Total synthesis of neokotalanol, a potent α-glucosidase inhibitor isolated from Salacia reticulata

Chinese Journal of Natural Medicines 2013, 11(6): 0676−0683

Chinese Journal of Natural Medicines

Total synthesis of neokotalanol, a potent α -glucosidase inhibitor isolated from Salacia reticulata XIE Wei-Jia 1, TANABE Genzoh 2, TSUTSUI Nozomi 2, WU Xiao-Ming 1, MURAOKA Osamu 2* 1 2

Department of Medicinal Chemistry, China Pharmaceutical University, Nanjing 210009, China; School of Pharmacy, Kinki University, 3-4-1 Kowakae, Higashi-osaka, Osaka 577-8502, Japan Available online 20 Nov. 2013

[ABSTRACT] Neokotalanol, a potent α-glucosidase inhibitor isolated from Salacia reticulata, was synthesized through a key coupling reaction between a perbenzylated thiosugar and an appropriately protected perseitol triflate derived from D-mannose. This key step was found to be quite temperature dependent, and a simultaneous cyclization of the triflate leading to a characteristic 2,4,7-trioxabicyclo[4.2.1]nonane system was detected. [KEY WORDS] Neokotalanol; Total synthesis; Salacia reticulata; α-glucosidase inhibitor

[CLC Number] R284.1

1

[Document code] A

[Article ID] 1672-3651(2013)06-0676-08

Introduction

Glucosidases play a fundamental role in catalyzing the hydrolysis of complex carbohydrates, which is closely related to various physiological and biological processes in living systems. The inhibition of glucosidases has long been considered a promising approach to treat a variety of diseases, such as diabetes, obesity, glycosphingolipid lysosomal storage disease, HIV infections, cancer, and Gaucher’s disease [1-10] . Thus, identifying and designing small molecules with potent glucosidase inhibitory activity has been of great interest for medicinal chemists for a long time. In the late 1990’s, salacinol (1) was isolated from Salacia reticulata, a large woody climbing plant widespread in Sri

[Received on] 10-May-2013 [Research Funding] This project was supported by the "High-Tech Research Center" Project for Private Universities: matching fund subsidy from MEXT (Ministry of Education, Culture, Sports. Science and Technology), 2007–2011, Fundamental Research Funds for the Central Universities (JKZ2011003), the National Natural Science Foundation of China (No. 81202409), Natural Science Foundation of Jiangsu Province (SBK201240392), Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education (2013) and Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (2013). [*Corresponding author] MURAOKA Osamu: Prof., E-mail: [email protected] These authors have no conflict of interest to declare. Published by Elsevier B.V. All rights reserved

Lanka and South India. The aqueous extracts of this plant have traditionally been used for the treatment of the type II diabetes in the Ayurvedic system of Indian traditional medicine [11-12]. The structure of salacinol is quite unique, bearing a permanent positive charge as the thiosugar sulfonium sulfate inner salt comprised of 1-deoxy-4-thio-D-arabinofranosyl cation and a 3’-sulfate anion, as shown in Fig. 1. The α-glucosidase inhibitory activity of 1 was tested in vitro, and it was revealed to be as potent as voglibose and acarbose, which are widely used in the clinic as antidiabetic agents these days [11-12]. Since the discovery of salacinol (1), related sulfonium sulfates, kotalanol [13] (2), ponkoranol [14] (3) salaprinol [14] (4), as well as their de-O-sulfonated sulfonium analogs, neosa-

Fig. 1 Sulfonium salts isolated from Salacia species as a new class of α-glucosidase inhibitors

XIE Wei-Jia, et al. /Chinese Journal of Natural Medicines 2013, 11(6):676−683

Fig. 2

Attempted synthesis of kotalanol (2) and the strategy to synthesize neokotalanol (6)

lacinol [15-16] (5), neokotalanol [17-19] (6), neoponkoranol [20-21] (7), and neosalaprinol [20-21] (8) were subsequently isolated from the same genus (Fig. 1). Other than 4 and 8, these sulfonium salts showed potent α-glucosidase inhibitory activities, composing a new class of naturally occurring α-glycosidase inhibitors, and much attention has been focused on the synthesis and structure-activity relationships (SAR) of these sulfonium salts [22]. In this series of natural inhibitors, kotalanol (2) and neokotalanol (6) were the most attractive targets, because the exact stereostructure of their long side chains had not been clarified until recently, when it was elucidated through the total synthesis by Jayakanthan and co-workers [23], and our degradation study of natural kotalanol (2) [24]. Interestingly, in the course of a synthetic study on kotalanol, Jayakanthan and co-workers designed a well-protected cyclic sulfate of per-

perseitol (10) as the key precursor of the coupling reaction [23]. The subsequent deprotection of the resulted sulfonium salt (11) unexpectedly caused desulfonation, and led to neokotalanol (6, Fig. 2). At about the same time, neokotalanol (6) was revealed to be the most potent inhibitor among these naturally occurring sulfonium salts [17-19]. In addition, substitution of the hydrophilic sulfate moiety on the 3’-position of salacinol (1) to hydrophobic groups was found to be an efficient way to substantially increase the α-glycosidase inhibitory activity [25-26] . For further SAR studies on neokotalanol (6), it is necessary to develop a coupling strategy which could directly format the de-O-sulfonated sulfonium salt structure (A, Fig. 2). In 2011, we succeeded in the total synthesis of neoponkoranol (7) through the coupling reaction between the perbenzylated thiosugar and a triflate of protected glucose [20].

