.14 (dd, J = 9.4, 5.9 Hz, 1H), 3.31 (d, J = 9.3 Hz, 1H), 1.70 (s, 1H), 1.47 (s, 3H), 1.40 (d, J = 6.5 Hz, 1H), 1.35 (s, 3H), 1.05–1.25 (m, 1H), 0.98 (s, 4H), 0.96–0.82 (m, 1H); ¹³C NMR (100 MHz, Chloroform-d): δ 135.8, 118.5, 109.1, 80.3, 78.2, 75.9, 34.8, 28.2, 26.1, 26.1, 25.6. HRMS (ESI): calcd. For C₁₁H₂₀O₃Na⁺[M + Na]⁺, 223.1305; found 223.1306.

3.2.4. Synthesis of Compound 7b

To a solution of compound 9 (500 mg, 1.59 mmol) in anhydrous THF (10 mL) was added t-BuLi (3 mL, 1.3 M in Pentane, 3.9 mmol) at −78 °C under a nitrogen atmosphere. The mixture was stirred at the same temperature for 30 min, and n-PentMgBr was added (3 mL, 2 M in THF, 6.372 mmol). The mixture was stirred at the same temperature for 2 h. After completion of the reaction, the mixture was quenched with saturated aqueous NH₄Cl (5 mL) at room temperature. The mixture was extracted with EtOAc (40 mL). The organic phase was dried over anhydrous Na₂SO₄ and concentrated in vacuum. Purification of the crude product by flash column chromatography (petroleum ether/EtOAc 10:1) gave the compound 7b (192 mg, 53%) and minor diastereomer cis-7b (80 mg, 22%) as colorless oil, respectively. ${\left[\alpha \right]}_D^{25}=$ +26.3 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 6.03 (ddd, J = 17.2, 10.2, 7.6 Hz, 1H), 5.41 (dt, J = 17.2, 1.4 Hz, 1H), 5.30 (dt, J = 10.2, 1.4 Hz, 1H), 4.67–4.59 (m, 1H), 3.96 (dd, J = 8.2, 6.4 Hz, 1H), 3.65 (td, J = 8.4, 2.8 Hz, 1H), 1.82–1.59 (m, 2H), 1.56–1.49 (m, 1H), 1.47 (s, 3H), 1.43–1.38 (m, 1H), 1.36 (s, 3H), 1.34–1.30 (m, 4H), 0.93–0.82 (m, 4H); ¹³C NMR (100 MHz, Chloroform-d): δ 134.8, 118.5, 108.7, 80.7, 79.0, 70.0, 33.7, 31.8, 27.8, 25.3, 24.9, 22.7, 14.1. IR (film): 3427.2, 2954.6, 2932.8, 2865.3, 1461.4, 1379.3, 1369.4, 1240.9, 1215.4, 1167.5, 1056.9, 1038.7, 972.6, 924.5, 872.99, 512.3 cm⁻¹. HRMS (ESI): calcd. For C₁₃H₂₄O₃Na⁺[M + Na]⁺, 251.1618; found 251.1618.

For cis-7b: ${\left[\alpha \right]}_D^{25}=$ 103.8 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 5.97 (ddd, J = 17.2, 10.2, 8.2 Hz, 1H), 5.38–5.29 (m, 2H), 4.56 (dd, J = 8.2, 6.8 Hz, 1H), 4.01 (dd, J = 6.8, 5.4 Hz, 1H), 3.55 (dt, J = 6.8, 5.4 Hz, 1H), 2.05 (br s, 1H), 1.51 (s, 3H), 1.42 (d, J = 7.2 Hz, 3H), 1.38 (s, 3H), 1.32–1.26 (m, 5H), 0.88 (t, J = 6.8, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 134.1, 119.3, 108.6, 80.8, 79.1, 69.7, 33.8, 31.7, 27.5, 25.2, 25.1, 22.6, 14.0. HRMS (ESI): calcd. For C₁₃H₂₄O₃Na⁺[M + Na]⁺, 251.1618; found 251.1618.

