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Total synthesis of (−)-phaeosphaeride B by a biomimetic conversion from (−)-phaeosphaeride A
Accepted Manuscript Total synthesis of (−)-phaeosphaeride B by a biomimetic conversion from (−)phaeosphaeride A Kenichi Kobayashi, Risako Kunimura, Kosaku Tanaka, III, Osamu Tamura, Hiroshi Kogen PII:
S0040-4020(17)30246-6
DOI:
10.1016/j.tet.2017.03.020
Reference:
TET 28526
To appear in:
Tetrahedron
Received Date: 28 January 2017 Revised Date:
6 March 2017
Accepted Date: 7 March 2017
Please cite this article as: Kobayashi K, Kunimura R, Tanaka III K, Tamura O, Kogen H, Total synthesis of (−)-phaeosphaeride B by a biomimetic conversion from (−)-phaeosphaeride A, Tetrahedron (2017), doi: 10.1016/j.tet.2017.03.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Total synthesis of (–)-phaeosphaeride B by a biomimetic conversion from (–)-phaeosphaeride A
Graduate School of Pharmaceutical Sciences, Meiji Pharmaceutical University, 2-522-1 Noshio,
Kiyose, Tokyo 204-8588, Japan b
Showa Pharmaceutical University, 3-3165 Higashi-tamagawagakuen, Machida, Tokyo 194-8543,
Japan * Corresponding authors. E-mail address: [email protected] (K. Kobayashi).
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E-mail address: [email protected] (H. Kogen).
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a
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Kenichi Kobayashi a, *, Risako Kunimura a, Kosaku Tanaka III a, Osamu Tamura b, Hiroshi Kogen a, *
Abstract We have accomplished the first total synthesis of STAT3 inhibitory (–)-phaeosphaeride A and its stereoisomer (–)-phaeosphaeride B. This work confirms the configurational assignment of these natural products. Notably, TFA-mediated dehydrative stereoinversion of phaeosphaeride A afforded phaeosphaeride A.
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phaeosphaeride B, based on our hypothesis of the biosynthesis mechanism of phaeosphaeride B from
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Keywords Phaeosphaeride A, Phaeosphaeride B, Total synthesis, Asymmetric synthesis, Biomimetic conversion
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1. Introduction Phaeosphaerides A (proposed structure 1) and B (2) are nitrogen-containing bicyclic natural products that were isolated from the endophytic fungus FA39 (Phaeosphaeria avenaria) by Clardy and
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co-workers in 2006 (Figure 1).1 Phaeosphaeride A is an inhibitor of STAT3-DNA binding with an IC50 of 0.61 mM and also inhibits cell growth in STAT3-dependent U266 multiple myeloma cells with an IC50
Structures of phaeosphaerides.
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Fig 1.
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of 6.7 µM in vitro, whereas its stereoisomer phaeosphaeride B (2) has no STAT3 inhibitory activity.
In 2011 and 2012, our group2 and Sarli’s group3 independently demonstrated the total synthesis of the proposed structure of phaeosphaeride A (1), which revealed that the structure of phaeosphaeride A was misassigned. Stereochemical considerations suggested that the correct structure of phaeosphaeride A is C-7 epimer 3 of the originally proposed structure (1) or its enantiomer (4). The first total synthesis of 3 was achieved by our group in 2015, providing a corrected configuration (4) for phaeosphaeride A,4 and this result was supported by the X-ray crystal structure of natural phaeosphaeride A reported by Abzianidze et al.5 During the course of these studies, Sarli and colleagues and Abzianidze et al. also conducted
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biological studies of their synthetic compounds.3,6,7 Sarli and colleagues synthesized and biologically evaluated the stereoisomers of phaeosphaeride A, which suggested that these compounds are upstream inhibitors of a tyrosine kinase in the JAK/STAT pathway.6 Abzianidze et al. prepared the C-6 acyl derivatives from the isolated natural phaeosphaeride A, and a chloroacetyl derivative exhibited more
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potent cytotoxicity (EC50 = 33 ± 7 µM) against A549 cancer cells than natural phaeosphaeride A (EC50 = 46 ± 5 µM).7 Their studies highlighted phaeosphaeride A as a potential seed compound for STAT3 inhibitors in anti-cancer drug discovery,8 and further screening of unnatural phaeosphaeride A derivatives may provide compounds that are more biologically active. Furthermore, the ambiguous biosynthetic pathway for phaeosphaerides A and B was expected to be clarified. Based on the structures
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of the phaeosphaeride family, we assumed that proton-mediated biosynthetic conversion occurred
Possible biosynthetic conversion between phaeosphaerides A and B.
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Scheme 1.
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between phaeosphaerides A and B (Scheme 1).
In this paper, we describe the total synthesis of (–)-phaeosphaeride B (2) by a hypothetical biomimetic conversion from (–)-phaeosphaeride A through a TFA-mediated stereoinversion.
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2. Results and discussion 2.1.Synthesis of (–)-phaeosphaerides A To synthesize (–)-phaeosphaeride B (2) from (–)-phaeosphaeride A (4) by a hypothetical
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stereoinversion at C-6, we first prepared (–)-phaeosphaeride A (4) by following our synthetic path to ent-phaeosphaeride A (3).4 The synthesis of (–)-phaeosphaerides A (4) began with Sharpless asymmetric dihydroxylation of known (Z)-5,4 synthesized by the Z-selective Horner−Wadsworth−Emmons reaction (Scheme 2).9 Ester (Z)-5 was dihydroxylated to (2S,3S)-diol 6, an enantiomer of the known diol,4 by Sharpless asymmetric dihydroxylation using AD-mix-α10 in high yield (87%) and enantioselectivity
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(98% ee, determined by modified Mosher’s method).11
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obtain 8 in 57% yield (two steps).
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According to our synthetic scheme for 3, diol 6 was converted to ester 8. The two hydroxy groups in 6 were protected with a Bn group12 for the secondary alcohol and a MOM group for the tertiary alcohol to
Scheme 2.
Synthesis of ester 8.
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Ester 8 was reduced with DIBALH to provide the alcohol in 86% yield, which was then protected to give the TIPS ether in quantitative yield (Scheme 3). Removal of the Bn group by hydrogenation using catalytic Pd/C produced desired secondary alcohol 9 in 99% yield. Oxa-Michael addition of alcohol 9 to 10 using catalytic n-BuLi gave (E)-11 in 86% yield. TIPS ether (E)-11 was then converted to aldehyde
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12 by desilylation with HF-pyridine to give an alcohol, followed by Dess-Martin oxidation. To construct a dihydropyran ring, aldehyde 12 was subjected to a sodium hexamethyldisilazide-mediated vinyl anion aldol reaction to generate desired product 13a (56%), its TMS ether 13b (4%), and C-6 diastereomer byproduct 13c (15%). During this reaction, alcohol 13a and TMS ether 13b appeared to be stereoselectively formed through the transition state shown in parentheses in Scheme 3.13 W-type
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long-range coupling14 between H-6 and H-8 in 13a and 13b was observed to confirm the required S configuration at the C-6 stereocenter.
Scheme 3.
Synthesis of dihydropyran derivatives 13a-c.
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Without protection of the C-6 alcohol in 13a, exposure of diester 13a to 1 M aqueous NaOH in MeOH caused regioselective hydrolysis of one of the two ester groups to afford a mono acid,15 which was immediately subjected to amidation with MeONH2 (14) to give maleimide derivative 15 with spontaneous cyclization in 48% yield, accompanied by amide 16 in 22% yield (Scheme 4). Amide 16
by HCl in 1,4-dioxane. The 1H and
13
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was easily converted to 15 in 81% yield by treating with Et3N in DMF at 50 °C. Total synthesis of 4 was achieved by exo-methylene formation through methylation by using MeMgBr and dehydration mediated C NMR spectra of synthetic 4 matched the literature data1 for
natural phaeosphaeride A, and the optical rotation of 4 ([α]D25 –98.6 (c 0.85, CH2Cl2)) was the same sign as the natural product ([α]D25 –93.6 (c 2.0, CH2Cl2)). Thus, we identified the structure of natural
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phaeosphaeride A as compound 4 through its total synthesis.5
Scheme 4.
Completion of the total synthesis of phaeosphaeride A (4).
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2.2.Hypothetical biosynthetic pathway from phaeosphaeride A to phaeosphaeride B With synthetic phaeosphaeride A (4) in hand, our next challenge was preparing and assigning the configuration of phaeosphaeride B.1,6,16 We assumed that a biosynthetic transformation occurred from
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phaeosphaeride A (4) to phaeosphaeride B (2) (Scheme 5). When exposed to a protic acid, phaeosphaeride A (4) would be dehydrated; the oxygen lone pair would help to form the oxonium cation A, and then a nucleophile (such as H2O) would preferentially attack the C-6 center from the more accessible β-face to afford phaeosphaeride B (2) as the major product. In addition, phaeosphaeride B (2) would be sterically favored over phaeosphaeride A (4), because the X-ray crystal structure of
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phaeosphaeride A (4) suggested steric repulsion between the C-1 carbonyl group and the pseudo-equatorial oriented C-6 hydroxy group as a result of the nearly planar structure of the
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phaeosphaeride skelton.5
Scheme 5.