Scheme 1 Reagents and conditions: (a) (Ph3P)3RhCl, iPr2NEt, EtOH, reflux, then HgO, HgCl2, acetone/H2O, 9/1, r.t.; (b) Ph3PCH3Br, nBuLi THF, 0 °C, then 45 °C; (c) CH2Br2, tBu4N+Br–, 50% aqueous NaOH, 60 °C; (d) AD-mix-, tBuOH, H2O, 0 °C; (e) BnBr, NaH, DMF, 0 °C–r.t.; (f) BCl3, CH2Cl2, –78–0 °C; (g) TBAF, THF–H2O, 0 °C–r.t.; (h) Tf2O, 2,6-lutidine, –200 °C

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As a continuing synthetic study on this series of compounds, this paper describes the direct synthesis of neokotalanol (6) through the coupling reaction of the protected thiosugar and a perseitol triflate derived from D-mannose.

2

Results and Discussion

In the course of synthetic studies on kotalanol (2) and neokotalanol (6), which contain the 7-carbon side chain, the authors have encountered numerous obstacles. The biggest problem addressed the stability of the polyprotected perseitol as a precursor of the coupling reaction. Due to the long side chain structure, most of the designed precursors easily underwent decomposition or intramolecular cyclization. As the 1, 3-dioxepane structure of 10 designed by Pinto and co-workers showed good stability under the coupling reaction conditions, this structure was used as the basic scaffold in the present work.

3

Preparation of Triflate (20)

In light of the elucidated side chain stereostructure of neokotalanol (6), D-mannose was selected as the starting material, and was converted to known allylmannnoside, allyl -3, 4-dibenzyl-6-O-tert-butyldiphenylsilyl-α-and-β-D-mannopyranoside [27-28] (12) in good yield. The allyl group was removed by isomerization of the terminal olefin moiety followed by hydrolysis of the resulting vinyl ether with HgO and HgCl2 to give an anomeric mixture of 3, 4-dibenzyl-6-Otert-butyldiphenylsilyl-D-mannopyranose (13) in 90% yield. The mixture was then subjected to the Wittig reaction with methyltriphenylphosphonium bromide (Ph3P+CH3Br–) to give a terminal olefin, 1-O-tert-butyldiphenylsilyl-3, 4-di-O- benzyl-D-manno-hept-6-enitol (14), in 74% yield. The olefin (14) was treated with dibromomethane (CH2Br2) in the presence of aqueous sodium hydroxide and tetra-n-butylammonium bromide (tBu4N+Br–) as a phase transfer catalyst to give a 1, 3-dioxepane derivative, 1-O-tert-butyldiphenylsilyl-3, 4-diO-benzyl-2, 5-O-methylene-D-manno-hept-6-enitol (15), in 54% yield. The acetal (15) was then subjected to the Sharpless asymmetric dihydroxylation using AD-mix-β to give a difficult to separate mixture of 3, 6-O-methylene-4, 5-di-O- benzyl-7-O-tert-butyldiphenylsilyl-D-glycero-D-galacto-heptitol (16), and its 2’-epimer (2-epi-16) in 74% yield (16/2-epi-16, ca. 8/1). After treatment of the epimeric mixture with benzyl bromide in the presence of sodium hydroxide, the major epimer,1,2,4,5-tetra-O-benzyl-3,6-O-methylene-7-O-tert-butyldiphenylsilyl-D-glycero-D-galacto-heptitol (17) was isolated in pure form with a yield of 53% in two steps. The stereochemistry of 17 was confirmed after leading to the known compound, D-glycero-D-galacto-heptitol [29-31] (18). Selective deprotection of the silyl group in compound 17 was accomplished by using TBAF in THF to give 1,2,4,5etra-O-benzyl-3, 6-O-methylene-D-glycero-D-galacto-heptitol (19) in 86% yield. Triflation of the primary hydroxyl of 19 with trifluoromethanesulfonic anhydride (Tf2O) in the pres-

