Synthesis of l-Ascorbic Acid Lactone Derivatives.

A small focused library which comprised of l-AA lactone derivatives was built with a facile method. This reported method was optimized by modifying the acidity of the solvent. As a result, 12 l-AA lactones were synthesized. Among these lactones, lactones 8-12 were new compounds. The cytotoxicity of these synthetic compounds were investigated.

Introduction

l-Ascorbic acid (l-AA), one form of vitamin C, plays an important role in both plant and animal physiology. The foremost biologically functions of l-AA are centred around the antioxidant properties. Considerable evidence has been accruing in the last two decades about the importance of l-AA not only in protecting the plant from oxidative stress, but also in protecting mammals from various chronic diseases that have their origins in oxidative stress [1]. Derivatives of l-AA were found showing wide range of bioactivities including antiviral [2-5], cytotoxicity [6], inhibitory activities against tyrosinase-catalyzed melanin formation [7], increasing skin permeability [8, 9], and neurotropic activity [10]. Among them, octanoyl-6-O-ascorbic acid could enhance the solubility of many poorly water soluble drugs [11]. Because of these properties, l-AA derivatives were applicable in cosmetics and medicine [12, 13].

Many bioactive l-AA derivatives were found in nature [14-16]. For example, bioactive-oriented isolation of dilaspirolactone aglycon (1) and delesserrine (2) (Fig. 1) from Delesseriaceae family were reported [17, 18]. Our research group are interested in fern plants for a long time. A lot of species were systematically studied towards chemical components and their bioactivities [19-23], which led to the isolate of dichotomains A and B (3, 4) (Fig. 1), two l-AA derivatives, from Dicranopteris dichotoma. And dichotomain B (4) was confirmed as a weak HIV-1 inhibitor [24]. These compounds with a fragment of l-AA lactone showed different bioactivities. Attracted by this difference and the unique structure of l-AA derivatives, we would like to build a small focused library of l-AA lactone derivatives to explore their bioactivities.

Fig. 1

Natural l-AA lactone derivatives

Results and Discussion

Tang et al. [25] reported a short total synthesis of l-AA lactone compounds leucodrin and leudrin through a organocatalystic 1,4-conjugate addition of l-AA to α,β-unsaturated aldehydes. Although it effectively synthesized 5/5/5 spirodilactone l-AA derivatives, it could not access to other l-AA lactone derivatives. Poss et al. [26] reported that treating l-AA with different 4-hydoxy benzyl alcohols in hot water resulted in l-AA lactone derivatives (scheme 1). With this method, we obtained some l-AA lactone derivatives (5, 7, 13, 14, 15, Fig. 2), but a number of l-AA lactone derivatives (6, 8, 9, 10, 11, 12, 16, Fig. 2) could not formed by using this methods. The failure probably was caused by acidity of the solvent [25, 27], so, we modified the condition by applying the phosphate-citrate buffer solution (PH 5.0) as solvent. As a result, compounds 6, 8, 9, 10, 11, 12, 16 were successfully synthesized.

Scheme 1

A synthetic route to l-AA lactone derivatives reported by Poss et al

Fig. 2

Synthetic l-AA lactone derivatives

All 4-hydroxy benzyl alcohols were synthesized by reduction of corresponding aldehydes with NaBH4 except B9 and B10 (Table 1). Without following the Ref. [26], 4-hydroxy benzaldehyde was protected with Bn group, and then reacted with methyl acetate through an aldol condensation. At last, removal of Bn group gave B9 in 89 % yield. It is interesting that B13 could not react with l-AA to yield lactone compound. However, it worked with methyl ether in stead of ethyl ether. An air oxidative product B14 was detected in methanolysis reaction of B13, which was reducted by NaBH4 to afford B10 (Scheme 2).

Table 1

Synthesis of l-AA lactone derivatives

Entry	4-hydroxy benzyl alcohol	Condition	Product (yields)
1		H20, 50 °C, 50 h	5 (85 %)
2		buffer, 40 °C, 72 h	6 (53 %)
3		H20, 50 °C, 72 h	7 (35 %)
4		buffer, 60 °C, 36 h	8 (72 %)
5		buffer, 60 °C, 60 h	9 (65 %) 10:1 of two benzylic siomers
6		buffer, 60 °C, 72 h	10 (42 %) 3.4:1 of two benzylic siomers
7		buffer, 40 °C, 48 h	11 (78 %)
8		buffer, 60 °C, 72 h	12 (58 %)
9		H20, 50 °C, 72 h	13 (35 %) 15 (40 %)
10		H20, 50 °C, 72 h	14 (30 %)
11		buffer/EtOH, r.t, 8 h	16 (57 %)
12		H2O, 50 °C, 72 h	nr
		buffer, 60 °C, 72 h
		buffer/EtOH, r.t., 72 h