Our proposed biosynthesis of phaeosphaeride B (2) from phaeosphaeride A (4).
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2.3.Biomimetic transformation from phaeosphaerides A to phaeosphaerides B Based on our hypothesis, we tried a protic acid-mediated dehydrative nucleophilic substitution at the
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C-6 stereocenter (Scheme 6). Upon treating synthetic 4 with TFA, starting alcohol 4 was fully consumed to form less polar compounds, as monitored by thin-layer chromatography.18 After concentration, the crude products were treated with aqueous NaHCO3 in THF to afford phaeosphaerides B (2) and A (4) in 75% and 19% yields, respectively. The spectroscopic data for synthetic 2, including 1H and 13C NMR spectra, fully matched those reported by Clardy and colleagues1 and Sarli and colleagues.6 The [α]D
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value of synthetic 2 ([α]D25 –142.8 (c 0.43, CH2Cl2)) has the same sign as the natural product ([α]D25 – 157.2 (c 2.0, CH2Cl2));1 therefore, we established the absolute configuration of natural phaeosphaeride B
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as shown in Scheme 6.
Scheme 6.
Biomimetic transformation from phaeosphaeride A (4) to phaeosphaeride B (2).
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Synthetic compound 12 was similarly converted to 17 in 83% yield, accompanied by recovery of 1 in 12% yield (Scheme 7). The 1H and 13C NMR spectra for 17 corresponded to the literature data reported by Sarli and colleagues.6 Additionally, the absolute configuration of synthetic 17 was identified as
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shown in Scheme 7, because the optical rotation of 17 ([α]D25 +167.8 (c 0.46, CH2Cl2)) has the opposite sign to the literature data reported by Sarli and colleagues ([α]D25 –13.5 (c 2.0, CH2Cl2)).6,9 Consequently,
Conversion of synthetic 1 to 17.
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3. Conclusion
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Scheme 7.
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compound 17 is formed through stereochemical inversion at C-6 from compound 1.
In conclusion, we have accomplished the first total synthesis of STAT3 inhibitory phaeosphaeride A (4) and its inactive stereoisomer, phaeosphaeride B (2). This synthesis enabled us to identify the relative and absolute configurations of these natural products. Additionally, we identified the remarkable biomimetic transformation from phaeosphaeride A (4) to phaeosphaeride B (2), and from compound 1 to 17, by trifluoroacetylation with inversion at the C-6 stereocenter followed by basic hydrolysis. This work confirmed the hypothetical biosynthesis of phaeosphaeride B (2) from phaeosphaeride A (4). We are currently working on the synthesis and biological evaluation of phaeosphaeride A analogues, and the structure-STAT3 inhibitory activity relationships will be reported at a future date.
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4. Experimental section 4.1. General 1
H NMR spectra were measured on a JEOL JNM-AL500 (500 MHz) instrument. The chemical shifts
are expressed in ppm relative to tetramethylsilane (δ = 0) as an internal standard (CDCl3 or DMSO-d6
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solution). Splitting patterns are indicated as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad peak.13C NMR spectra were measured at 125 MHz. The chemical shifts are reported in ppm, relative to the central line of the triplet at 77.0 ppm for CDCl3 or the quintet at 39.5 ppm for DMSO-d6. Infrared spectra (IR) were measured on a JASCO VALOR-III and are reported in wavenumbers (cm-1). High resolution mass spectra (HRMS) were recorded on a JEOL JMS 700
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instrument with a direct inlet system. Optical rotations were measured on a Jasco P-2200 polarimeter using a cell with an optical path length of 100 mm. Column chromatography was carried out on silica gel 60-F plates. 4.2. Synthesis of phaeosphaeride A
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gel (40−100 mesh). Analytical thin-layer chromatography (TLC) was performed using 0.25 mm silica
4.2.1. Ethyl (2S,3S)-2,3-Dihydroxy-2-methyloctanoate (6)
To a stirred solution of AD-mix-α (16.0 g) and methanesulfonamide (1.09 g, 11.5 mmol) in t-BuOH
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(47 mL) and H2O (57 mL) was added (Z)-5 (2.11 g, 11.5 mmol) in t-BuOH (10 mL) at 0 °C. The reaction mixture was stirred at the same temperature for 22 h. For workup, Na2SO3 (17.2 g) was added, and the mixture was allowed to warm to room temperature, and stirring was continued for 40 min. The whole was extracted with AcOEt (50 mL × 2). The combined extracts were washed with 1 M aqueous NaOH (45 mL) and brine (45 mL), dried (Na2SO4), and concentrated in vacuo. The crude product was
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purified by column chromatography on silica gel (hexane–AcOEt, 7:3) to afford 6 (2.18 g, 87%) as a
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colorless oil. The spectroscopic data of compound 6 was identical to that of ent-64) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –14.1 (c 0.13, CHCl3). Determination of stereochemistry at C-3 and estimation of the optical purity of 6 were conducted by using modified Mosher’s method. Derivatization of 6 to the corresponding (R)- or (S)-MTPA ester was conducted according to a usual method using the corresponding acid chlorides and pyridine. Since 1H NMR of each crude product did not show the signals of the other diastereomer, the optical purity of 6 was estimated to be more than 98% ee. 4.2.2 Ethyl (2S,3S)-3-(Benzyloxy)-2-hydroxy-2-methyloctanoate (7a) To a stirred solution of 6 (2.15 g, 9.85 mmol) in 1,4-dioxane (49 mL) were added TriBOT (1.58 g, 3.94 mmol) and MS5Å (1.23 g). TfOH (0.17 mL, 1.9 mmol) was added slowly, and the reaction mixture
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was stirred for 19 h. The reaction was quenched by addition of Et3N (0.69 mL), and stirring was continued for 5 min. The mixture was filtered through a pad of Celite, and the residue was washed with AcOEt (30 mL). The filtrate was concentrated in vacuo. The residual oil was dissolved in AcOEt (100 mL) and washed with saturated aqueous NaHCO3 (50 mL), H2O (50 mL), brine (50 mL), dried (MgSO4),
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and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 19:1→9:1→17:3→7:3) to afford 7a (1.76 g, 58%) as a pale brown oil, 7b (639 mg, 16%) as a pale brown oil, and 6 (532 mg, 25%) as a pale brown oil. The spectroscopic data of compound
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7a was identical to that of ent-7a4) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –10.5 (c 0.13, CHCl3). 4.2.3. Ethyl (2S,3S)-2,3-Dihydroxy-2-methyloctanoate (6) from 7b
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A mixture of 7b (635 mg, 1.59 mmol) and 10% Pd on carbon (95.3 mg) in EtOH (16 mL) was stirred at 35 °C under hydrogen for 5 h. To the reaction mixture was added 10% Pd on carbon (95.3 mg), and the reaction was continued at 45 °C under hydrogen for 15 h. The mixture was filtered, and the filtrate was concentrated in vacuo to afford 6 (375 mg) as a pale yellow oil.
4.2.4. Ethyl (2S,3S)-3-(Benzyloxy)-2-(methoxymethoxy)-2-methyloctanoate (8) To a stirred solution of 7a (2.42 g, 7.84 mmol) and i-Pr2NEt (4.0 mL, 24 mmol) in CH2Cl2 (31 mL) was added MOMCl (1.5 mL, 20 mmol) at 0 °C. The resultant mixture was allowed to warm to 35 °C,
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and stirring was continued for 4 days. After concentration, the mixture was diluted with Et2O (150 mL) and washed with 0.5 M aqueous HCl (30 mL), saturated aqueous NaHCO3 (20 mL), brine (20 mL), dried (Na2SO4), and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 9:1) to afford 8 (2.71 g, 98%) as a pale yellow oil. The spectroscopic data
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of compound 8 was identical to that of ent-84) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –11.9 (c 0.10, CHCl3).
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4.2.5. (2S,3S)-3-(Benzyloxy)-2-(methoxymethoxy)-2-methyloctan-1-ol (S1) To a stirred solution of 8 (2.29 g, 6.50 mmol) in THF (43 mL) was added DIBALH (1.02 M solution in hexane, 19.1 mL, 19.5 mmol) at 0 °C. After being stirred at the same temperature for 30 min, the mixture was allowed to warm to room temperature, and stirring was continued for 2 h. To the reaction mixture was added 1 M aqueous roschelle salt (78 mL) at 0 °C, and the mixture was stirred at room temperature for 1 h. The whole was extracted with Et2O (50 mL × 2). The combined extracts were dried (Na2SO4), and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 9:1) to afford S1 (1.74 g, 86%) as a pale yellow oil. The spectroscopic data of compound S1 was identical to that of ent-S14) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 +2.1 (c 0.13, CHCl3).