ence of 2, 6-lutidine gave 1, 2, 4, 5-tetra-O-benzyl-3, 6-Omethylene-7-O-trifluoromethanesulfonyl-D-glycero-D-galactoheptitol (20) in 83% yield. Triflate 20 was found to be unstable even at r.t., and was subjected to the next coupling reaction immediately (Scheme 1). Coupling reaction between triflate (20) and thiosugar (9b) The mode of the coupling reaction between triflate 20 and thiosugar (9b) was found to be quite temperature dependent. At room temperature, no formation of the desired sulfonium salt, 2, 3, 5-tri-O-benzyl-1, 4-dideoxy-[(R)-7-deoxy-1, 2, 4, 5-tetra-O-benyl-3, 6-O-methylene-D-glycero-D-galactoheptitol-1yl]-D-arabinitol trifluoromethanesulfonate (21), was detected at all, while the starting material 20 was totally converted to a bicyclic compound, 4,7-anhydro-1,2, 5-triO-benzyl-3, 6-O-methylene-D-glycero-D-galacto-heptitol (22), accompanied by a formation of the S-benzylated thiosugar, 2, 3, 5-tri-O-benzyl-1, 4-dideoxy-1, 4-[(R)-benzyl episulfoniumylidene]-D-arabinitoltrifluoromethanesulfonate (23), in less than one hour. At lower temperature (0 °C), the coupling reaction progressed slowly, and it took one week for the starting material (20) to be totally consumed. The desired sulfonium salt (21, 24% from 20) was obtained together with bicyclic compound (22, 70% from 20) and sulfonium salt (23, 70% from 20) (Scheme 2). When the reaction temperature was set below –20 °C, the reaction did not proceed, and the reactants were totally recovered. A plausible mechanism of formation of the bicyclic compound 22 is presented in Fig. 3. In this reaction, two different modes (routes a and b, in Figure 3) of nucleophilic attack were proposed to take place. At 0 °C (route a), direct attack of the sulfur atom of the thiosugar 9b to the methylene carbon at C-7 bearing the leaving group (TfO–) took place through the thermodynamically stable conformer 20-A (–390 kcal·mol −1) [32] providing the desired sulfonium salt 21, while at room temperature, a kinetically advantageous intramolecular cyclization of 20, which was triggered by the attack of the sulfur atom of 9b on the methylene carbon of the benzyloxy moiety at C-4 of compound 20 (route b), mainly proceeded to provide bicyclic compound 22. The formation of product 23 well supported the reaction mechanism. Thus, it is of interest to note that the chemoselectivity in the reaction between 20 and 9b entirely varied depending on the temperature. The FAB mass spectrum of 22, run in a positive ion mode, showed a peak at m/z 499 corresponding to the sodium adduct ion of the molecule [M + Na]+, which lost one benzyl moiety from reactant 20. A NOE correlation between H-7b and the endo-proton on the acetal moiety, as well as HMBC correlations between positions 4 and 7, supported the structure 22. On the other hand, the FAB mass spectrum of the sulfonium salt 23 in a positive ion mode showed a peak at m/z 511, which indicated the introduction of one more benzyl moiety to reactant 9b. The negative-ion FAB mass spectrum of 23 showed a peak at m/z 149, corresponding to the triflate anion moiety.

XIE Wei-Jia, et al. /Chinese Journal of Natural Medicines 2013, 11(6):676−683

Scheme 2

Fig. 3

Scheme 3

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Reagents and conditions: thiosugar (9b), THF, –5–0 °C, 1 week

Plausible mechanism of the coupling reaction between triflate 20 and thiosugar 9b

Reagents and conditions: (a) BCl3, CH2Cl2, –78 °C; (b) IRA 400 J (Cl– form), MeOH, r.t.

Deprotection of Sulfonium Salt (21)

The resulted sulfonium salt (21) was then treated with BCl3 in CH2Cl2 to remove all the protecting groups, and subsequent treatment of the product with ion exchange resin IRA 400 J (Cl– form) to change any anions to chlorine gave the target compound (6) in 60% yield in two steps (Scheme 3). The 1H and 13C NMR spectra of 6 were in good accord with those of an authentic specimen isolated from Salacia reticulata [17-19].

5

Conclusions

Neokotalanol (6), the de-O-sulfonated sulfonium salt of kotalanol (2) bearing a 7-carbon-polyhydroxylated side chain, was synthesized by the direct coupling reaction between the perbenzylated thiosugar (9b) and a perseitol triflate (20) as the key reaction. It is interesting to note that the coupling reaction between these two substrates proceeded via different two conformations of 20, depending on the reaction temperature. A characteristic intramolecular cyclization via an

unstable conformation of 20 leading to the trioxabicyclic compound (22) competed with the desired direct coupling between 20 and 9b, even at 0 °C. The above coupling strategy to construct the 3’-de-O-sulfonated sulfonium salt is of great importance for SAR studies on this type of α-glucosidase inhibitor. Further optimization of the thiosugar - triflate coupling reaction, and the structure modification of neokotalanol using this strategy as the key reaction, are now in progress.