Scheme 2

Synthetic route to compounds B9 and B10

With all designed 4-hydoxy benzyl alcohols in hand, we built a small focused library which contained l-AA lactone drivatives 5–16. We found that 4-hydoxy benzyl alcohols like B1, B4, B5, B7, and B11 with good water solubility could react well with l-AA to give lactone derivatives in good yield except B3. This might be that two phenolic hydroxyl groups in B3 made it be easily oxidized by air. B12 could not react with l-AA in all conditions applied in this article, probably because the reactivity of lone pair electron at S atom of B12 was lower than that of phenol hydroxyl. So, it could not react like other 4-hydoxy benzyl alcohols. Furthermore, all l-AA lactones were obtained as a single compound except lactones 9 and 10 which were mixtures of two isomers at benzylic position. 1H NMR indicated their ratio is about 10:1 for 9 and 3.4:1 for 10, and we found that one isomer of 9 and 10 were unstable and could transform from semi-ketal into ketone at C3 position at room temperature or under NMR condition spontaneously.

All synthetic L-AA lactone derivatives were evaluated on five human tumor cell lines, including HL-60, SMMC-7721, A-549, MCF-7 and SW480, using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method (Table 2). Anticancer drug cisplatin (DDP) was used as the positive control. To our disappointment, none of these compounds showed cytotoxicity.

Table 2

In vitro anti-tumor assay of the synthetic compounds

Entry	IC50(μM)
	HL-60	SMMC-7721	A-549	MCF-7	SW480
5	>40	>40	>40	>40	>40
6	>40	>40	>40	>40	>40
7	>40	>40	>40	>40	>40
8	>40	>40	>40	>40	>40
9	>40	>40	>40	>40	>40
10	>40	>40	>40	>40	>40
11	>40	>40	>40	>40	>40
12	>40	>40	>40	>40	>40
13	>40	>40	>40	>40	>40
14	>40	>40	>40	>40	>40
15	>40	>40	>40	>40	>40
16	>40	>40	>40	>40	>40
DPP	1.05	6.76	6.01	15.38	16.31
Taxol	<0.008	<0.008	<0.008	<0.008	<0.008

Experiment Section

General Experimental Procedures

HRESIMS were performed on a Agilent 6540 Q-TOF. 1H and 13C NMR spectra were recorded on Bruker Avance III-400 and Bruker Avance III-600 MHz spectrometers. Chemical shifts (δ) were expressed in ppm with reference to the TMS resonance. Column chromatography was performed using Silica gel [(200–300) mesh, Qingdao Marine Chemical, Inc, Qingdao, China]. Reactions were monitored by TLC and spots were visualized by heating the silica gel plates sprayed with 10 % H2SO4 in EtOH.

Synthesis of 4-Hydroxy Benzyl Alcohols (B1–B8, B11–B12)

4-Hydroxy benzyl alcohols B1–B8, B11–B12 were synthesized by reduction of corresponding commercial available aldehydes with NaBH4 in MeOH at 0 °C for 1–4 h.

B3: Brown foam, 90 % yields, 1H NMR (400 MHz, D2O) δ 6.50–6.56 (m, 3H), 4.34 (s, 2H).

B5: Light yellow oil, 96 % yields, 1H NMR (400 MHz, acetone-D6) δ 8.29 (s, 1H), 7.20 (d, J = 8.4 Hz, 1H), 6.77 (d, J = 8.4 Hz, 1H), 4.75 (m, 1H), 4.04 (d, J = 4 Hz, 1H), 1.36 (d, J = 6.4 Hz, 3H).

B7: White solid, 92 % yields, 1H NMR (400 MHz, CDCl3) δ 9.72 (s, 1H), 7.57 (d, J = 1.6 Hz, 1H), 7.30 (d, J = 1.6 Hz, 1H), 6.48 (s, 1H), 3.91 (s, 3H).

B8: White solid, 82 % yields, 1H NMR (400 MHz, CDCl3) δ 7.02 (d, J = 1.0 Hz, 1H), 6.79 (d, J = 1.0 Hz, 1H), 5.86 (br. s, 1H), 4.53 (s, 2H), 3.85 (s, 3H).