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4.2.6. (2R,3S)-3-(Benzyloxy)-2-(methoxymethoxy)-2-methyl-1-triisopropylsilyloxyoctane (S2) To a stirred solution of S1 (1.52 g, 4.88 mmol) in CH2Cl2 (49 mL) were added 2,6-lutidine (1.0 mL, 8.6 mmol) and TIPSOTf (1.6 mL, 6.0 mmol) at 0 °C. The resulting mixture was stirred at the same
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temperature for 1 h. To the reaction mixture were added H2O (25 mL) and 1 M aqueous HCl (5.0 mL), and the layers were separated. The organic phase was washed with brine (25 mL), dried (Na2SO4), and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 49:1→97:3) to afford S2 (2.28 g, quant.) as a colorless oil. The spectroscopic data of
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compound S2 was identical to that of ent-S24) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –69.1 (c 0.15, CHCl3). 4.2.7. (2R,3S)-2-(Methoxymethoxy)-2-methyl-1-((triisopropylsilyl)oxy)octan-3-ol (9)
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A mixture of S2 (2.23 g, 4.79 mmol) and 10% Pd on carbon (335 mg) in EtOH (48 mL) was stirred at 30 °C under hydrogen for 16 h. The mixture was filtered, and the residue was washed with AcOEt (30 mL). The filtrate was concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 19:1) to afford 9 (1.79 g, 99%) as a colorless oil. The spectroscopic data of
4.2.8.
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compound 9 was identical to that of ent-94) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –15.9 (c 0.16, CHCl3). Dimethyl
2-(((2R,3S)-2-(Methoxymethoxy)-2-methyl-1-((triisopropylsilyl)oxy)octan-3-yl)oxy)maleate ((E)-11) To a stirred solution of 9 (1.74 g, 4.62 mmol) in THF (70 mL) was added n-BuLi (1.61 M solution in hexane, 0.57 mL, 0.92 mmol) at –78 °C. After stirring at the same temperature for 15 min, a solution of
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freshly distilled 10 (984 mg, 6.93 mmol) in THF (7.0 mL) was added slowly at –78 °C. The mixture was stirred at the same temperature for 2 h, and then allowed to warm to –60 °C, and stirring was continued for 15 min. The reaction was quenched with saturated aqueous NH4Cl (35 mL) and H2O (35 mL). The
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whole was extracted with Et2O (50 mL × 2). The combined extracts were washed with brine (70 mL), dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 19:1→47:3→93:7) to afford (E)-11 (2.05 g, 86%) as a pale yellow oil. The spectroscopic data of compound (E)-11 was identical to that of ent-(E)-114) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 +20.7 (c 0.12, CHCl3). 4.2.9. Dimethyl 2-(((2R,3S)-1-Hydroxy-2-(methoxymethoxy)-2-methyloctan-3-yl)oxy)maleate (S3) A solution of (E)-11 (1.99 g, 3.84 mmol) in HF·pyr./pyr./THF (1:2:2) (38 mL) was stirred at 35 °C for 3 days. The reaction mixture was poured into saturated aqueous NaHCO3 (160 mL), and the mixture was stirred for 15 min. The whole was extracted with Et2O (160 mL and 80 mL), and the combined extracts
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were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 13:7) to afford S3 (1.39 g, quant.) as a colorless oil. The
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spectroscopic data of compound S3 was identical to that of ent-S34) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 +17.9 (c 0.18, CHCl3). 4.2.10. Dimethyl 2-(((2S,3S)-2-(Methoxymethoxy)-2-methyl-1-oxooctan-3-yl)oxy)maleate (12)
To a stirred solution of S3 (1.36 g, 3.74 mmol) in CH2Cl2 (37 mL) was added Dess–Martin periodinane (3.18 g, 7.49 mmol) at room temperature. The reaction mixture was stirred for 1.5 h and poured into a mixture of 5% aqueous Na2SO3 (50 mL), saturated aqueous NaHCO3 (50 mL), and H2O
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(50 mL). The whole was extracted with Et2O (50 mL and 30 mL). The combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 4:1→3:1) to afford 12 (1.13 g, 84%) as a colorless oil. The spectroscopic data
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of compound 12 was identical to that of ent-124) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –6.8 (c 0.12, CHCl3). 4.2.11.
Dimethyl
(2S,3R,4S)-4-Hydroxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydro-2H-pyran-5,6-dicarboxylate (13a) and Dimethyl arboxylate (13b) and Dimethyl
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(2S,3S,4S)-3-(Methoxymethoxy)-3-methyl-2-pentyl-4-((trimethylsilyl)oxy)-3,4-dihydro-2H-pyran-5,6-dic
(13c)
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(2S,3R,4R)-4-Hydroxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydro-2H-pyran-5,6-dicarboxylate To a stirred solution of NaHMDS (1.0 M solution in THF, 4.6 mL, 4.6 mmol) in THF (20 mL) was
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added a solution of 12 (1.10 g, 3.04 mmol) in THF (10 mL) at –78 °C, and the mixture was stirred at the same temperature for 1 h. To the reaction mixture were added saturated aqueous NH4Cl (20 mL) and H2O (20 mL). The whole was extracted with Et2O (30 mL × 2), and the combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 3:1→13:7) to afford 13a (608 mg, 56%) as a colorless oil, 13b (58.5 mg, 4%) as a colorless oil, and 13c (160 mg, 15%) as a colorless oil. The spectroscopic data of compounds 13a, 13b, and 13c was identical to that of ent-13a,4) ent-13b,4) and ent-13c4) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D25 –28.1 (c 0.11, CHCl3) for 13a, [α]D24 –31.5 (c 0.14, CHCl3) for 13b, and [α]D25 +2.0 (c 0.29, CHCl3) for 13c.
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4.2.12. (2S,3R,4S)-4-Hydroxy-6-methoxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydropyrano[2,3-c]pyrr ole-5,7(2H,6H)-dione (15) and Methyl
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(2S,3R,4S)-4-Hydroxy-6-(methoxycarbamoyl)-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydro-2H-p yran-5-carboxylate (16)
To a stirred solution of 13a (572 mg, 1.59 mmol) in MeOH (19 mL) was added 1 M aqueous NaOH (2.4 mL, 2.4 mmol) at room temperature. The resulting mixture was stirred at 35 °C for 12 h and then concentrated in vacuo. The residual oil was dissolved in H2O (24 mL), and 1 M aqueous HCl (7.2 mL)
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was added to this solution. The whole was extracted with AcOEt (30 mL × 2), and the combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude acid was carried forward without further purification. To a stirred solution of the crude product in CH2Cl2 (53 mL) were added MeONH2·HCl
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(332 mg, 3.97 mmol), Et3N (0.89 mL, 6.4 mmol), HOBt·H2O (486 mg, 3.18 mmol), and WSC·HCl (609 mg, 3.18 mmol) at room temperature. The mixture was stirred for 22 h and poured into 0.2 M aqueous HCl (30 mL). The layers were separated, and the aqueous phase was extracted with CH2Cl2 (30 mL). The organic phase and extract were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 7:3→13:7→1:1→7:13→3:7) to afford 15 (262 mg, 48%) as a pale yellow oil and 16 (130 mg, 22%) as a colorless oil. The spectroscopic data of compounds 15 and 16 was identical to that of ent-154) and ent-164) and exhibited an optical
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rotation of similar magnitude with opposite sign: [α]D25 –40.7 (c 0.13, CHCl3) for 15 and [α]D25 –14.3 (c 0.13, CHCl3) for 16. 4.2.13.