6

Experimental

Mps were determined on a Yanagimoto MP-3S micromelting point apparatus, and mps and bps are uncorrected. IR spectra were measured on a Shimadzu FTIR-8600PC spectrophotometer. NMR spectra were recorded on a JEOL JNM-ECA 500 (500 MHz 1H, 125 MHz 13C) or a JEOL JNM-ECA 600 (600 MHz 1H, 150 MHz 13C) or a JEOL JNM-ECA 700 (700 MHz 1H, 175 MHz 13C) spectrometer. Chemical shifts (δ) and coupling constants (J) are given in

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ppm and Hz, respectively. Low-resolution and high- resolution mass spectra were recorded on a JEOL JMS-HX 100 spectrometer. Optical rotations were determined with a JASCODIP-370 digital polarimeter. Column chromatography was effected over Fuji Silysia silica gel BW-200. All the organic extracts were dried over anhydrous sodium sulfate prior to evaporation. 3, 4-Dibenzyl-6-O-tert-butyldiphenylsilyl-α-and -β-Dmannopyranose (13) To a solution of 12 [27-28] (7.0 g, 11.0 mmol) in EtOH (150 mL) was added (Ph3P)3RhCl (0.55 g) and iPr2NEt (17 mL), and the mixture was heated under reflux for 1.5 h. After removal of the solvent under reduced pressure, the residue was diluted with CH2Cl2 (200 mL), and the resulting mixture was washed with brine, and concentrated to give a brown oil (6.7 g), which was then treated with HgO (4.5 g) and HgCl2 (4.5 g) in a mixture of acetone/water (9/1, 400 mL) at r.t. for 12 h. The reaction mixture was evaporated under reduced pressure, and the residue was diluted with water (200 mL). The resulting mixture was extracted with ether. The extract was washed with brine and condensed under reduced pressure to give a pale brown oil (6.8 g), which on column chromatography (n-hexane/AcOEt, 15/1) gave the title compound (13, 5.9 g, 90%) as a pale yellow oil. 1-tert-Butyldiphenylsilyl-3, 4-di-O-benzyl-D-mannohept-6-enitol (14) To a solution of methyltriphenylphosphonium bromide (Ph3PCH3Br, 17.6 g, 49.3 mmol) in THF (160 mL) was added 1.6 mol·L−1 nBuLi (44 mL, 70 mmol) at 0 °C. The mixture was stirred at r.t. for 2.5 h. To the resulting mixture was added dropwise a solution of 13 (4.9 g, 8.2 mmol) in THF (100 mL) at 0 °C, and the mixture was stirred at 45 °C for 30 min. After the reaction was quenched by the addition of acetone, the resulting mixture was diluted with water, and concentrated under reduced pressure. The aqueous residue was extracted with ether, washed with brine, and condensed to give a crude (5.6 g) as a yellow oil, which, on column chromatography (n-hexane/AcOEt, 15/1), gave the title compound (14, 3.6 g, 74% yield) as a pale yellow oil. [α]25D +8.4 (c 1.48, CHCl3). IR (neat): 3 435, 2 928, 2 359, 1 471, 1 456, 1 429, 1 213, 1 096, 1 074 cm–1. 1H NMR (700 MHz, CDCl3) δ: 1.06 [9H, s, C(CH3)3], 3.70 (1H, dd J = 5.2, 3.0, H-4), 3.77 (1H, dd J = 10.2, 5.0, H-1a), 3.81 (1H, dd J = 10.2, 5.5, H-1b), 3.93 (1H, dd J = 6.5, 3.0, H-3), 4.01 (1H, ddd-like, J = 6.5, 5.5, 5.0, H-2), 4.48 (1H, dddd, J = 5.2, 5.2, 1.6, 1.6, H-5), 4.52/4.67 (each 1H, d, J = 11.3, PhCH2), 4.56 (2H, s, PhCH2), 5.23 (1H, ddd, J = 10.8, 1.6, 1.6, H-7a), 5.41 (1H, dd J =17.0, 1.6, 1.6, H-7b), 5.91(1H, ddd, J = 17.0, 10.8, 5.2, H-6), 7.18–7.65 (20H, m, arom.). 13C NMR (175 MHz, CDCl3) δ: 19.2 [C(CH3)3], 26.9 [C(CH3)3], 64.8 (C-1), 71.1 (C-2), 71.8 (C-5), 72.6/73.0 (PhCH2), 78.0 (C-3), 80.1 (C-4), 116.2 (C-7), 137.8 (C-6), 127.8/127.9/128.3/128.35/128.41/ 128.43/129.9/135.57/ 135.62 (d, arom.), 132.98/133.04/ 137.45/137.51 (s, arom.). FABMS m/z 619, [M + Na]+ (pos.).

