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][3][4][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][15][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][20][21][22][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.
All 4-hydroxy benzyl alcohols were synthesized by reduction of corresponding aldehydes with NaBH 4 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 NaBH 4 to afford B10 (Scheme 2).
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. 1 H 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 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.

General Experimental Procedures
HRESIMS were performed on a Agilent 6540 Q-TOF. 1   refluxed for 4 h. Cooled to room temperature, 20 mL icewater was added and extracted with EtOAc (3 9 100 mL). The organic layers were combined and washed by brine (3 9 50 mL), dried over Na 2 SO 4 (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 NH 4 Cl aqueous solution (50 mL), warmed to room temperature, and stirred for an additional 6 h. The solution was extracted with EtOAc (3 9 100 mL), and the organic layers were combined and washed by brine (3 9 50 mL), dried over Na 2 SO 4 (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)-3hydroxypropanoate 26 g (91 %) as a white solid. 1  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 H 2 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. 1    To a solution of B14 (5 g, 24 mmol) in 30 mL MeOH at 0°C was added NaBH 4 (1.82 g, 48 mmol), the mixture was stirred at 0°C for 2 h. The reaction was quenched with saturated NH 4 Cl aqueous solution (5 mL) at 0°C. The resulting mixture was extracted with EtOAc (3 9 30 mL), and the organic layers were combined and washed by brine (3 9 50 mL), dried over Na 2 SO 4 (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. 1  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