Synthesis of 4-Hydroxy Benzyl Alcohols (B9, B10)

B9: To a solution of 4-hydroxy benzldehyde (12.2 g, 0.1 mol) in 100 mL acetone, K2CO3 (20.7 g, 0.15 mol) and BnCl (12.6 mL, 0.11 mol) was added, which was refluxed for 4 h. Cooled to room temperature, 20 mL ice-water was added and extracted with EtOAc (3 × 100 mL). The organic layers were combined and washed by brine (3 × 50 mL), dried over Na2SO4 (s), then evaporated the solvent under the reduced pressure to give 4-benzyloxy benzldehyde as a light yellow solid (21 g, 99 %). This compound was used in next step without further purification.

To a solution of freshly distilled diisopropylamine (14 mL, 0.12 mol) in 100 mL dry THF at −78 °C was added n-BuLi (50 mL of 2 M in hexane, 0.1 mol) and stirred for 15 min. Freshly distilled methyl acetate (8.6 mL, 0.11 mol) was added. The reaction stirred for 1 h at −78 °C, and 4-benzyloxyl benzldehyde (21 g, 0.1 mol) in 100 mL dry THF was added. After 1 h at −78 °C, the reaction was quenched with saturated NH4Cl aqueous solution (50 mL), warmed to room temperature, and stirred for an additional 6 h. The solution was extracted with EtOAc (3 × 100 mL), and the organic layers were combined and washed by brine (3 × 50 mL), dried over Na2SO4 (s), evaporated the solvent under the reduced pressure to give crude product as a yellow solid, which was purified by flash chromatography with petroleum ether/EtOAc (20/1) to give methyl 3-(4-(benzyloxy)phenyl)-3-hydroxypropanoate 26 g (91 %) as a white solid. 1H NMR (400 MHz, CDCl3) δ 7.29 (m, 7H), 6.96 (dd, J = 1.9, 8.0 Hz, 2H), 5.08 (dd, J = 3.7, 9.2 Hz, 1H), 5.06 (s, 2H), 3.72 (s, 3H), 2.66–2.80 (m, 2H).

To a solution of methyl 3-(4-(benzyloxy)phenyl)-3-hydroxypropanoate (5 g, 17.48 mmol) in 50 mL EtOH, 500 mg 10 % Pd/C was added, which then stirred under H2 atmosphere overnight at room temperature. The reaction mixture was passed through a short pad of Celite to remove Pd/C and evaporated the solvent under the reduced pressure to give 3.4 g (100 %) of B9 as a white foam. 1H NMR (400 MHz, CDCl3) δ 7.22 (d, J = 8.5 Hz, 2H), 6.78 (d, J = 8.5 Hz, 2H), 5.07 (dd, J = 3.7, 9.2 Hz, 1H), 3.72 (s, 3H), 2.66–2.81 (m, 2H).

B10: To a solution of B13 (10 g, 44.6 mmol) in anhydrous MeOH, MeONa (240 mg, 4.46 mmol) was added. The mixture was stirred at 40 °C for 24 h. Evaporated the solvent under the reduced pressure, the residue was dissolved in 100 mL EtOAc, washed by water (3 × 50 mL), brine (3 × 50 mL), dried over Na2SO4 (s). Evaporated the solvent under the reduced pressure to give 9.28 g (100 %) of B14 as a light yellow oil. 1H NMR (400 MHz, CDCl3) δ 7.91 (d, J = 8.8 Hz, 2H), 6.93 (d, J = 8.8 Hz, 2H), 3.95 (s, 2H), 3.86 (s, 3H), 3.73 (s, 3H).

To a solution of B14 (5 g, 24 mmol) in 30 mL MeOH at 0 °C was added NaBH4 (1.82 g, 48 mmol), the mixture was stirred at 0 °C for 2 h. The reaction was quenched with saturated NH4Cl aqueous solution (5 mL) at 0 °C. The resulting mixture was extracted with EtOAc (3 × 30 mL), and the organic layers were combined and washed by brine (3 × 50 mL), dried over Na2SO4 (s). Evaporated the solvent under the reduced pressure to give crude product as a colorless oil, and the crude product was purified by flash chromatography with petroleum ether/EtOAc (20/1) to give 4.7 g (93 %) of B10 as a colorless oil. 1H NMR (400 MHz, CDCl3) δ 7.30 (dd, J = 2.8, 8.6 Hz, 2H), 6.90 (dd, J = 2.8, 8.6 Hz, 2H), 5.08 (dd, J = 3.7, 9.3 Hz, 1H), 3.80 (s, 3H), 3.71 (s, 3H), 2.65–2.80 (m, 2H).

