First total synthesis of salvianolic acid C, tournefolic acid A, and tournefolal

First total synthesis of the natural product salvianolic acid C, tournefolal and tournefolic acid A has been described. The key benzofuran skeletons are prepared via selective iodination and Sonogashira reaction


Introduction
Salvia miltiorrhiza Bunge (Dan-shen) is widely used as a Chinese traditional medicine for the treatment of myocardial infarction, atherosclerosis, and thrombus. 1The hydrosoluble salvianolic acids isolated from water-soluble part of Dan-shen are considered as the main pharmacological active ingredients for the activities of anti-oxidative, anticoagulant, antithrombotic, anti-HIV, anti-tumor, and so on. 2Salvianolic acid C (1), one of the Salvianolic acids, is constituted of danshensu (2) 3 and 2-phenyl-benzofuran neolignan tournefolic acid A (3) 4 linked by ester bond.Recently, Liang et al. 5 have reported that salvianolic acid C (1) displays anti-proliferative activity against HepG2 cells with IC50 value of 20 M through apoptosis, and the mechanism is concerned with inhibition of tubulin polymerization.Furthermore, neolignan tournefolic acid A (3) and tournefolal (4) express valuable anti-lipidperoxidative acitivity. 4However, the low contents of salvianolic acid C (1), 6 tournefolic acid A (3) as well as tournefolal (4) limit for further pharmacological property research.In view of their importance, the development of a route for the synthesis of salvianolic acid C (1), tournefolic acid A (3) as well as tournefolal (4) are of importance.
According to the literature, [4][5][6] although several groups have isolated salvianolic acid C (1), tournefolic acid A (3), and tournefolal (4) to study their biological activities, the total synthesis of them have not yet been reported.Our group has focused on total synthesis and pharmacological research of natural products for many years. 14Herein, we wish to report a route first time on the total synthesis of salvianolic acid C (1), tournefolic acid A (3), and tournefolal (4).And the approach is also suitable for synthesis the C-4 substituted 2-phenyl-benzofuran compounds.

Results and Discussion
The retrosynthetic analysis of salvianolic acid C (1) is shown in Scheme 1.We hypothesized that 1 could be structured from tournefolic acid A (3) through esterification with danshensu (2).And 3 could be obtained by Knoevenagel condensation from 4. 15 The key benzofuran intermediate tournefolal ( 4) could be synthesized by Sonogashira coupling of 3-hydroxy-2-iodobenzaldehyde (6) and the ethynylbenzene analogues (7).Compound (6) could be prepared from 4dihydroxybenzaldehyde (8), and Compound (7) could be prepared from pyrocatechol (9).To investigate the feasibility of the analysis above, the related experiments were carried out.

Scheme 1. The retrosynthetic analysis of salvanolic acid C.
As illustrated in Scheme 2, the synthesis of the intermediate 6 was initiated with the selective benzylation at C-4(OH) of 8 by treatment with NaI, and NaHCO3 at 40 o C for 2 days, 16 followed by using iodine chloride to introduce iodine at C-2. 17 In order to avoid over iodination, the dose of iodine chloride should not exceed 1 eqiv.The other key building block 7 18 was prepared via four steps including benzylation from 9, iodination catalyzed by iodine and Ag 2 SO 4 , Sonogashira reaction with ethynyltrimethylsilane at r.t., and alkaline hydrolysis reaction.It was noted that the amount of iodine should be subjected to 0.75 eqiv to obtain 10 in a quantitative yield.If the dose was exceeded, the byproduct of over iodination would be generated.The Sonogashira coupling 13 of 6 with 7 was catalyzed by Pd(Ph3P)2Cl2 (3 mol%) and CuI (2 mol%) to afford benzofuran aldehyde 11 as a yellow solid in 63% yield.Compound 12 was prepared via Knoevenagel condensation of 11 with malonic acid in pyridine in 85% yield with excellent E/Z ratio (95:5).Compound 13, which was prepared from Sodium Danshensu 5 via benzylation and methyl esterification, was esterified with 12 to produce 14 in 87% yield.Considering about the acrylic acid ester bond of 14, we chose neutral reagent Me 3 SnOH as the catalyst for demethylation to afford 15 in 43% yield. 14Taking into account the tolerance of double-bond, the debenzylation of 15 was catalyzed by Lewis acid BBr3 at -78 o C to afford 1.And we succeeded in obtaining 1 through purification by Sephadex LH-20 in 40% yield.This method of debenzylation could be applied in the preparation of tournefolic acid A (3) and tournefolal (4) from compound 12 and 11, respectively.

Conclusions
In summary, we have developed a method first time for the total synthesis of salvianolic acid C (1) (4.5% yield, in thirteen steps), tournefolic acid A (3) (12.1% yield, in nine steps), and tournefolal (4) (31.5% yield, in eight steps).This approach also can be applied to build the C-4 substituted 2-phenyl-benzofuran construction.

Experimental Section
General.Reagents and all solvents were analytically pure grade and were used without further purification.Column chromatography (CC) was performed on Sephadex LH-20, silica gel (200-300 mesh) and RP-18 (20-45 μm). 1 H (300 MHz) and 13 C (75 MHz) NMR spectra were recorded on a Varian Mercury 300 spectrometer in the solvent indicated.Chemical shifts are reported in ppm relative to the internal reference.ESIMS were obtained on a Bruker Esquire 3000 Plus spectrometer, and HRESIMS on a Micromass Q-Tof Global mass spectrometer.

Salvianolic acid C (1).
To a stirred solution of 15 (100 mg, 0.106 mmol) in dry DCM (5 mL), BBr3 (2 M in DCM, 0.53 mL, 1.06 mmol) was added at -78 o C. Then the mixture was stirred at the same temperature for 2h.After completion of the reaction, 0.5 mol/L Na2HPO4 (5 mL) was added to quench the reaction.The aqueous layer was extracted with EtOAc (3×15 mL).The combined organic layers were dried, filtered, and concentrated under reduced pressure.The crude product was purified by Sephadex LH-20 to afford a yellow solid 1 (20 mg, 40%).