1-tert-Butyldiphenylsilyl-2, 5-O-methylene-3, 4-di-Obenzyl-D-manno-hept-6-enitol (15) To a solution of 14 (3.6 g, 6.0 mmol) and nBu4N+Br(250 mg) in CH2Br2 (60 mL) was added 50% aqueous NaOH solution (130 g), and the mixture was stirred at 60 °C for 45 min. After addition of CH2Cl2 (150 mL), the resulting mixture was diluted with water (150 mL), and washed with brine. The organic layer was condensed under reduced pressure to give a yellow oil (4.2 g), which, on column chromatography, gave the title compound (15, 2.0 g, 54%) as a pale yellow oil. [α] 25 D −29.7 (c 0.7, CHCl3). IR (neat): 3 069, 2 930, 2 888, 2 357, 1 472, 1 456, 1 427, 1 213, 1 096, 1 073 cm–1. 1H NMR (700 MHz, CDCl3) δ: 1.05 [9H, s, C(CH3)3], 3.42 (1H, dd, = 9.5, 7.5, H-4), 3.71 (1H, ddd, J = 9.8, 4.5, 2.0, H-2), 3.73 (1H, dd, J = 9.8, 7.5, H-3), 3.90 (1H, dd, J = 10.0, 4.5, H-1a), 3.95 (1H, dd, J = 10.0, 2.0 H-1b), 4.19 (1H, dddd-like, J = 9.5, 6.0, 1.5, 1.5, H-5), 4.67/4.73 (each 1H, d, J = 10.5, PhCH2), 4.69/4.82 (each 1H, d, J = 10.9, PhCH2), 4.80/4.87 (each 1H, d, J = 4.4, OCH2O), 5.28 (1H, ddd, J = 10.5, 1.5, 1.5, H-7a), 5.44 (1H, ddd, J = 17.0, 1.5, 1.5, H-7b), 6.07 (1H, ddd, J = 17.0, 10.5, 6.0, H-6), 7.23–7.73 (20H, m, arom.). 13C NMR (175 MHz, CDCl3) δ: 19.3 [C(CH3)3], 26.7 [C(CH3)3], 63.8(C-1), 74.9/75.1 (PhCH2), 75.0 (C-5), 75.6 (C-2), 81.8 (C-3), 86.7 (C-4), 92.7 (OCH2O), 116.9 (C-7), 135.8 (C-6), 127.5/127.6/127.69/127.73/127.9/128.3/128.4/129.6/135.6/ 135.9 (d, arom), 133.3/133.7/ 138.2/138.3 (s, arom.). FABMS m/z 631, [M + Na]+ (pos.).

7

Dihydroxylation of Terminal Olefin (15)

To a solution of 15 (1.4 g, 2.3 mmol) in a mixture of tert-butyl alcohol and water (1/1, 10 mL) was added AD-mix-β (3.5 g). The reaction mixture was stirred vigorously at 0 °C for 7 days. The reaction was quenched by the addition of solid sodium sulfite (4 g), and the resulting mixture was extracted with AcOEt (3 × 100 mL). The extract was washed successively with water (50 mL) and brine (50 mL), and condensed under reduced pressure to give a pale yellow oil (1.56 g), which, on column chromatography, (n-hexane/AcOEt, 10/1) gave a ca. 8/1 mixture of (16/2-epi16, 1.1 g, 74%) as a pale yellow oil. Benzylation of diols 16/2-epi-16 To a mixture of sodium hydride (NaH, 375 mg, 9.4 mmol, 60% in liquid paraffin) and benzyl bromide (BnBr, 1.36 mL, 11.2 mmol), and dry DMF (8 mL) was added a solution of 16/2-epi-16 (1.0 g) in dry DMF (5 mL) at 0 °C. After stirring at rt for 2 h, the reaction mixture was poured into ice-water, and extracted with ether. The extract was washed with brine, and condensed under reduced pressure to give a yellow oil (1.4 g), which, on column chromatography (n-hexane/AcOEt, 20/1), gave 7-O-tert-butyldiphenylsilyl-1, 2,4,5-tetra-O-benzyl-3,5-O-methylene-D-glycero-D-galactoheptitol (17, 0.91 g, 53% from 15) and a ca. 1/1 mixture of (17/2-epi-17, 0.26 g, 15% from 15).