3.2.5. Synthesis of Compounds 13a and 13b

To a solution of compound 7a (120 mg, 0.6 mmol) in THF (15 mL) at room temperature was added NaHMDS (0.6 mL, 2.0 M in THF, 1.2 mmol) under a nitrogen atmosphere. The mixture was stirred at the same temperature for 15 min, and 8 was added (152 mg in 3 mL THF, 0.9 mmol). The mixture was stirred at the same temperature for 5 min. After completion of the reaction, the mixture was quenched with saturated aqueous NH₄Cl (3 mL) at room temperature. The mixture was extracted with EtOAc (20 mL). The organic phase was dried over anhydrous Na₂SO₄ and concentrated in vacuum to afford crude compound 13. The ester 13 was unstable and decomposed rapidly on a flash column or standing in air. Thus, the ester 13 was converted into the corresponding acetyl ester 13a or 13b under standard acetylation conditions or Keck esterification condition.

For compound 13a: To a solution of the crude 13 in anhydrous DCM (15 mL) were added sorbic acid (200 mg, 1.8 mmol), DCC (491 mg, 2.4 mmol), and DMAP (36 mg, 0.3 mmol). The mixture was stirred at room temperature for 13 h and quenched with MeOH (3 mL). The mixture was concentrated under reduced pressure and purified via flash chromatography on silica gel (petroleum ether/EtOAc 20:1) to afford 13a (149 mg, 61% over two steps) as a colorless oil. ¹H NMR (400 MHz, Chloroform-d): δ 7.31–7.24 (m, 1H), 6.24–6.18 (m, 2H), 5.87–5.70 (m, 3H), 5.36 (d, J = 17.2 Hz, 1H), 5.25 (d, J = 10.4 Hz, 1H), 5.04–4.93 (m, 4H), 4.62 (d, J = 6.8 Hz, 1H), 4.18 (t, J = 6.8 Hz, 1H), 2.17 (d, J = 7.4 Hz, 2H), 1.92 (d, J = 8.0 Hz, 2H), 1.85 (d, J = 5.2 Hz, 3H), 1.68–1.58 (m, 2H), 1.46 (s, 3H), 1.35 (s, 3H), 1.28–1.17 (m, 2H), 0.88 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 169.5, 166.4, 146.2, 140.2, 136.7, 132.6, 129.7, 119.1, 117.8, 115.9, 108.8, 78.7, 78.0, 72.8, 71.3, 33.0, 30.4, 29.4, 27.5, 25.3, 18.7, 17.7, 14.0. HRMS (ESI): calcd. For C₂₃H₃₄O₆Na⁺[M + Na]⁺, 429.2248; found 429.2250.

For compound 13b: The above transesterifications were carried out with 7a (40 mg, 0.2 mmol) and 8 (17 mg, 0.1 mmol). To a solution of the crude 13 in anhydrous pyridine (5 mL) was added acetic anhydride (0.165 mL, 1.8 mmol) at room temperature. The mixture was stirred at the same temperature for another 1 h and quenched with methanol (3 mL). The mixture was concentrated under vacuum. The resulting residue was purified using flash chromatography on silica gel (petroleum ether/EtOAc 8:1) to afford 13b (24 mg, 66% over two steps) as a colorless oil. ¹H NMR (400 MHz, Chloroform-d): δ 5.84–5.79 (m, 2H), 5.38 (d, J = 16.8 Hz, 1H), 5.27 (d, J = 10.4 Hz, 1H), 5.08–4.96 (m, 4H), 4.63 (t, J = 6.8 Hz, 1H), 4.19 (t, J = 6.8 Hz, 1H), 2.24–2.15 (m, 2H), 2.13 (s, 3H), 1.95–1.82 (m, 2H), 1.65 (q, J = 7.2 Hz, 2H), 1.48 (s, 3H), 1.37 (s, 3H), 1.34–1.21 (m, 2H), 0.90 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 170.3, 169.3, 136.6, 132.5, 119.1, 116.0, 108.8, 78.7, 78.0, 73.0, 71.5, 33.0, 30.2, 29.4, 27.5, 25.3, 20.6, 17.8, 14.0. HRMS (ESI): calcd. For C₂₁H₃₄O₆Na⁺[M + Na]⁺, 377.1935; found 377.1936.