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(2S,3R,4S)-4-Hydroxy-6-methoxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydropyrano[2,3-c]pyrr ole-5,7(2H,6H)-dione (15) from 16
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A solution of 16 (57.0 mg, 0.15 mmol) and Et3N (62 µL, 0.45 mmol) in DMF (1.5 mL) was stirred at 50 °C for 6.5 h. After concentration, the crude product was purified by column chromatography on silica gel (hexane–AcOEt, 7:3) to afford 15 (41.7 mg, 81%) as a pale yellow oil. 4.2.14. Phaeosphaeride A (4)
To a stirred solution of 15 (232 mg, 0.674 mmol) in Et2O (6.7 mL) was added MeMgBr (3.0 M solution in Et2O, 0.79 mL, 2.4 mmol) at –78 °C. The mixture was stirred at the same temperature for 1 h. To the reaction mixture was added MeMgBr (3.0 M solution in Et2O, 0.22 mL, 0.66 mmol), and the stirring was continued for 2 h. The reaction was quenched with saturated aqueous NH4Cl (15 mL) and extracted with Et2O (15 mL × 3). The combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude hemiaminal was carried forward without further purification. The crude product was
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dissolved in a solution of HCl (ca. 4 mol/L solution in 1,4-dioxane, 6.7 mL) at room temperature. The reaction mixture was stirred at the same temperature for 1.5 h. After concentration, the crude product was purified by column chromatography on silica gel (hexane–AcOEt, 3:2→1:1→9:11) to afford
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phaeosphaeride A (4, 45.5 mg, 23%) as a pale yellow oil. [α]D25 –98.6 (c 0.85, CH2Cl2); IR (CHCl3) 3378, 2960, 2932, 2862, 1707, 1638, 1440, 1086, 1055 cm–1; 1H NMR (500 MHz, DMSO-d6) δ 5.40 (d, J = 5.5 Hz, 1H), 4.97 (s, 2H), 4.88 (s, 1H), 4.07 (d, J = 11.5 Hz, 1H), 3.87 (d, J = 5.5 Hz, 1H), 3.79 (s, 3H), 1.82 (m, 1H), 1.59–1.39 (m, 2H), 1.38–1.13 (m, 5H), 1.18 (s, 3H), 0.86 (t, J = 6.5 Hz, 3H);
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NMR (125 MHz, DMSO-d6) δ 166.5, 155.3, 137.1, 104.8, 90.7, 86.2, 70.9, 64.3, 63.7, 30.8, 27.5, 26.0,
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21.9, 20.3, 13.8; HRMS (EI+) [M]+ calcd for C15H23NO5 297.1576, found 297.1571. 4.3. Biomimetic transformation from phaeosphaeride A to phaeosphaeride B
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4.3.1. Phaeosphaeride B (2)
Phaeosphaeride A (4, 10.7 mg, 0.036 mmol) was dissolved in TFA (0.4 mL), and the mixture was stirred for 1 h at room temperature. After concentration under vacuum, the crude product was dissolved in THF (0.4 mL). To this solution was added a half saturated aqueous solution of NaHCO3 (0.2 mL). The reaction mixture was stirred for 30 min at room temperature, and then diluted with brine (5.0 mL). The mixture was extracted with Et2O (10 mL × 2), and the combined organic layers were dried over Na2SO4, filtered, and concentrated in vacuo. The crude product was purified by column chromatography on silica
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gel (hexane–AcOEt, 7:3→3:2→9:11) to afford phaeosphaeride B (2, 8.0 mg, 75%) as a colorless oil and phaeosphaeride A (4, 2.0 mg, 19%) as a colorless oil. [α]D25 –142.8 (c 0.43, CH2Cl2); IR (CHCl3) 3377, 2959, 2930, 2860, 1702, 1635, 1451, 1058, 1012 cm–1; 1H NMR (500 MHz, DMSO-d6) δ 5.48 (d, J = 6.0 Hz, 1H), 5.02 (d, J = 2.0 Hz, 1H), 5.01 (d, J = 2.0 Hz, 1H), 4.57 (s, 1H), 3.98 (dd, J = 10.5, 1.5 Hz,
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1H), 3.79 (s, 3H), 3.76 (d, J = 6.0 Hz, 1H), 1.85 (m, 1H), 1.67–1.49 (m, 2H), 1.46–1.27 (m, 5H), 0.89 (t, J = 7.0 Hz, 3H), 0.88 (s, 3H); 13C NMR (125 MHz, DMSO-d6) δ 165.4, 157.0, 136.2, 105.1, 91.5, 80.6, 69.7, 63.9, 63.7, 31.0, 26.8, 25.4, 21.9, 18.0, 13.8; HRMS (EI+) [M]+ calcd for C15H23NO5 297.1576,
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found 297.1574.
4.3.2. (6S,7R,8R)-17
Compound 1 (14.9 mg, 0.0501 mmol) was dissolved in TFA (0.6 mL), and the mixture was stirred for 3 h at room temperature. After concentration under vacuum, the crude product was dissolved in THF (0.6 mL). To this solution was added a half saturated aqueous solution of NaHCO3 (0.3 mL). The reaction mixture was stirred for 30 min at room temperature, and then diluted with brine (5.0 mL). The mixture was extracted with Et2O (10 mL × 2), and the combined organic layers were dried over Na2SO4, filtered, and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 3:2→11:9→2:3) to afford compound (6S,7R,8R)-17 (12.4 mg, 83%) as a colorless
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oil and compound 1 (1.8 mg, 12%) as a white solid. [α]D25 +167.8 (c 0.46, CH2Cl2); IR (CHCl3) 3394, 2958, 2930, 2862, 1709, 1637, 1453, 1086, 1052 cm–1; 1H NMR (500 MHz, CDCl3) δ 5.11 (d, J = 1.5 Hz, 1H), 5.04 (d, J = 1.5 Hz, 1H), 4.14 (br s, 1H), 4.09 (dd, J = 10.5, 2.0 Hz, 1H), 3.89 (s, 3H), 3.65 (br s, 1H), 2.16 (br s, 1H), 1.97-1.80 (m, 2H), 1.76–1.62 (m, 2H), 1.47 (m, 1H), 1.41–1.29 (m, 3H), 1.36 (s,
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3H), 0.92 (t, J = 7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 166.8, 157.8, 136.5, 104.2, 92.6, 81.5, 71.3, 65.1, 64.4, 31.6, 27.4, 25.3, 22.5, 21.4, 14.0; HRMS (EI+) [M]+ calcd for C15H23NO5 297.1576, found 297.1574.
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Acknowledgment This work was supported by JSPS KAKENHI Grant Number 25860015 and partially by a grant from the Dementia Drug Resource Development Center Project S1511016, the Ministry of Education, Culture,
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Sports Science and Technology (MEXT), Japan.
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://.
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References and Notes 1. Maloney KN, Hao W, Xu J, Gibbons J, Hucul J, Roll D, Brady SF, Schroeder FC, Clardy J. Org Lett. 2006;8:4067. 2. Kobayashi K, Okamoto I, Morita N, Kiyotani T, Tamura O. Org Biomol Chem. 2011;9:5825.
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3. Chatzimpaloglou A, Yavropoulou MP, Rooij KE, Biedermann R, Mueller U, Kaskel S, Sarli V. J Org Chem. 2012;77:9659.
4. Kobayashi K, Kobayashi Y, Nakamura M, Tamura O, Kogen H. J Org Chem. 2015;80:1243. 5. Abzianidze VV, Poluektova EV, Bolshakova KP, Panikorovskii TL, Bogachenkov AS, Berestetskiy AO. Acta Crystallogr. 2015;E71:o625.
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6. Chatzimpaloglou A, Kolosov M, Eckols TK, Tweardy DJ, Sarli V. J Org Chem. 2014;79:4043. 7. Abzianidze VV, Prokofieva DS, Chisty LA, Bolshakova KP, Berestetskiy AO, Panikorovskii TL, Bogachenkov AS, Holder AA. Bioorg Med Chem Lett. 2015;25:5566.
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8. For other small-molecule STAT3 inhibitors, see: (a) Weidler M, Rether J, Anke T, Erkel G. FEBS Lett. 2000;484:1;
(b) Bharti AC, Donato N, Aggarwal BB. J Immunol. 2003;171:3863; (c) McMurray JS. Chem Biol. 2006;13:1123;
(d) Shin DS, Kim HN, Shin KD, Yoon YJ, Kim SJ, Han DC, Kwon BM. Cancer Res. 2009;69:193; (e) Chen C, Qiang S, Lou L, Zhao W. J Nat Prod. 2009;72:824;
(f) Matsuno K, Masuda Y, Uehara Y, Sato H, Muroya A, Takahashi O, Yokotagawa T, Furuya T, Chem Lett. 2010;1:371;
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Okawara T, Otsuka M, Ogo N, Ashizawa T, Oshita C, Tai S, Ishii H, Akiyama Y, Asai A. ACS Med (g) Li CS, Ding Y, Yang BJ, Miklossy G, Yin HQ, Walker LA, Turkson J, Cao S. Org Lett. 2015;17:3556.
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9. Still WC, Gennari C. Tetrahedron Lett. 1983;24:4405. 10. Sharpless KB, Amberg W, Bennani YL, Crispino GA, Hartung J, Jeong K-S, Kwong H-L, Morikawa K, Wang Z-M, Xu D, Zhang X-L. J Org Chem. 1992;57:2768.
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11. Ohtani I, Kusumi T, Kashman Y, Kakisawa H. J Am Chem Soc. 1991;113:4092. 12. Yamada K, Fujita H, Kunishima M. Org Lett. 2012;14:5026. 13. Zimmerman HE, Pushechnikov A. Eur J Org Chem. 2006;3491. 14. Güenther H, Jikeli G. Chem Rev. 1977;77:599. 15. Under the hydrolysis conditions, TMS ether 13b also gave the acid with concomitant cleavage of the TMS group. 16. The absolute configuration of phaeosphaeride B remains unknown. 17. Johnson F, Malhotra SK. J Am Chem Soc. 1965;87:5492. 18. (a) The trifluoroacetates were prone to partial decomposition and hydrolysis during purification by column chromatography. Therefore, they were used in the next step without purification.
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(b) We also tried the reaction with a non-nucleophilic acid (TfOH and HBF4), however, the reaction did not proceed to recover the starting material.
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19. The optical rotation of synthetic 17 is much larger than the reported data (see ref. 6), the reason for which is unknown.