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Compound 17: Colorless oil. [α]25D −3.7 (c 0.82, CHCl3). IR (neat): 1 497, 1 454, 1 427, 1 397, 1 096, 1 090, 1 076, 1 043 cm-1. 1H NMR (700 MHz, CDCl3) δ: 1.05 [9H, s, C(CH3)3],3.70 (1H, dd, J = 9.2, 6.8, H-1a), 3.74 (1H, ddd, J = 9.8, 4.3, 2.0, H-6), 3.76 (1H, dd, J = 9.8, 6.7, H-5), 3.80(1H, dd, J = 9.2, 5.2, H-1b), 3.91 (1H, dd, J = 10.8, 4.3, H-7a), 3.90-3.93 (2H, m, H-3 and H-4), 3.94 (1H, dd, J = 10.8, 2.0, H-7b), 4.20 (1H, ddd, J = 6.8, 5.2, 1.0, H-2), 4.50/4.78 (each 1H, d, J = 11.6, PhCH2), 4.51/4.56 (each 1H, d, J = 11.8, PhCH2), 4.59/4.88 (each 1H, d, J = 11.4, PhCH2),4.64/4.80 (each 1H, d, J = 4.2, OCH2O), 4.70/4.73 (each 1H, d, J = 10.8, PhCH2), 7.05–7.73 (20H, m, arom.). 13C NMR (175 MHz, CDCl3) δ: 19.3 [C(CH3)3], 26.9 [C(CH3)3], 63.7(C-7), 69.2 (C-1), 72.3/73.5/74.2/74.5 (PhCH2), 73.3 (C-3), 75.4 (C-6), 76.1 (C-2), 81.5 (C-4), 82.8 (C-5), 93.4 (OCH2O), 127.0/127.36/127.45/127.52/127.55/127.62/127.66/127.70/12 7.75/127.81/128.27/128.36/128.44/129.5/129.6/135.6/135.9 (d, arom.), 133.3/133.7/138.0/138.2/138.4/138.8 (s, arom.). FABMS m/z 845, [M + Na]+ (pos.). 1, 2, 4, 5-Tetra-O-benzyl-3, 6-O-methylene-D-glycero-D-galacto-heptitol (19) To a solution of 17 (0.9 g, 1.1 mmol) in THF (10 mL) was added dropwise 1.0 mol·L−1 TBAF in THF (10 mL) at 0 °C. After being stirred at r.t. for 12 h, the reaction mixture was condensed under reduced pressure. The residue was diluted with water, and extracted with AcOEt. The extract was washed with brine and condensed under reduced pressure to give a pale yellow oil (0.85 g), which, on column chromatography, gave the title compound (19, 0.55 g, 86%) as a pale yellow oil. [α]25D −17.5 (c 0.4, CHCl3). IR (neat): 3 030, 2 959, 1 497, 1 454, 1 395, 1 362, 1 261, 1 121, 1 092, 1 082, 1 069, 1 045 cm–1. 1H NMR (700 MHz, CDCl3) δ: 3.66 (1H, dd, J = 8.4, 7.9, H-5), 3.68 (1H, dd, J = 9.1, 7.0, H-1a), 3.73–3.76 (2H, m, H-6 and H-7a), 3.77 (1H, dd, J = 9.1, 5.2, H-1b), 3.88 (1H, dd, J = 9.8, 1.6, H-3), 3.88-3.91 (1H, m, H-7b), 3.93 (1H, dd, J = 9.8, 7.9, H-4), 4.16 (1H, ddd, J = 7.0, 5.2, 1.6, H-2), 4.48/4.55 (each 1H, d, J = 11.8, PhCH2), 4.49/4.76 (each 1H, d, J = 11.6, PhCH2), 4.59/4.89 (each 1H, d, J = 11.4, PhCH2), 4.62/4.82 (each 1H, d, J = 4.3, OCH2O), 4.69/4.78 (each 1H, d, J = 10.8, PhCH2), 7.22–7.73 (20H, m, arom.). 13C NMR (175 MHz, CDCl3): δ: 62.8 (C-7), 68.8 (C-1), 72.4/73.5/ 74.1/74.8 (PhCH2), 73.7 (C-3), 74.3 (C-6), 75.9 (C-2), 81.1 (C-4), 83.3 (C-5), 93.4 (OCH2O), 127.5/127.6/127.7/127.8/127.9/128.0/128.1/128.2/128.3/128. 5 (d, arom.), 137.9/138.0/138.3/138.7 (s, arom.). FABMS m/z 607, [M + Na]+ (pos.). Coupling reaction between triflate (20) and thiosugar (9b) Under an argon atmosphere, Tf2O (0.22 mL, 1.3 mmol) was added to a solution of 2, 6-lutidine (0.15 mL, 1.3 mmol) in CH2Cl2 (10 mL) at –20 °C, and the mixture was stirred at that temperature for 5 min. To the resulting mixture was added dropwise a solution of 19 (0.5 g, 0.86 mmol) in CH2Cl2 (10 mL) at –20 °C. After being stirred at 0°C for 30