General Procedure for the Preparation of l-AA Lactone Derivatives (5–16)

Method A [26] for lactones 5, 7, 13, 14, 15: To l-AA (3 eq.) in 2 mL water was added corresponding alcohol B1 or B3 or B9 or B10 (0.5 mmol, 1 eq.), and the solution stirred at 50 °C for 72 h. The reaction was evaporated under reduced pressure and the residue was purified by column chromatography with DCM/MeOH (50/1–20/1) to afford lactone 5 or 7 or 13, 15 or 14, respectively.

5 (85 %), white foam: 1H NMR (600 MHz, acetone-d6) δ 8.31 (s, 1H), 7.14 (d, J = 8.5 Hz, 2H), 6.73 (d, J = 8.5 Hz, 2H), 5.86 (s, 1H), 4.66 (s, 1H), 4.45 (s, 1H), 4.30 (s, 1H), 4.09 (dd, J = 9.7, 5.5 Hz, 1H), 4.00 (dd, J = 9.7, 3.1 Hz, 1H), 3.77 (s, 1H), 3.09 (d, J = 13.5 Hz, 1H), 2.92 (d, J = 13.5 Hz, 1H). 13C NMR (150 MHz, acetone-d6) δ 175.88, 157.41, 132.83, 125.75, 115.60, 108.31, 86.97, 80.77, 75.51, 75.45, 55.05, 40.62. HRESIMS m/z 305.0636 (calcd for C13H14O7 [M + Na]+, 305.0632).

7 (35 %), yellow oil: 1H NMR (600 MHz, acetone-d6) δ 6.81 (d, J = 2.0 Hz, 1H), 6.70 (d, J = 8.1 Hz, 1H), 6.65 (dd, J = 8.1, 2.0 Hz, 1H), 5.85 (s, 1H), 4.66 (s, 1H), 4.42 (s, 1H), 4.30 (s, 1H), 4.09 (dd, J = 9.7, 5.5 Hz, 1H), 3.99 (dd, J = 9.7, 3.2 Hz, 1H), 3.04 (d, J = 13.4 Hz, 1H), 2.87 (s, 1H), 2.85 (d, J = 13.4 Hz, 1H). 13C NMR (150 MHz, acetone-d6) δ 175.95, 145.25, 145.17, 126.45, 123.30, 118.81, 115.60, 108.34, 86.97, 80.68, 75.49, 75.47, 40.90. HRESIMS m/z 333.0377 (calcd for C13H14O8 [M + Na]+, 333.0383).

13 (35 %), white foam: 1H NMR (600 MHz, acetone-d6) δ 8.67 (s, 1H), 7.29 (d, J = 8.7 Hz, 2H), 6.86 (d, J = 8.7 Hz, 2H), 6.56 (s, 1H), 4.79 (s, 1H), 4.25–4.40 (m, 2H), 4.21 (dd, J = 9.7, 6.0 Hz, 1H), 4.02 (dd, J = 9.7, 3.7 Hz, 1H), 3.90 (s, 1H), 3.17 (dd, J = 17.3, 13.2 Hz, 1H), 2.90 (dd, J = 17.3, 8.5 Hz, 1H). 13C NMR (150 MHz, acetone-d6) δ 174.37, 172.06, 158.70, 130.91, 124.09, 116.48, 106.38, 90.12, 88.98, 76.19, 74.81, 45.66, 33.97. HRESIMS m/z 321.0607 (calcd for C15H14O8 [M–H]−, 321.0616).

15 (40 %), light yellow oil: 1H NMR (600 MHz, acetone-d6) δ 8.46 (s, 1H), 7.25 (d, J = 9.0 Hz, 2H), 6.74 (d, J = 9.0 Hz, 2H), 5.78 (s, 1H), 4.72 (s, 1H), 4.26 (m, 1H), 4.02 (m, 2H), 3.62 (m, 1H), 3.47 (s, 3H), 3.29 (d, J = 1.5 Hz, 2H). HRESIMS m/z 377.0845 (calcd for C16H18O9 [M + Na]+, 377.0843).