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S0040-4020(17)30246-6
DOI:
10.1016/j.tet.2017.03.020
Reference:
TET 28526
To appear in:
Tetrahedron
Received Date: 28 January 2017 Revised Date:
6 March 2017
Accepted Date: 7 March 2017
Please cite this article as: Kobayashi K, Kunimura R, Tanaka III K, Tamura O, Kogen H, Total synthesis of (−)-phaeosphaeride B by a biomimetic conversion from (−)-phaeosphaeride A, Tetrahedron (2017), doi: 10.1016/j.tet.2017.03.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Total synthesis of (–)-phaeosphaeride B by a biomimetic conversion from (–)-phaeosphaeride A
Graduate School of Pharmaceutical Sciences, Meiji Pharmaceutical University, 2-522-1 Noshio,
Kiyose, Tokyo 204-8588, Japan b
Showa Pharmaceutical University, 3-3165 Higashi-tamagawagakuen, Machida, Tokyo 194-8543,
Japan * Corresponding authors. E-mail address: [email protected] (K. Kobayashi).
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E-mail address: [email protected] (H. Kogen).
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a
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Kenichi Kobayashi a, *, Risako Kunimura a, Kosaku Tanaka III a, Osamu Tamura b, Hiroshi Kogen a, *
Abstract We have accomplished the first total synthesis of STAT3 inhibitory (–)-phaeosphaeride A and its stereoisomer (–)-phaeosphaeride B. This work confirms the configurational assignment of these natural products. Notably, TFA-mediated dehydrative stereoinversion of phaeosphaeride A afforded phaeosphaeride A.
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phaeosphaeride B, based on our hypothesis of the biosynthesis mechanism of phaeosphaeride B from
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Keywords Phaeosphaeride A, Phaeosphaeride B, Total synthesis, Asymmetric synthesis, Biomimetic conversion
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1. Introduction Phaeosphaerides A (proposed structure 1) and B (2) are nitrogen-containing bicyclic natural products that were isolated from the endophytic fungus FA39 (Phaeosphaeria avenaria) by Clardy and
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co-workers in 2006 (Figure 1).1 Phaeosphaeride A is an inhibitor of STAT3-DNA binding with an IC50 of 0.61 mM and also inhibits cell growth in STAT3-dependent U266 multiple myeloma cells with an IC50
Structures of phaeosphaerides.
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Fig 1.
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of 6.7 µM in vitro, whereas its stereoisomer phaeosphaeride B (2) has no STAT3 inhibitory activity.
In 2011 and 2012, our group2 and Sarli’s group3 independently demonstrated the total synthesis of the proposed structure of phaeosphaeride A (1), which revealed that the structure of phaeosphaeride A was misassigned. Stereochemical considerations suggested that the correct structure of phaeosphaeride A is C-7 epimer 3 of the originally proposed structure (1) or its enantiomer (4). The first total synthesis of 3 was achieved by our group in 2015, providing a corrected configuration (4) for phaeosphaeride A,4 and this result was supported by the X-ray crystal structure of natural phaeosphaeride A reported by Abzianidze et al.5 During the course of these studies, Sarli and colleagues and Abzianidze et al. also conducted
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biological studies of their synthetic compounds.3,6,7 Sarli and colleagues synthesized and biologically evaluated the stereoisomers of phaeosphaeride A, which suggested that these compounds are upstream inhibitors of a tyrosine kinase in the JAK/STAT pathway.6 Abzianidze et al. prepared the C-6 acyl derivatives from the isolated natural phaeosphaeride A, and a chloroacetyl derivative exhibited more
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potent cytotoxicity (EC50 = 33 ± 7 µM) against A549 cancer cells than natural phaeosphaeride A (EC50 = 46 ± 5 µM).7 Their studies highlighted phaeosphaeride A as a potential seed compound for STAT3 inhibitors in anti-cancer drug discovery,8 and further screening of unnatural phaeosphaeride A derivatives may provide compounds that are more biologically active. Furthermore, the ambiguous biosynthetic pathway for phaeosphaerides A and B was expected to be clarified. Based on the structures
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of the phaeosphaeride family, we assumed that proton-mediated biosynthetic conversion occurred
Possible biosynthetic conversion between phaeosphaerides A and B.
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Scheme 1.
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between phaeosphaerides A and B (Scheme 1).
In this paper, we describe the total synthesis of (–)-phaeosphaeride B (2) by a hypothetical biomimetic conversion from (–)-phaeosphaeride A through a TFA-mediated stereoinversion.
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2. Results and discussion 2.1.Synthesis of (–)-phaeosphaerides A To synthesize (–)-phaeosphaeride B (2) from (–)-phaeosphaeride A (4) by a hypothetical
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stereoinversion at C-6, we first prepared (–)-phaeosphaeride A (4) by following our synthetic path to ent-phaeosphaeride A (3).4 The synthesis of (–)-phaeosphaerides A (4) began with Sharpless asymmetric dihydroxylation of known (Z)-5,4 synthesized by the Z-selective Horner−Wadsworth−Emmons reaction (Scheme 2).9 Ester (Z)-5 was dihydroxylated to (2S,3S)-diol 6, an enantiomer of the known diol,4 by Sharpless asymmetric dihydroxylation using AD-mix-α10 in high yield (87%) and enantioselectivity
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(98% ee, determined by modified Mosher’s method).11
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obtain 8 in 57% yield (two steps).
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According to our synthetic scheme for 3, diol 6 was converted to ester 8. The two hydroxy groups in 6 were protected with a Bn group12 for the secondary alcohol and a MOM group for the tertiary alcohol to
Scheme 2.
Synthesis of ester 8.
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Ester 8 was reduced with DIBALH to provide the alcohol in 86% yield, which was then protected to give the TIPS ether in quantitative yield (Scheme 3). Removal of the Bn group by hydrogenation using catalytic Pd/C produced desired secondary alcohol 9 in 99% yield. Oxa-Michael addition of alcohol 9 to 10 using catalytic n-BuLi gave (E)-11 in 86% yield. TIPS ether (E)-11 was then converted to aldehyde
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12 by desilylation with HF-pyridine to give an alcohol, followed by Dess-Martin oxidation. To construct a dihydropyran ring, aldehyde 12 was subjected to a sodium hexamethyldisilazide-mediated vinyl anion aldol reaction to generate desired product 13a (56%), its TMS ether 13b (4%), and C-6 diastereomer byproduct 13c (15%). During this reaction, alcohol 13a and TMS ether 13b appeared to be stereoselectively formed through the transition state shown in parentheses in Scheme 3.13 W-type
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long-range coupling14 between H-6 and H-8 in 13a and 13b was observed to confirm the required S configuration at the C-6 stereocenter.
Scheme 3.
Synthesis of dihydropyran derivatives 13a-c.
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Without protection of the C-6 alcohol in 13a, exposure of diester 13a to 1 M aqueous NaOH in MeOH caused regioselective hydrolysis of one of the two ester groups to afford a mono acid,15 which was immediately subjected to amidation with MeONH2 (14) to give maleimide derivative 15 with spontaneous cyclization in 48% yield, accompanied by amide 16 in 22% yield (Scheme 4). Amide 16
by HCl in 1,4-dioxane. The 1H and
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was easily converted to 15 in 81% yield by treating with Et3N in DMF at 50 °C. Total synthesis of 4 was achieved by exo-methylene formation through methylation by using MeMgBr and dehydration mediated C NMR spectra of synthetic 4 matched the literature data1 for
natural phaeosphaeride A, and the optical rotation of 4 ([α]D25 –98.6 (c 0.85, CH2Cl2)) was the same sign as the natural product ([α]D25 –93.6 (c 2.0, CH2Cl2)). Thus, we identified the structure of natural
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phaeosphaeride A as compound 4 through its total synthesis.5
Scheme 4.
Completion of the total synthesis of phaeosphaeride A (4).
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2.2.Hypothetical biosynthetic pathway from phaeosphaeride A to phaeosphaeride B With synthetic phaeosphaeride A (4) in hand, our next challenge was preparing and assigning the configuration of phaeosphaeride B.1,6,16 We assumed that a biosynthetic transformation occurred from
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phaeosphaeride A (4) to phaeosphaeride B (2) (Scheme 5). When exposed to a protic acid, phaeosphaeride A (4) would be dehydrated; the oxygen lone pair would help to form the oxonium cation A, and then a nucleophile (such as H2O) would preferentially attack the C-6 center from the more accessible β-face to afford phaeosphaeride B (2) as the major product. In addition, phaeosphaeride B (2) would be sterically favored over phaeosphaeride A (4), because the X-ray crystal structure of
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phaeosphaeride A (4) suggested steric repulsion between the C-1 carbonyl group and the pseudo-equatorial oriented C-6 hydroxy group as a result of the nearly planar structure of the
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phaeosphaeride skelton.5
Scheme 5.
Our proposed biosynthesis of phaeosphaeride B (2) from phaeosphaeride A (4).