min, the reaction mixture was poured into ice-cooled water and extracted with CH2Cl2. The extract was condensed under reduced pressure to give a pale yellow oil (0.61 g), which, on column chromatography (n-hexane/AcOEt, 20/1), gave the triflate (20, 0.51 g, 83%) as a pale yellow oil. Owing to the instability of 20, even at rt, it was subjected to the coupling reaction with thiosugar (9b) immediately. Thus, under an argon atmosphere, to a solution of triflate (20, 0.45 g, 0.62 mmol) in THF (3 mL) was added a solution of thiosugar (9b, 0.78 g, 1.86 mmol) in THF (2 mL) at –15 °C, and the mixture was stirred at 0 °C for 7 days. The reaction mixture was condensed under reduced pressure to give a pale yellow oil (1.20 g), which, on column chromatography (CHCl3/MeOH, 200/1) gave 2,3,5-tri-O-benzyl-1,4-dideoxy-[(R)-7-deoxy- 1,2,4,5tetra-O-benzyl-3,6-O-methylene-D-glycero-D-galacto-heptito l-1yl]-D-arabinitol trifluoromethanesulfonate (21, 0.17 g, 24% from 20), together with the bicyclic compound, 4, 7-anhydro-1, 2, 5-tri-O-benzyl-3,6-O-methylene-D-glyceroD-galacto-heptitol (22, 210 mg, 70% from 20) and 1, 4-dideoxy-1, 4-[(R)-benzylepisulfoniumylidene]-D-arabinitoltrifluoromethanesulfonate (23, 290 mg,70% from 20) as pale yellow oils. Compound 21: [α]25D −1.8 (c 0.65, CHCl3). IR (neat): 1 497, 1 454, 1 360, 1 279, 1 263, 1 147, 1 117, 1 088, 1 030 cm–1. 1H NMR (700 MHz, CDCl3) δ: 3.57–3.63 (2H, m, H-5a/H-5b), 3.58 (1H, dd, J = 9.5, 8.0, H-3’), 3,61 (1H, dd, J = 9.0, 5.5, H-7’a), 3.64 (1H, dd, J = 12.5, 9.5, H-1’a), 3.69 (1H, dd-like, J = 12.5, 3.6, H-1a), 3.70 (1H, dd, J = 12.5, 2.5, H-1’b), 3.75 (1H, dd, J = 9.0, 5.5, H-7’b), 3.83 (1H, dd. J = 9.6, 1.5, H-5’), 3.91 (1H, dd, J = 9.6, 8.0, H-4’), 3.98 (1H, ddd, J = 9.5, 9.5, 2.5, H-2’), 4.09 (1H, ddd, J = 5.5, 5.5, 1.5, H-6’), 4.11 (1H, br d-like, J = 12.5, H-1b), 4.12 (1H, br m, H-3), 4.44–4.83 (14H, m, PhCH2), 4.47/4.66 (each 1H, d, J = 4.4, OCH2O), 4.56 (1H, br m, H-2), 7.13–7.82 (35H, m, arom.). 13C NMR (175 MHz, CDCl3) δ: 48.4 (C-1), 49.4 (C-1’), 66.4 (C-5), 66.8 (C-4), 68.5 (C-7’), 70.1 (C-2’), 72.17/72.22/72.6/73.5/73.7/73.9/75.1(PhCH2), 74.6 (C-5’), 75.7 (C-6’), 81.1 (C-4’), 82.2 (C-2), 82.7 (C-3), 84.2 (C-3’), 93.3 (OCH2O), 126.9/127.45/127.53/127.59/127.64/127.7/ 127.85/127.87/127.96/127.99/128.07/128.12/128.25/128.28/1 28.31/128.38/128.43/128.46/128.49/128.68/128.72/128.86/13 3.4/133.5/135.06/135.08(d, arom.), 135.8/135.9/136.3/ 137.4/ 137.7/138.0/138.1(s, arom.). FABMS m/z 987 [M – CF3SO3–]+ (pos.), 149 [CF3SO3]– (neg.). Compound 22: [α]25D −5.67 (c 1.50, CHCl3). IR (neat): 1 497, 1 456, 1 369, 1 340, 1 215, 1 157, 1 144, 1 086, 1 024 cm–1. 1H NMR (700 MHz, CDCl3) δ: 3.51 (1H, dd, J = 10.8, 5.2, H-1a), 3.65 (1H, dd, J = 10.8, 4.0, H-1b), 3.91 (1H, dd, 10.9, 4.0, H-7a), 3.92 (1H, ddd-like, J = 7.8, 5.2, 4.0, H-2), 4.06 (1H, d, J = 7.8, H-3), 4.14 (1H,d-like, J = 3.5, H-4), 4.15 (1H, dd, J = 10.9, 4.0, H-7b), 4.43/4.46 (each 1H, d, J = 11.6, PhCH2), 4.44 (1H, dd, J = 4.0, 4.0, H-6), 4.47 (1H, d, J = 3.5, H-5). 4.51/4.72 (each 1H, d, J = 12.0, PhCH2), 4.66/4.76 (each 1H, d, J = 11.6, PhCH2), 5.08/5.21 (each 1H, d, J = 8.0, OCH2O), 7.23–7.34 (15H, m, arom.). 13C NMR (175 MHz,