3.2.6. Synthesis of Compound 14

To a solution of the crude compound 13a (28 mg, 0.069 mmol) in MeOH (8 mL) was added PTSA (2.3 mg, 0.014mmol). The mixture was stirred at room temperature for 10 h and quenched with Et₃N (0.3 mL). The mixture was concentrated under reduced pressure and purified via flash chromatography on silica gel (petroleum ether/EtOAc 1:1) to afford 14 (23 mg, 90%) as a colorless oil. ¹H NMR (400 MHz, Chloroform-d): δ 7.28–7.25 (m, 1H), 6.21–6.18 (m, 2H), 6.04–5.90 (m, 1H), 5.89–5.73 (m, 2H), 5.52 (dd, J = 6.8, 4.0 Hz, 1H), 5.37 (m, 1H), 5.34 (d, J = 10.6 Hz, 1H), 5.14–4.93 (m, 3H), 3.73 (dd, J = 6.4, 4.0 Hz, 1H), 3.63–3.54 (m, 1H), 2.22–2.17 (m, 4H), 2.05–1.95 (m, 2H), 1.90–1.84 (m, 3H), 1.73–1.63 (m, 2H), 1.41–1.25 (m, 2H), 0.93 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 170.3, 166.9, 146.6, 140.6, 136.6, 136.0, 129.7, 117.9, 117.6, 116.0, 75.4, 74.2, 73.2, 71.9, 31.7, 30.3, 29.3, 18.8, 18.3, 13.9. HRMS (ESI): calcd. For C₂₀H₃₀O₆Na⁺[M + Na]⁺, 389.1935; found 389.1940.

3.2.7. Synthesis of Pinolidoxin (1) and 14a (Z Isomer) from Compound 14

To a stirred solution of 14 (30 mg, 0.082 mmol) in anhydrous DCM (20 mL) was added the Grubbs second-generation catalyst (4.25 mg, 0.05 mmol). The mixture was refluxed for 2 h and concentrated under reduced pressure. Purification of the crude product using flash chromatography on silica gel (petroleum ether/EtOAc 2:1) afforded pinolidoxin (5 mg, 17.5%) as a colorless oil and compound 14a (Z isomer of pinolidoxin) (12 mg, 40%) as a colorless oil.

For pinolidoxin (1): ${\left[\alpha \right]}_D^{25}$ = +133.6 (c 0.2, MeOH) {Lit.¹² ${\left[\alpha \right]}_D^{25}$ = +142.9 (c 0.31, MeOH)}; ¹H NMR (400 MHz, Chloroform-d): δ 7.31 (dd, J = 15.4, 9.6 Hz, 1H), 6.24–6.21 (m, 2H), 5.88 (d, J = 15.4 Hz, 1H), 5.65 (dd, J = 15.6, 1.4 Hz, 1H), 5.53 (td, J = 15.6, 7.4 Hz, 1H), 5.26 (dd, J = 5.8, 1.8 Hz, 1H), 5.04 (td, J = 9.4, 2.8 Hz, 1H), 4.44 (br s, 1H), 3.52 (dd, J = 10.0, 2.4 Hz, 1H), 2.43–2.40 (m, 1H), 2.26–2.16 (m, 2H), 2.08–2.00 (m, 1H), 1.89 (d, J = 5.6 Hz, 3H), 1.88–1.78 (m, 1H), 1.53–1.49 (m, 1H), 1.37–1.31 (m, 1H), 1.25–1.21 (m, 1H), 0.87 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 171.9, 166.0, 145.9, 140.3, 132.3, 129.7, 123.0, 118.1, 73.2, 73.1, 71.3, 69.7, 33.6, 29.8, 27.4, 18.7, 17.4, 13.8. IR (film): 3473.9, 2960.3, 2927.6, 2871.6, 1714.6, 1643.5, 1297.4, 1242.1, 1182.4, 1133.9, 1100.6, 1076.3, 1026.9, 997.8, 955.3, 866.7, 735.1, 698.5, 501.5 cm⁻¹. HRMS (ESI): calcd. For C₁₈H₂₆O₆Na⁺[M + Na]⁺, 361.1622; found 361.1622;

For compound 14a (Z isomer of pinolidoxin): ¹H NMR (400 MHz, Chloroform-d): δ 7.37–7.25 (m, 1H), 6.22 (dd, J = 6.8, 3.2 Hz, 2H), 5.84 (d, J = 15.4 Hz, 1H), 5.61–5.54 (m, 2H), 5.03 (t, J = 3.6 Hz, 1H), 4.76–4.59 (m, 2H), 3.92 (dd, J = 9.6, 4.0 Hz, 1H), 2.77 (d, J = 11.8 Hz, 1H), 2.55 (s, 1H), 2.18- 2.10 (m, 1H), 2.08–2.00 (m, 2H), 1.97–1.86 (m, 3H), 1.80–1.72 (m, 1H), 1.65–1.51 (m, 2H), 1.42–1.23 (m, 2H), 0.89 (t, J = 7.2 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 169.4, 166.4, 146.0, 140.4, 133.8, 129.6, 126.8, 117.8, 74.9, 73.9, 71.1, 66.7, 35.2, 30.5, 21.8, 18.7, 17.7, 14.0. IR (film): 3431.9, 2958.7, 2925.4, 1716.2, 1645.2, 1265.2, 1175.2, 1135.3, 1026.8, 999.5, 735.1 cm⁻¹. HRMS (ESI): calcd. For C₁₈H₂₆O₆Na⁺[M + Na]⁺, 361.1622; found 361.1623.