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2.3.Biomimetic transformation from phaeosphaerides A to phaeosphaerides B Based on our hypothesis, we tried a protic acid-mediated dehydrative nucleophilic substitution at the
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C-6 stereocenter (Scheme 6). Upon treating synthetic 4 with TFA, starting alcohol 4 was fully consumed to form less polar compounds, as monitored by thin-layer chromatography.18 After concentration, the crude products were treated with aqueous NaHCO3 in THF to afford phaeosphaerides B (2) and A (4) in 75% and 19% yields, respectively. The spectroscopic data for synthetic 2, including 1H and 13C NMR spectra, fully matched those reported by Clardy and colleagues1 and Sarli and colleagues.6 The [α]D
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value of synthetic 2 ([α]D25 –142.8 (c 0.43, CH2Cl2)) has the same sign as the natural product ([α]D25 – 157.2 (c 2.0, CH2Cl2));1 therefore, we established the absolute configuration of natural phaeosphaeride B
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as shown in Scheme 6.
Scheme 6.
Biomimetic transformation from phaeosphaeride A (4) to phaeosphaeride B (2).
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Synthetic compound 12 was similarly converted to 17 in 83% yield, accompanied by recovery of 1 in 12% yield (Scheme 7). The 1H and 13C NMR spectra for 17 corresponded to the literature data reported by Sarli and colleagues.6 Additionally, the absolute configuration of synthetic 17 was identified as
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shown in Scheme 7, because the optical rotation of 17 ([α]D25 +167.8 (c 0.46, CH2Cl2)) has the opposite sign to the literature data reported by Sarli and colleagues ([α]D25 –13.5 (c 2.0, CH2Cl2)).6,9 Consequently,
Conversion of synthetic 1 to 17.
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3. Conclusion
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Scheme 7.
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compound 17 is formed through stereochemical inversion at C-6 from compound 1.
In conclusion, we have accomplished the first total synthesis of STAT3 inhibitory phaeosphaeride A (4) and its inactive stereoisomer, phaeosphaeride B (2). This synthesis enabled us to identify the relative and absolute configurations of these natural products. Additionally, we identified the remarkable biomimetic transformation from phaeosphaeride A (4) to phaeosphaeride B (2), and from compound 1 to 17, by trifluoroacetylation with inversion at the C-6 stereocenter followed by basic hydrolysis. This work confirmed the hypothetical biosynthesis of phaeosphaeride B (2) from phaeosphaeride A (4). We are currently working on the synthesis and biological evaluation of phaeosphaeride A analogues, and the structure-STAT3 inhibitory activity relationships will be reported at a future date.
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4. Experimental section 4.1. General 1
H NMR spectra were measured on a JEOL JNM-AL500 (500 MHz) instrument. The chemical shifts
are expressed in ppm relative to tetramethylsilane (δ = 0) as an internal standard (CDCl3 or DMSO-d6
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solution). Splitting patterns are indicated as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad peak.13C NMR spectra were measured at 125 MHz. The chemical shifts are reported in ppm, relative to the central line of the triplet at 77.0 ppm for CDCl3 or the quintet at 39.5 ppm for DMSO-d6. Infrared spectra (IR) were measured on a JASCO VALOR-III and are reported in wavenumbers (cm-1). High resolution mass spectra (HRMS) were recorded on a JEOL JMS 700
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instrument with a direct inlet system. Optical rotations were measured on a Jasco P-2200 polarimeter using a cell with an optical path length of 100 mm. Column chromatography was carried out on silica gel 60-F plates. 4.2. Synthesis of phaeosphaeride A
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gel (40−100 mesh). Analytical thin-layer chromatography (TLC) was performed using 0.25 mm silica
4.2.1. Ethyl (2S,3S)-2,3-Dihydroxy-2-methyloctanoate (6)
To a stirred solution of AD-mix-α (16.0 g) and methanesulfonamide (1.09 g, 11.5 mmol) in t-BuOH
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(47 mL) and H2O (57 mL) was added (Z)-5 (2.11 g, 11.5 mmol) in t-BuOH (10 mL) at 0 °C. The reaction mixture was stirred at the same temperature for 22 h. For workup, Na2SO3 (17.2 g) was added, and the mixture was allowed to warm to room temperature, and stirring was continued for 40 min. The whole was extracted with AcOEt (50 mL × 2). The combined extracts were washed with 1 M aqueous NaOH (45 mL) and brine (45 mL), dried (Na2SO4), and concentrated in vacuo. The crude product was
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purified by column chromatography on silica gel (hexane–AcOEt, 7:3) to afford 6 (2.18 g, 87%) as a
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colorless oil. The spectroscopic data of compound 6 was identical to that of ent-64) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –14.1 (c 0.13, CHCl3). Determination of stereochemistry at C-3 and estimation of the optical purity of 6 were conducted by using modified Mosher’s method. Derivatization of 6 to the corresponding (R)- or (S)-MTPA ester was conducted according to a usual method using the corresponding acid chlorides and pyridine. Since 1H NMR of each crude product did not show the signals of the other diastereomer, the optical purity of 6 was estimated to be more than 98% ee. 4.2.2 Ethyl (2S,3S)-3-(Benzyloxy)-2-hydroxy-2-methyloctanoate (7a) To a stirred solution of 6 (2.15 g, 9.85 mmol) in 1,4-dioxane (49 mL) were added TriBOT (1.58 g, 3.94 mmol) and MS5Å (1.23 g). TfOH (0.17 mL, 1.9 mmol) was added slowly, and the reaction mixture
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was stirred for 19 h. The reaction was quenched by addition of Et3N (0.69 mL), and stirring was continued for 5 min. The mixture was filtered through a pad of Celite, and the residue was washed with AcOEt (30 mL). The filtrate was concentrated in vacuo. The residual oil was dissolved in AcOEt (100 mL) and washed with saturated aqueous NaHCO3 (50 mL), H2O (50 mL), brine (50 mL), dried (MgSO4),
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and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 19:1→9:1→17:3→7:3) to afford 7a (1.76 g, 58%) as a pale brown oil, 7b (639 mg, 16%) as a pale brown oil, and 6 (532 mg, 25%) as a pale brown oil. The spectroscopic data of compound
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7a was identical to that of ent-7a4) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –10.5 (c 0.13, CHCl3). 4.2.3. Ethyl (2S,3S)-2,3-Dihydroxy-2-methyloctanoate (6) from 7b
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A mixture of 7b (635 mg, 1.59 mmol) and 10% Pd on carbon (95.3 mg) in EtOH (16 mL) was stirred at 35 °C under hydrogen for 5 h. To the reaction mixture was added 10% Pd on carbon (95.3 mg), and the reaction was continued at 45 °C under hydrogen for 15 h. The mixture was filtered, and the filtrate was concentrated in vacuo to afford 6 (375 mg) as a pale yellow oil.
4.2.4. Ethyl (2S,3S)-3-(Benzyloxy)-2-(methoxymethoxy)-2-methyloctanoate (8) To a stirred solution of 7a (2.42 g, 7.84 mmol) and i-Pr2NEt (4.0 mL, 24 mmol) in CH2Cl2 (31 mL) was added MOMCl (1.5 mL, 20 mmol) at 0 °C. The resultant mixture was allowed to warm to 35 °C,
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and stirring was continued for 4 days. After concentration, the mixture was diluted with Et2O (150 mL) and washed with 0.5 M aqueous HCl (30 mL), saturated aqueous NaHCO3 (20 mL), brine (20 mL), dried (Na2SO4), and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 9:1) to afford 8 (2.71 g, 98%) as a pale yellow oil. The spectroscopic data
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of compound 8 was identical to that of ent-84) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –11.9 (c 0.10, CHCl3).
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4.2.5. (2S,3S)-3-(Benzyloxy)-2-(methoxymethoxy)-2-methyloctan-1-ol (S1) To a stirred solution of 8 (2.29 g, 6.50 mmol) in THF (43 mL) was added DIBALH (1.02 M solution in hexane, 19.1 mL, 19.5 mmol) at 0 °C. After being stirred at the same temperature for 30 min, the mixture was allowed to warm to room temperature, and stirring was continued for 2 h. To the reaction mixture was added 1 M aqueous roschelle salt (78 mL) at 0 °C, and the mixture was stirred at room temperature for 1 h. The whole was extracted with Et2O (50 mL × 2). The combined extracts were dried (Na2SO4), and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 9:1) to afford S1 (1.74 g, 86%) as a pale yellow oil. The spectroscopic data of compound S1 was identical to that of ent-S14) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 +2.1 (c 0.13, CHCl3).