14 (30 %), white foam: 1H NMR (600 MHz, acetone-d6) δ 8.67 (s, 1H), 7.38 (d, J = 9 Hz, 2H), 6.96 (d, J = 9 Hz, 2H), 6.56 (s, 1H), 4.79 (s, 1H), 4.25–4.40 (m, 2H), 4.21 (dd, J = 9.7, 6.0 Hz, 1H), 4.02 (dd, J = 9.7, 3.7 Hz, 1H), 3.90 (s, 1H), 3.81 (s, 3H), 3.17 (dd, J = 17.3, 13.2 Hz, 1H), 2.90 (dd, J = 17.3, 8.5 Hz, 2H). 13C NMR (150 MHz, acetone-d6) δ 174.32, 171.98, 160.88, 130.87, 125.34, 115.02, 106.38, 90.05, 88.95, 76.22, 74.78, 55.62, 45.57, 33.98. HRESIMS m/z 359.0740 (calcd for C16H16O8 [M + Na]+, 359.0737).

Method B for lactones 6, 8, 9, 10, 11, 12: To l-AA (4 eq.) in 4 mL phosphate-citrate buffer (pH = 5.0) was added corresponding alcohol B2 or B4 or B5 or B6 or B7 or B8 (1 mmol, 1 eq.), and the solution stirred at 40–60 °C for 36–72 h. The reaction was evaporated under reduced pressure and the residue was purified by colum chromatography with DCM/MeOH (50/1–20/1) to afford lactone 6 or 8 or 9 or 10 or 11 or 12, respectively.

6 (53 %), yellow foam: 1H NMR (600 MHz, acetone-d6) δ 7.22 (d, J = 8.7 Hz, 2H), 6.82 (d, J = 8.7 Hz, 2H), 5.90 (s, 1H), 4.68 (s, 1H), 4.50 (s, 1H), 4.31 (dd, J = 5.3, 3.2 Hz, 1H), 4.09 (dd, J = 9.7, 5.5 Hz, 1H), 4.01 (dd, J = 9.7, 3.1 Hz, 1H), 3.82 (s, 1H), 3.77 (s, 3H), 3.11 (d, J = 13.6 Hz, 1H), 2.96 (d, J = 13.6 Hz, 1H). 13C NMR (150 MHz, acetone-d6) δ 175.74, 159.76, 132.78, 127.07, 114.09, 108.30, 87.03, 80.64, 75.57, 75.44, 55.42, 40.51. HRESIMS m/z 319.0788 (calcd for C14H16O7 [M + Na]+, 319.0787).

8 (72 %), white foam: 1H NMR (600 MHz, acetone-d6) δ 6.83 (d, J = 1.4 Hz, 1H), 6.76 (dt, J = 16.3, 4.7 Hz, 2H), 5.96 (d, J = 1.7 Hz, 2H), 5.90 (s, 1H), 4.71 (s, 1H), 4.56 (s, 1H), 4.34 (dd, J = 4.6, 3.2 Hz, 1H), 4.11 (dd, J = 9.7, 5.5 Hz, 1H), 4.03 (m, overlaped, 2H), 3.08 (d, J = 13.7 Hz, 1H), 2.96 (d, J = 13.7 Hz, 1H). 13C NMR (150 MHz, acetone-d6) δ 175.49, 148.05, 147.55, 129.05, 124.87, 111.96, 108.47, 108.31, 101.88, 87.10, 80.48, 75.70, 75.41, 40.91. HRESIMS m/z 333.0586 (calcd for C14H14O8 [M + Na]+, 333.0581).

9 (65 %), colorless oil: main isomer: 1H NMR (600 MHz, acetone-d6) δ 8.35 (s, 1H), 7.17 (d, J = 8.8 Hz, 2H), 6.74 (d, J = 8.8 Hz, 2H), 5.71 (s, 1H), 4.57 (d, J = 3.7 Hz, 1H), 4.38 (s, 1H), 4.23 (d, J = 2.3 Hz, 1H), 4.04 (dd, J = 9.7, 5.4 Hz, 1H), 3.95 (dd, J = 9.7, 2.9 Hz, 1H), 3.34 (q, J = 7.2 Hz, 1H), 3.13 (s, 1H), 1.40 (d, J = 7.2 Hz, 3H). 13C NMR (150 MHz, acetone-d6) δ 175.36, 157.37, 131.85, 129.12, 116.30, 115.48, 108.61, 86.28, 81.86, 75.30, 75.25, 43.24, 16.40. HRESIMS m/z 295.0816 (calcd for C14H16O7 [M − H]−,295.0823).