3.2.8. Synthesis of Compound 6a

To a stirred solution of 8 (200 mg, 1.18 mmol) and compound 7a (470 mg, 2.35 mmol) in anhydrous DCM (30 mL) was added Grubbs second-generation catalyst (49.9 mg, 0.05 mmol). The mixture was refluxed for 8 h and concentrated under reduced pressure. Purification of the crude product via flash chromatography on silica gel (petroleum ether/EtOAc 5:1) afforded compound 6a (358 mg, 89%) as a colorless oil. ${\left[\alpha \right]}_D^{25}$ = +45.7 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 5.82 (dd, J = 15.2, 6.4 Hz, 1H), 5.72 (dd, J = 15.2, 8.0 Hz, 1H), 4.61 (t, J = 7.2 Hz, 1H), 4.40 (dd, J = 7.6, 4.2 Hz, 1H), 3.92 (dd, J = 8.0, 6.4 Hz, 1H), 3.65 (td, J = 8.4, 2.8 Hz, 1H), 2.00 (ddt, J = 15.4, 4.4, 7.6 Hz, 2H), 1.84 (dq, J = 14.6, 7.6 Hz, 1H), 1.73–1.62 (m, 3H), 1.60 (s, 3H), 1.53 (s, 3H), 1.46 (s, 3H), 1.38 (m, 3H), 1.35 (s, 3H), 0.94 (t, J = 7.0 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 173.1, 133.4, 127.8, 110.6, 108.5, 80.7, 78.6, 73.3, 69.8, 35.9, 30.9, 27.9, 27.7, 27.2, 25.7, 25.3, 18.4, 14.1. IR (film): 3458.3, 2960.3, 2932.3, 1874.7, 1778.9, 1379.8, 1214.6, 1167.4, 1097.1, 1065.5, 1031.3, 926.6, 871.8, 736.5, 702.9, 511.8 cm⁻¹. HRMS (ESI): calcd. For C₁₈H₃₀O₆Na⁺[M + Na]⁺, 365.1935; found 365.1935.

3.2.9. Synthesis of Compound 15

To a solution of compound 6a (104 mg, 0.3 mmol) in THF (15 mL) at room temperature under a nitrogen atmosphere was added NaHMDS (0.3 mL, 2.0 M in THF, 0.6 mmol). The mixture was stirred at room temperature for 20 min. After completion of the reaction, the mixture was quenched with saturated aqueous NH₄Cl (3 mL) at room temperature. The mixture was extracted with EtOAc (30 mL). Then, the solution mixture was evaporated under a reduced pressure to afford the crude lactone 15a. The lactone 15a was unstable and decomposed rapidly on the flash column or standing in air. Thus, 15a was converted into the corresponding sorbate 15 under the standard Keck esterification condition. To a solution of the crude compound 15a in anhydrous DCM (15 mL) were added DMAP (8.6 mg, 0.07 mmol), DCC (184 mg, 0.95 mmol), and sorbic acid (79 mg, 0.71 mmol) at room temperature. The mixture was stirred at the same temperature for another 16 h and quenched with 3 mL methanol. The mixture was concentrated under vacuum. The residue was purified using flash chromatography on silica gel (petroleum ether/EtOAc 30:1) to afford compound 15 (56 mg, 49% over 2 steps) as a colorless oil. ${\left[\alpha \right]}_D^{25}$ = +163.3 (c 0.1, CHCl₃); The NMR spectra of 15 show the presence of two conformers in a ratio of 4.3:1, which give rise to two sets of partly overlapped signals (the minor conformers are given in italics). ¹H NMR of two conformers (400 MHz, Chloroform-d): δ 7.33 (dd, J = 15.4, 9.2 Hz, 1.23H), 6.31–6.17 (m, 2.5H), 5.92–5.76 (m, 2H), 5.67 (ddd, J = 15.6, 10.2, 3.2 Hz, 1.23H), 5.55 (d, J = 8.2 Hz, 0.23H), 5.29 (d, J = 6.0 Hz, 1H), 5.15 (t, J = 6.0 Hz, 0.23H), 4.95 (t, J = 9.0 Hz, 1H), 4.81 (s, 0.23H), 4.65 (s, 1H), 4.72 (t, J = 7.4 Hz, 0.23H), 4.07 (dd, J = 10.0, 6.7 Hz, 0.23H), 3.93 (dd, J = 10.0, 4.6 Hz, 1H), 2.48–2.32 (m, 2.5H), 2.25 (td, J = 15.6, 11.4 Hz, 2.5H), 2.03 (dd, J = 11.4, 6.0 Hz, 1.23H), 1.89 (d, J = 5.6 Hz, 3.7H), 1.72 (t, J = 7.2 Hz, 1.23H), 1.53 (s, 3.7H), 1.36 (s, 3.7H), 0.89 (t, J = 7.2 Hz, 3.7H); ¹³C NMR of major conformer (100 MHz, Chloroform-d): δ 172.2, 166.0, 145.9, 140.3, 129.7, 128.8, 123.0, 118.2, 109.2, 77.9, 76.1, 71.7, 70.1, 34.0, 30.6, 28.5, 27.7, 26.2, 18.8, 17.6, 13.9. HRMS (ESI): calcd. For C₂₁H₃₀O₆Na⁺[M + Na]⁺, 401.1935 found: 401.1934.