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4.2.6. (2R,3S)-3-(Benzyloxy)-2-(methoxymethoxy)-2-methyl-1-triisopropylsilyloxyoctane (S2) To a stirred solution of S1 (1.52 g, 4.88 mmol) in CH2Cl2 (49 mL) were added 2,6-lutidine (1.0 mL, 8.6 mmol) and TIPSOTf (1.6 mL, 6.0 mmol) at 0 °C. The resulting mixture was stirred at the same
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temperature for 1 h. To the reaction mixture were added H2O (25 mL) and 1 M aqueous HCl (5.0 mL), and the layers were separated. The organic phase was washed with brine (25 mL), dried (Na2SO4), and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 49:1→97:3) to afford S2 (2.28 g, quant.) as a colorless oil. The spectroscopic data of
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compound S2 was identical to that of ent-S24) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –69.1 (c 0.15, CHCl3). 4.2.7. (2R,3S)-2-(Methoxymethoxy)-2-methyl-1-((triisopropylsilyl)oxy)octan-3-ol (9)
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A mixture of S2 (2.23 g, 4.79 mmol) and 10% Pd on carbon (335 mg) in EtOH (48 mL) was stirred at 30 °C under hydrogen for 16 h. The mixture was filtered, and the residue was washed with AcOEt (30 mL). The filtrate was concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 19:1) to afford 9 (1.79 g, 99%) as a colorless oil. The spectroscopic data of
4.2.8.
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compound 9 was identical to that of ent-94) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –15.9 (c 0.16, CHCl3). Dimethyl
2-(((2R,3S)-2-(Methoxymethoxy)-2-methyl-1-((triisopropylsilyl)oxy)octan-3-yl)oxy)maleate ((E)-11) To a stirred solution of 9 (1.74 g, 4.62 mmol) in THF (70 mL) was added n-BuLi (1.61 M solution in hexane, 0.57 mL, 0.92 mmol) at –78 °C. After stirring at the same temperature for 15 min, a solution of
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freshly distilled 10 (984 mg, 6.93 mmol) in THF (7.0 mL) was added slowly at –78 °C. The mixture was stirred at the same temperature for 2 h, and then allowed to warm to –60 °C, and stirring was continued for 15 min. The reaction was quenched with saturated aqueous NH4Cl (35 mL) and H2O (35 mL). The
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whole was extracted with Et2O (50 mL × 2). The combined extracts were washed with brine (70 mL), dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 19:1→47:3→93:7) to afford (E)-11 (2.05 g, 86%) as a pale yellow oil. The spectroscopic data of compound (E)-11 was identical to that of ent-(E)-114) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 +20.7 (c 0.12, CHCl3). 4.2.9. Dimethyl 2-(((2R,3S)-1-Hydroxy-2-(methoxymethoxy)-2-methyloctan-3-yl)oxy)maleate (S3) A solution of (E)-11 (1.99 g, 3.84 mmol) in HF·pyr./pyr./THF (1:2:2) (38 mL) was stirred at 35 °C for 3 days. The reaction mixture was poured into saturated aqueous NaHCO3 (160 mL), and the mixture was stirred for 15 min. The whole was extracted with Et2O (160 mL and 80 mL), and the combined extracts
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were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 13:7) to afford S3 (1.39 g, quant.) as a colorless oil. The
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spectroscopic data of compound S3 was identical to that of ent-S34) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 +17.9 (c 0.18, CHCl3). 4.2.10. Dimethyl 2-(((2S,3S)-2-(Methoxymethoxy)-2-methyl-1-oxooctan-3-yl)oxy)maleate (12)
To a stirred solution of S3 (1.36 g, 3.74 mmol) in CH2Cl2 (37 mL) was added Dess–Martin periodinane (3.18 g, 7.49 mmol) at room temperature. The reaction mixture was stirred for 1.5 h and poured into a mixture of 5% aqueous Na2SO3 (50 mL), saturated aqueous NaHCO3 (50 mL), and H2O
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(50 mL). The whole was extracted with Et2O (50 mL and 30 mL). The combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 4:1→3:1) to afford 12 (1.13 g, 84%) as a colorless oil. The spectroscopic data
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of compound 12 was identical to that of ent-124) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D24 –6.8 (c 0.12, CHCl3). 4.2.11.
Dimethyl
(2S,3R,4S)-4-Hydroxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydro-2H-pyran-5,6-dicarboxylate (13a) and Dimethyl arboxylate (13b) and Dimethyl
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(2S,3S,4S)-3-(Methoxymethoxy)-3-methyl-2-pentyl-4-((trimethylsilyl)oxy)-3,4-dihydro-2H-pyran-5,6-dic
(13c)
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(2S,3R,4R)-4-Hydroxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydro-2H-pyran-5,6-dicarboxylate To a stirred solution of NaHMDS (1.0 M solution in THF, 4.6 mL, 4.6 mmol) in THF (20 mL) was
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added a solution of 12 (1.10 g, 3.04 mmol) in THF (10 mL) at –78 °C, and the mixture was stirred at the same temperature for 1 h. To the reaction mixture were added saturated aqueous NH4Cl (20 mL) and H2O (20 mL). The whole was extracted with Et2O (30 mL × 2), and the combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 3:1→13:7) to afford 13a (608 mg, 56%) as a colorless oil, 13b (58.5 mg, 4%) as a colorless oil, and 13c (160 mg, 15%) as a colorless oil. The spectroscopic data of compounds 13a, 13b, and 13c was identical to that of ent-13a,4) ent-13b,4) and ent-13c4) and exhibited an optical rotation of similar magnitude with opposite sign: [α]D25 –28.1 (c 0.11, CHCl3) for 13a, [α]D24 –31.5 (c 0.14, CHCl3) for 13b, and [α]D25 +2.0 (c 0.29, CHCl3) for 13c.
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4.2.12. (2S,3R,4S)-4-Hydroxy-6-methoxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydropyrano[2,3-c]pyrr ole-5,7(2H,6H)-dione (15) and Methyl
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(2S,3R,4S)-4-Hydroxy-6-(methoxycarbamoyl)-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydro-2H-p yran-5-carboxylate (16)
To a stirred solution of 13a (572 mg, 1.59 mmol) in MeOH (19 mL) was added 1 M aqueous NaOH (2.4 mL, 2.4 mmol) at room temperature. The resulting mixture was stirred at 35 °C for 12 h and then concentrated in vacuo. The residual oil was dissolved in H2O (24 mL), and 1 M aqueous HCl (7.2 mL)
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was added to this solution. The whole was extracted with AcOEt (30 mL × 2), and the combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude acid was carried forward without further purification. To a stirred solution of the crude product in CH2Cl2 (53 mL) were added MeONH2·HCl
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(332 mg, 3.97 mmol), Et3N (0.89 mL, 6.4 mmol), HOBt·H2O (486 mg, 3.18 mmol), and WSC·HCl (609 mg, 3.18 mmol) at room temperature. The mixture was stirred for 22 h and poured into 0.2 M aqueous HCl (30 mL). The layers were separated, and the aqueous phase was extracted with CH2Cl2 (30 mL). The organic phase and extract were dried (Na2SO4) and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 7:3→13:7→1:1→7:13→3:7) to afford 15 (262 mg, 48%) as a pale yellow oil and 16 (130 mg, 22%) as a colorless oil. The spectroscopic data of compounds 15 and 16 was identical to that of ent-154) and ent-164) and exhibited an optical
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rotation of similar magnitude with opposite sign: [α]D25 –40.7 (c 0.13, CHCl3) for 15 and [α]D25 –14.3 (c 0.13, CHCl3) for 16. 4.2.13.