XIE Wei-Jia, et al. /Chinese Journal of Natural Medicines 2013, 11(6): 676−683

CDCl3) δ: 69.3 (C-7), 71.0 (C-1), 72.3/73.2/73.5 (PhCH2), 74.0 (C-6), 76.6 (C-5), 77.9 (C-2), 81.0 (C-4), 86.3 (C-3), 89.7 (OCH2O), 127.4/127.5/127.6/127.9/128.2/ 128.3/ 128.4 (d, arom.), 137.6/138.3/138.8 (s, arom.). FABMS m/z 499, [M + Na]+ (pos.). Compound 23: Colorless oil, [α]25D –1.6 (c 1.26, CHCl3). IR (neat): 1 497, 1 454, 1 366, 1 339, 1 273, 1 215, 1 154, 1 088, 1 026 cm−1. 1H NMR (700 MHz, CDCl3) δ: 3.58 (1H, dd, J = 10.2, 10.2, H-5a), 3.66 (1H, dd, J = 13.4, 3.8, H-1a), 3.67 (1H, dd, J = 10.2, 5.8, H-5b), 3.89 (1H, br dd, J = 10.2, 5.8, H-4), 4.17 (1H, dd-like, J = ca. 1.5, 1.5, H-3), 4.28 (1H, dd, J = 13.4, 1.4, H-1b), 4.30/4.34 (each 1H, d, J = 11.8, PhCH2), 4.41/4.58 (each 1H, d, J = 11.8, PhCH2O), 4.48–4.50 (1H, m, H-2), 4.49/4.55 (each 1H, d, J = 11.8, PhCH2O), 4.63/5.04 (each 1H, d, J = 12.7, PhCH2S+), 7.04–7.39 (20H, m, arom.). 13 C NMR (175 MHz, CDCl3) δ: 46.0 (C-1), 49.3 (PhCH2S+), 63.2 (C-4), 66.8 (C-5), 72.0/ 72.6/73.4 (PhCH2O), 82.1 (C-2), 83.2 (C-3), 120.7 (q, J = 320, CF3SO3–), 127.7/128.1/128.50/ 128.54/128.7/128.8/128.9/129.7/130.3/130.7 (d, arom.), 127.6/135.7/135.9/136.5 (s, arom.). FABMS m/z 511 [M – CF3SO3–]+ (pos.), 149 [CF3SO3]– (neg.). Deprotection of sulfonium salt (21) To a solution of compound 21 (0.05 g, 0.04 mmol) in CH2Cl2 (2 mL) was added 1.0 mol·L1 BCl3 in CH2Cl2 (1.0 mL) at –78 °C. The mixture was allowed to reach room temperature, and stirred for 12 h. The reaction was quenched by the addition of MeOH, and the resulting mixture was condensed under reduced pressure. The residue was dissolved in water (2 mL), and the resulting solution was washed with CH2Cl2 (3 × 2 mL). The water layer was condensed under reduced pressure to give a colorless oil, which was treated with ion exchange resin IRA 400J (Cl– form, 100 mg) at r.t. for 3 h. The resins were filtered off and washed with methanol. The combined filtrate and the washings were condensed to give a colorless oil, which on column chromatography (CHCl3/MeOH/water, 20/4/1) gave neokotalanol (6, 10 mg, 60%), the 1H and 13CNMR spectroscopic properties of which were in accord with those reported [17-19].

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

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五层龙属植物中提取的具有 α-葡萄糖苷酶抑制活性的天然产物 neokotalanol 的全合成 谢唯佳 1，田边元三 2，筒井望 2，吴晓明 1，村冈修 2* 1

中国药科大学药学院，南京 210009，中国;

2

近畿大学药学部，大阪府东大阪市小若江 3-4-1, 577-5802，日本

【摘 要】 以苄基保护的硫糖和由 D-甘露糖为起始原料, 合成得到的三氟甲磺酸酯间的偶合反应作为关键反应合成了从五 层龙属植物根茎部提取的具有很强α-葡萄糖苷酶抑制活性的天然产物 neokotalanol。研究发现该关键的偶合反应的产物是由反应 温度直接决定的，反应除了得到目标锍糖化合物的同时, 还分离出了通过经典的分子内成环反应所得到 2, 4, 7-三氧代二环壬烷 类衍生物。 【关键词】 Neokotalanol; 全合成; 五层龙; α-葡萄糖苷酶抑制剂 【基金项目】 中央高校基本科研业务费(JKZ2011003), 国家自然科学基金青年基金(No. 81202409), 江苏省自然科学基金青年基 金 (SBK201240392), 教育部留学回国人员启动经费（2013）, 人事部留学人员科技活动项目择优资助经费（2013）