3.2.10. Synthesis of Pinolidoxin (1) from Compound 15

To a solution of compound 15 (15 mg, 0.04 mmol) in MeOH (5 mL) was added PTSA (2 mg, 0.009 mmol). The mixture was stirred at room temperature for 10 h and quenched with Et₃N (0.3 mL). The mixture was concentrated under reduced pressure. Purification of the crude product using flash chromatography on silica gel (petroleum ether/EtOAc 1:1) afforded pinolidoxin (1) (12 mg, 91%) as a colorless oil.

3.2.11. Synthesis of Compound 6b

Compounds 7b and 8 were subjected to the same cross-metathesis to furnish 6b in a yield of 81%. ${\left[\alpha \right]}_D^{25}=$ +127.4 (c 0.1, CHCl₃); ¹H NMR (400 MHz, Chloroform-d): δ 5.90–5.77 (m, 1H), 5.77–5.66 (m, 1H), 4.61 (dd, J = 8.0, 6.4 Hz, 1H), 4.40 (dd, J = 7.6, 4.4 Hz, 1H), 3.92 (dd, J = 8.2, 6.4 Hz, 1H), 3.63 (td, J = 8.4, 2.8 Hz, 1H), 2.52 (br s, 1H), 2.37–2.19 (m, 2H), 2.06–1.93 (m, 1H), 1.84 (dtd, J = 14.2, 8.0, 6.4 Hz, 1H), 1.70 (ddt, J = 15.2, 13.2, 2.8 Hz, 2H), 1.61 (s, 3H), 1.54 (s, 3H), 1.46 (s, 3H), 1.45–1.37 (m, 1H), 1.35 (s, 3H), 1.29 (s, 5H), 0.95 (t, J = 7.0 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 173.2, 133.5, 127.8, 110.7, 108.5, 80.7, 78.6, 73.3, 70.1, 33.7, 31.9, 30.9, 27.9, 27.7, 27.2, 25.8, 25.3, 24.9, 22.7, 14.1. HRMS (ESI): calcd. For C₂₀H₃₄O₆Na⁺[M + Na]⁺, 393.2248; found 393.2248.