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(2S,3R,4S)-4-Hydroxy-6-methoxy-3-(methoxymethoxy)-3-methyl-2-pentyl-3,4-dihydropyrano[2,3-c]pyrr ole-5,7(2H,6H)-dione (15) from 16
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A solution of 16 (57.0 mg, 0.15 mmol) and Et3N (62 µL, 0.45 mmol) in DMF (1.5 mL) was stirred at 50 °C for 6.5 h. After concentration, the crude product was purified by column chromatography on silica gel (hexane–AcOEt, 7:3) to afford 15 (41.7 mg, 81%) as a pale yellow oil. 4.2.14. Phaeosphaeride A (4)
To a stirred solution of 15 (232 mg, 0.674 mmol) in Et2O (6.7 mL) was added MeMgBr (3.0 M solution in Et2O, 0.79 mL, 2.4 mmol) at –78 °C. The mixture was stirred at the same temperature for 1 h. To the reaction mixture was added MeMgBr (3.0 M solution in Et2O, 0.22 mL, 0.66 mmol), and the stirring was continued for 2 h. The reaction was quenched with saturated aqueous NH4Cl (15 mL) and extracted with Et2O (15 mL × 3). The combined extracts were dried (Na2SO4) and concentrated in vacuo. The crude hemiaminal was carried forward without further purification. The crude product was
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dissolved in a solution of HCl (ca. 4 mol/L solution in 1,4-dioxane, 6.7 mL) at room temperature. The reaction mixture was stirred at the same temperature for 1.5 h. After concentration, the crude product was purified by column chromatography on silica gel (hexane–AcOEt, 3:2→1:1→9:11) to afford
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phaeosphaeride A (4, 45.5 mg, 23%) as a pale yellow oil. [α]D25 –98.6 (c 0.85, CH2Cl2); IR (CHCl3) 3378, 2960, 2932, 2862, 1707, 1638, 1440, 1086, 1055 cm–1; 1H NMR (500 MHz, DMSO-d6) δ 5.40 (d, J = 5.5 Hz, 1H), 4.97 (s, 2H), 4.88 (s, 1H), 4.07 (d, J = 11.5 Hz, 1H), 3.87 (d, J = 5.5 Hz, 1H), 3.79 (s, 3H), 1.82 (m, 1H), 1.59–1.39 (m, 2H), 1.38–1.13 (m, 5H), 1.18 (s, 3H), 0.86 (t, J = 6.5 Hz, 3H);
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NMR (125 MHz, DMSO-d6) δ 166.5, 155.3, 137.1, 104.8, 90.7, 86.2, 70.9, 64.3, 63.7, 30.8, 27.5, 26.0,
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21.9, 20.3, 13.8; HRMS (EI+) [M]+ calcd for C15H23NO5 297.1576, found 297.1571. 4.3. Biomimetic transformation from phaeosphaeride A to phaeosphaeride B
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4.3.1. Phaeosphaeride B (2)
Phaeosphaeride A (4, 10.7 mg, 0.036 mmol) was dissolved in TFA (0.4 mL), and the mixture was stirred for 1 h at room temperature. After concentration under vacuum, the crude product was dissolved in THF (0.4 mL). To this solution was added a half saturated aqueous solution of NaHCO3 (0.2 mL). The reaction mixture was stirred for 30 min at room temperature, and then diluted with brine (5.0 mL). The mixture was extracted with Et2O (10 mL × 2), and the combined organic layers were dried over Na2SO4, filtered, and concentrated in vacuo. The crude product was purified by column chromatography on silica
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gel (hexane–AcOEt, 7:3→3:2→9:11) to afford phaeosphaeride B (2, 8.0 mg, 75%) as a colorless oil and phaeosphaeride A (4, 2.0 mg, 19%) as a colorless oil. [α]D25 –142.8 (c 0.43, CH2Cl2); IR (CHCl3) 3377, 2959, 2930, 2860, 1702, 1635, 1451, 1058, 1012 cm–1; 1H NMR (500 MHz, DMSO-d6) δ 5.48 (d, J = 6.0 Hz, 1H), 5.02 (d, J = 2.0 Hz, 1H), 5.01 (d, J = 2.0 Hz, 1H), 4.57 (s, 1H), 3.98 (dd, J = 10.5, 1.5 Hz,
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1H), 3.79 (s, 3H), 3.76 (d, J = 6.0 Hz, 1H), 1.85 (m, 1H), 1.67–1.49 (m, 2H), 1.46–1.27 (m, 5H), 0.89 (t, J = 7.0 Hz, 3H), 0.88 (s, 3H); 13C NMR (125 MHz, DMSO-d6) δ 165.4, 157.0, 136.2, 105.1, 91.5, 80.6, 69.7, 63.9, 63.7, 31.0, 26.8, 25.4, 21.9, 18.0, 13.8; HRMS (EI+) [M]+ calcd for C15H23NO5 297.1576,
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found 297.1574.
4.3.2. (6S,7R,8R)-17
Compound 1 (14.9 mg, 0.0501 mmol) was dissolved in TFA (0.6 mL), and the mixture was stirred for 3 h at room temperature. After concentration under vacuum, the crude product was dissolved in THF (0.6 mL). To this solution was added a half saturated aqueous solution of NaHCO3 (0.3 mL). The reaction mixture was stirred for 30 min at room temperature, and then diluted with brine (5.0 mL). The mixture was extracted with Et2O (10 mL × 2), and the combined organic layers were dried over Na2SO4, filtered, and concentrated in vacuo. The crude product was purified by column chromatography on silica gel (hexane–AcOEt, 3:2→11:9→2:3) to afford compound (6S,7R,8R)-17 (12.4 mg, 83%) as a colorless
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oil and compound 1 (1.8 mg, 12%) as a white solid. [α]D25 +167.8 (c 0.46, CH2Cl2); IR (CHCl3) 3394, 2958, 2930, 2862, 1709, 1637, 1453, 1086, 1052 cm–1; 1H NMR (500 MHz, CDCl3) δ 5.11 (d, J = 1.5 Hz, 1H), 5.04 (d, J = 1.5 Hz, 1H), 4.14 (br s, 1H), 4.09 (dd, J = 10.5, 2.0 Hz, 1H), 3.89 (s, 3H), 3.65 (br s, 1H), 2.16 (br s, 1H), 1.97-1.80 (m, 2H), 1.76–1.62 (m, 2H), 1.47 (m, 1H), 1.41–1.29 (m, 3H), 1.36 (s,
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3H), 0.92 (t, J = 7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 166.8, 157.8, 136.5, 104.2, 92.6, 81.5, 71.3, 65.1, 64.4, 31.6, 27.4, 25.3, 22.5, 21.4, 14.0; HRMS (EI+) [M]+ calcd for C15H23NO5 297.1576, found 297.1574.
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Acknowledgment This work was supported by JSPS KAKENHI Grant Number 25860015 and partially by a grant from the Dementia Drug Resource Development Center Project S1511016, the Ministry of Education, Culture,
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Sports Science and Technology (MEXT), Japan.
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://.
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References and Notes 1. Maloney KN, Hao W, Xu J, Gibbons J, Hucul J, Roll D, Brady SF, Schroeder FC, Clardy J. Org Lett. 2006;8:4067. 2. Kobayashi K, Okamoto I, Morita N, Kiyotani T, Tamura O. Org Biomol Chem. 2011;9:5825.
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3. Chatzimpaloglou A, Yavropoulou MP, Rooij KE, Biedermann R, Mueller U, Kaskel S, Sarli V. J Org Chem. 2012;77:9659.
4. Kobayashi K, Kobayashi Y, Nakamura M, Tamura O, Kogen H. J Org Chem. 2015;80:1243. 5. Abzianidze VV, Poluektova EV, Bolshakova KP, Panikorovskii TL, Bogachenkov AS, Berestetskiy AO. Acta Crystallogr. 2015;E71:o625.
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6. Chatzimpaloglou A, Kolosov M, Eckols TK, Tweardy DJ, Sarli V. J Org Chem. 2014;79:4043. 7. Abzianidze VV, Prokofieva DS, Chisty LA, Bolshakova KP, Berestetskiy AO, Panikorovskii TL, Bogachenkov AS, Holder AA. Bioorg Med Chem Lett. 2015;25:5566.
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8. For other small-molecule STAT3 inhibitors, see: (a) Weidler M, Rether J, Anke T, Erkel G. FEBS Lett. 2000;484:1;
(b) Bharti AC, Donato N, Aggarwal BB. J Immunol. 2003;171:3863; (c) McMurray JS. Chem Biol. 2006;13:1123;
(d) Shin DS, Kim HN, Shin KD, Yoon YJ, Kim SJ, Han DC, Kwon BM. Cancer Res. 2009;69:193; (e) Chen C, Qiang S, Lou L, Zhao W. J Nat Prod. 2009;72:824;
(f) Matsuno K, Masuda Y, Uehara Y, Sato H, Muroya A, Takahashi O, Yokotagawa T, Furuya T, Chem Lett. 2010;1:371;
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Okawara T, Otsuka M, Ogo N, Ashizawa T, Oshita C, Tai S, Ishii H, Akiyama Y, Asai A. ACS Med (g) Li CS, Ding Y, Yang BJ, Miklossy G, Yin HQ, Walker LA, Turkson J, Cao S. Org Lett. 2015;17:3556.
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9. Still WC, Gennari C. Tetrahedron Lett. 1983;24:4405. 10. Sharpless KB, Amberg W, Bennani YL, Crispino GA, Hartung J, Jeong K-S, Kwong H-L, Morikawa K, Wang Z-M, Xu D, Zhang X-L. J Org Chem. 1992;57:2768.
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11. Ohtani I, Kusumi T, Kashman Y, Kakisawa H. J Am Chem Soc. 1991;113:4092. 12. Yamada K, Fujita H, Kunishima M. Org Lett. 2012;14:5026. 13. Zimmerman HE, Pushechnikov A. Eur J Org Chem. 2006;3491. 14. Güenther H, Jikeli G. Chem Rev. 1977;77:599. 15. Under the hydrolysis conditions, TMS ether 13b also gave the acid with concomitant cleavage of the TMS group. 16. The absolute configuration of phaeosphaeride B remains unknown. 17. Johnson F, Malhotra SK. J Am Chem Soc. 1965;87:5492. 18. (a) The trifluoroacetates were prone to partial decomposition and hydrolysis during purification by column chromatography. Therefore, they were used in the next step without purification.
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(b) We also tried the reaction with a non-nucleophilic acid (TfOH and HBF4), however, the reaction did not proceed to recover the starting material.
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19. The optical rotation of synthetic 17 is much larger than the reported data (see ref. 6), the reason for which is unknown.
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