10 (42 %), colorless oil, main isomer: 1H NMR (600 MHz, acetone-d6) δ 7.29 (d, J = 8.8 Hz, 2H), 6.82 (d, J = 8.8 Hz, 2H), 5.69 (s, 1H), 4.70 (s, 1H), 4.39 (d, J = 6.1 Hz, 1H), 4.32 (dd, J = 5.0, 3.1 Hz, 1H), 4.05–4.08 (m, overlaped, 1H), 3.97 (dd, J = 11.5, 3.0 Hz, 1H), 3.83 (s, 1H), 3.77 (s, 3H), 3.23 (q, J = 7.3 Hz, 1H), 1.51 (d, J = 7.3 Hz, 3H). 13C NMR (150 MHz, acetone-d6) δ 176.34, 159.64, 131.76, 130.42, 114.69, 113.74, 109.15, 87.03, 86.29, 75.31, 75.10, 55.41, 43.25, 16.39. HRESIMS m/z 333.0949 (calcd for C15H18O7 [M + Na]+,333.0945).

11 (78 %), colorless oil: 1H NMR (600 MHz, acetone-d6) δ 7.57 (s, 1H), 6.92 (d, J = 1.9 Hz, 1H), 6.80–6.67 (m, 2H), 5.87 (s, 1H), 4.69 (s, 1H), 4.51 (s, 1H), 4.31 (m, 1H), 4.09 (dd, J = 9.8, 5.4 Hz, 1H), 4.01 (dd, J = 9.7, 3.0 Hz, 1H), 3.82 (s, 1H), 3.78 (s, 3H), 3.11 (d, J = 13.5 Hz, 1H), 2.94 (d, J = 13.5 Hz, 1H). 13C NMR (150 MHz, acetone-d6) δ 175.94, 147.61, 146.55, 126.16, 124.37, 115.33, 115.21, 108.30, 86.97, 80.79, 75.48, 75.34, 56.14, 41.03. HRESIMS m/z 335.0742 (calcd for C14H16O8 [M + Na]+,335.0737).

12 (58 %), white foam: 1H NMR (600 MHz, acetone-d6) δ 8.22 (s, 1H), 7.05 (d, J = 1.9 Hz, 1H), 6.92 (d, J = 1.8 Hz, 1H), 5.93 (s, 1H), 4.73 (s, 1H), 4.60 (s, 1H), 4.35 (dd, J = 5.2, 3.3 Hz, 1H), 4.11 (dd, J = 9.8, 5.5 Hz, 1H), 4.03 (dd, J = 9.8, 3.0 Hz, 2H), 3.81 (s, 3H), 3.07 (d, J = 13.8 Hz, 1H), 2.97 (m, overlaped, 1H). 13C NMR (150 MHz, acetone-d6) δ 175.50, 148.28, 143.97, 127.58, 127.50, 114.28, 108.59, 108.29, 87.26, 80.38, 75.67, 75.38, 56.61, 40.46. HRESIMS m/z 388.9870, 390.9851 (M + 2-H) (calcd for C14H15BrO8 [M – H]−, 388.9878).

Method C [27] for lactone 16: To l-AA (2.3 eq.) in 10 mL phosphate-citrate buffer (PH 5.0) was added corresponding alcohol B11 (1 mmol, 1 eq.) in 1 mL EtOH, and the solution stirred at room temperature for 8 h. The reaction mixture was extracted with EtOAc, the extract was washed with water, dried over Na2SO4 (s), and the solvent was evaporated under reduced pressure and the residue was purified by column chromatography with DCM/MeOH (10/1) to afford lactone 16 (57 %) a gray foam. 1H NMR (600 MHz, acetone-d6) δ 10.18 (s, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.37 (d, J = 8.0 Hz, 1H), 7.29 (d, J = 2.3 Hz, 1H), 7.07 (t, J = 8.0, 1.0 Hz, 1H), 6.98 (t, J = 8.0, 1.0 Hz, 1H), 5.92 (s, 1H), 4.65 (s, 1H), 4.50 (s, 1H), 4.28 (m, 1H), 4.10 (dd, J = 9.7, 5.5 Hz, 1H), 4.01 (dd, J = 9.7, 3.2 Hz, 1H), 3.91 (s, 1H), 3.39 (d, J = 14.3 Hz, 1H), 3.25 (d, J = 14.3 Hz, 1H). 13C NMR (150 MHz, acetone-d6) δ 176.47, 137.13, 129.06, 126.46, 122.00, 120.06, 119.52, 111.99, 108.53, 107.89, 87.24, 80.22, 75.47, 75.45, 31.22. HRESIMS m/z 328.0795 (calcd for C15H15NO6 [M + Na]+, 328.0792).