3.2.12. Synthesis of Compound 16

Compound 6b was subjected to the same transesterification followed by esterification to afford lactone 16 in a yield of 46% over two steps. The NMR spectra of 16 also show the presence of two conformers in a ratio of 3.7:1, which give rise to two sets of partly overlapped signals (the minor conformers are given in italics).${\left[\alpha \right]}_D^{25}=$ +151.6 (c 0.1, CHCl₃); ¹H NMR of two conformers (400 MHz, Chloroform-d): δ 7.30 (dd, J = 15.2, 9.6 Hz, 1.27H), 6.31–6.13 (m, 2.5H), 5.92–5.61 (m, 2.27H), 5.64 (dd, J = 7.8, 2.6 Hz, 1.27H), 5.53 (dd, J = 16.0, 8.2 Hz, 0.27H), 5.29 (dd, J = 5.8, 1.6 Hz, 1H), 5.15 (dd, J = 6.8, 4.8 Hz, 0.27H), 4.94 (ddd, J = 10.8, 8.4, 3.0 Hz, 1H), 4.82 (s, 0.27H), 4.75–4.69 (m, 0.27H), 4.65 (dt, J = 5.4, 2.4 Hz, 1H), 4.07 (dd, J = 10.0, 6.6 Hz, 0.27H), 3.93 (dd, J = 10.0, 4.7 Hz, 1H), 2.48–2.32 (m, 2.5H), 2.32–2.18 (m, 3H), 2.04 (dt, J = 11.0, 3.0 Hz, 3.8H), 1.89 (d, J = 5.6 Hz, 3.8H), 1.73 (d, J = 13.4 Hz, 2.5H), 1.53 (s, 3.8H), 1.37 (s, 3.8H), 1.26 (d, J = 7.9 Hz, 5H), 0.85 (s, 3.8H); ¹³C NMR of major conformer (100 MHz, Chloroform-d): δ 172.2, 166.0, 145.9, 140.3, 129.7, 128.8, 123.0, 118.2, 109.2, 77.5, 76.1, 71.9, 70.1, 31.7, 31.5, 30.6, 28.5, 27.7, 26.3, 23.9, 22.5, 18.8, 14.0. HRMS (ESI): calcd. For C₂₃H₃₄O₆Na⁺[M + Na]⁺, 429.2248 found 429.2241.

3.2.13. Synthesis of Bellidisin C (2)

To a solution of compound 16 (17 mg, 0.042 mmol) in MeOH (5 mL) was added PTSA (2 mg, 0.009 mmol). The mixture was stirred at room temperature for 12 h and quenched with Et₃N (0.3 mL). The mixture was concentrated under reduced pressure and purified via flash chromatography on silica gel (petroleum ether/EtOAc 1:1) to afford bellidisin C (13.6 mg, 89%) as a colorless oil. ${\left[\alpha \right]}_D^{25}=$ +121.9 (c 0.2, MeOH); {Lit.¹¹ ${\left[\alpha \right]}_D^{25}$ = +138.14 (c 0.04, MeOH)}; ¹H NMR (400 MHz, Chloroform-d): δ 7.31 (dd, J = 15.4, 10.0 Hz, 1H), 6.25–6.20 (m, 2H), 5.87 (d, J = 15.4 Hz, 1H), 5.66 (dd, J = 15.6, 2.0 Hz, 1H), 5.54–5.52 (m, 1H), 5.26 (dd, J = 5.4, 2.0 Hz, 1H), 5.00 (td, J = 10.0, 2.8 Hz, 1H), 4.44 (br s, 1H), 3.52 (dd, J = 10.0, 2.6 Hz, 1H), 2.44–2.39 (m, 1H), 2.26–2.18 (m, 2H), 2.03–1.99 (m, 1H), 1.89 (d, J = 6.0 Hz, 3H), 1.82–1.78 (m, 1H), 1.52–1.48 (m, 1H), 1.38–1.22 (m, 8H), 0.85 (t, J = 6.4 Hz, 3H); ¹³C NMR (100 MHz, Chloroform-d): δ 172.0, 166.0, 145.9, 140.3, 132.3, 129.7, 123.1, 118.1, 73.1 (d), 71.5, 69.7, 31.5, 31.4, 29.9, 27.4, 23.7, 22.4, 18.8, 14.0. IR (film): 3408.5, 2927.8, 2857.5, 1716.7, 1645.0, 1258.3, 1242.6, 1134.9, 998.1cm⁻¹. HRMS (ESI): calcd. For C₂₀H₃₀O₆Na⁺[M + Na]⁺ 389.1935; found 389.1936.

4. Conclusions

In conclusion, an enantioselective total synthesis of pinolidoxin and bellidisin C has been accomplished from readily available L-ribose and L-malic acid in the longest linear sequence of eight steps. In particular, bellidisin C, which is the first total synthesis so far, was achieved in eight longest linear sequences (19.1% overall yield). The synthetic route highlights the efficiency of our chiron-based approach to access the decanolide skeleton through a sodium-alkoxide-promoted intramolecular lactonization and a cascade reaction of reductive elimination and nucleophilic addition in a one-pot process. Further bioactivity evaluation and the synthesis of other decanolide-containing natural products, such as herbarumin and bellidisins B and D, are currently being explored in our laboratory.