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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Nat Prod. Author manuscript; available in PMC 2010 July 1.
Published in final edited form as:
PMCID: PMC2829974
NIHMSID: NIHMS124470

Antineoplastic Agents 454. Synthesis of the Strong Cancer Cell Growth Inhibitors trans-Dihydronarciclasine and 7-Deoxy-trans-dihydronarciclasine1a

Abstract

To further pursue the antineoplastic leads offered by our isolation of trans-dihydronarciclasine (1a) and 7-deoxy-trans-dihydronarciclasine (1c) from two medicinal plant species of the Amaryllidaceae family, a practical palladium-catalyzed hydrogenation procedure was developed for synthesis of these isocarbostyrils from narciclasine (2a) and 7-deoxynarciclasine (2c).

From a 1982 collection (bulbs) of the Chinese medicinal plant Zephyranthes candida (Amaryllidaceae)1b we isolated the strong (ED50 0.0032 μg/mL) P388 lymphocytic leukemia cell growth inhibitor trans-dihydronarciclasine (1a). The structure was established by detailed spectroscopic analyses of its peracetate derivative (1b)1b and confirmed by comparison with the minor product from catalytic hydrogenation of narciclasine (2a).2 Hydrogenation afforded as the major product the expected cis-dihydronarciclasine (3a) accompanied by the trans isomer (1a) and iso-narciclasine (4a). Subsequently, trans-dihydronarciclasine (1a) was found to exhibit strong cancer cell growth inhibition (mean panel GI50 12.6 nM) against the U.S. National Cancer Institute (NCI) panel of cancer cell lines3a,b whereas its cis isomer (3a)3c was only very weakly active (mean panel GI50 3800 nM). Importantly, the trans isomer (1a) gave an active Compare correlation coefficient of 0.92 in respect to (+)-pancratistatin (5).3a The trans isomer (1a) also showed strong activity against a range of RNA viruses while the synthetic cis isomer (3a) was completely inactive.4

Because of the close structural and biological relationship of trans-dihydronarciclasine (1a) to (+)-pancratistatin (5), already in preclinical development, it became necessary to increase the availability of trans isomer 1a and the closely related 7-deoxy-trans-dihydronarciclasine (1c), by synthesis. The latter was isolated from Hymenocallis sp. (P388 ED50 0.02 μg/mL) and gave an active Compare correlation coefficient of 0.89 in respect to (+)-pancratistatin (5).3 Those structural and biological relationships of 1a and 1c to (+)-pancratistatin (5) are of special importance owing to its well known5 in vitro and in vivo anticancer activity augmented by the rapidly increasing knowledge of the very promising activation of the mitochondrial route to cancer cell apoptosis.6a Importantly, pancratistatin has been shown to induce apoptosis routinely in various cancer cell lines at sub-micromolar concentrations while being nontoxic to normal human fibroblasts and endothelial cells at the same drug concentrations.6b,c As anticipated, narciclasine (2a) has recently been found to induce the mitochondrial and/or caspase-8/caspase-10 death receptor pathway in human MCF-7 breast as well as PC-3 prostate cancer cell lines.5 Presumably, the strong cancer cell growth inhibitors 1a and 1c will also display related mechanisms of action selectively targeting cancer cells.

Synthetic approaches to the multifunctional carbostyril structures of (+)-pancratistatin (5) and the narciclasines have presented significant challenges. Over the past 30 years, nine total syntheses of natural 5 have been reported proceeding from those of Danishefsky7a in 1989 through Hudlicky7b and Trost7c in 1995, followed by Haseltine (1997),7d Magnus (1998),7e Rigby (2000),7f Pettit (2001),7g Kim (2002),7h and Li (2006)7i where the earliest was for (±)-5. Details of these syntheses have been examined in an important review by Kornienko7j which also leads to narciclasine chemistry and syntheses.7f A substantial number of useful SAR studies have been completed based on 2a and 5. Those of Hudlicky8a and McNulty8b are two of the most recent and supplemented by a detailed review by Kornienko.8c

The need to begin more detailed preclinical development of 1a and 1c led to our syntheses of the potentially useful phosphate prodrugs trans-dihydronarcistatin (6a) and 7-deoxy-trans-dihydronarcistatin (6b).9 In concert with the increased requirements for both precursors, it became important to uncover a selective hydrogenation reaction for conversion of the obvious precursors narciclasine (2a) and 7-deoxy-narciclasine (2c) readily available from certain Narcissus (Amaryllidaceae) species3a,9 to the required dihydro-derivatives. That approach appeared to be of more immediate practicality than multistep total syntheses.8c,10

Results and Discussion

Functional groups such as hydroxyl, ester and amide often direct the stereochemistry of hydrogenation.11a,b Homogenous hydrogenation of allylic alcohols usually occurs with high stereoselectivity. Catalysts used in such hydroxy-directed hydrogenation often include Wilkinson’s catalyst (7) [RhCl(PPh3)3] and Crabtree’s catalyst (8)[Ir(COD)(Pcy3)(py)]PF.11c Consequently, narciclasine, protected as its acetonide (2d),2 was treated with Crabtree’s catalyst (8) in dichloromethane, but failed to yield any product of hydrogenation. The reaction was also attempted with Wilkinson’s catalyst (7) in toluene and again narciclasine acetonide (2d) resisted hydrogenation. With a related trisubstituted styrene that proved unreactive towards hydrogenation with Wilkinson’s catalyst even under forcing conditions, it was successfully hydrogenated when first converted to its alkoxide, but that led exclusively to the cis isomer.12 This approach was again unsuccessful when using narciclasine acetonide (2d). Since a hydrogenation reaction is believed to be associated with the olefin’s ability to donate unshared electron pairs to unfilled surface orbitals of the catalyst metal,13 the double bond in narciclasine was considered too hindered to allow this type of hydroxyl group-directed reduction. Attention was therefore directed to ionic hydrogenation.

The ionic hydrogenation (CF3CO2H, Et3SiH) of a Δ8(9)-D-homosteroid is known to proceed stereospecifically to give a D-homosteroid with the trans-antitrans configuration .14 Such sterically hindered olefins have been hydrogenated at low temperature in dichloromethane with trifluoroacetic acid as a proton donor and triethylsilane as a hydride donor.15 Interestingly, in our experiments narciclasine (2a) and derivatives 2b and 2d resisted hydrogenation with triethylsilane-trifluoroacetic acid in dichloromethane at both −75 °C and at 25 °C. Consequently, it seemed best to study the palladium catalyzed hydrogenation of narciclasine (2a) in detail with the prospect of uncovering a technique that would favor the trans-isomer (1a).

While we and others2 have hydrogenated narciclasine (2a) using Adam’s catalyst (PtO2) in ethanol, 28% of the trans isomer (1a) was usually obtained, along with 58% of the cis isomer 3a and 13% of iso-narciclasine (4a). Since a related styrene-type olefin has also been shown to afford different ratios of cis:trans isomers by simply using various solvents for the reduction,16 the hydrogenation of narciclasine peracetate (2b)1719 was conducted in the presence of 5% Pd/C (20 mol%) at 1 atm using a variety of solvents: ethyl acetate, ethanol, acetic acid, hexane, tetrahydrofuran, pyridine and N,N-dimethylformamide. The results are shown in Table I. The trans:cis:iso ratios were determined by 1H NMR and were based on a 100% conversion of starting material to product. The best ratio observed (51:47:2 respectively) was with acetic acid as solvent on a small scale (approx. 0.020 g of narciclasine peracetate). Scaleup with this solvent gave a wide variation in results. When 5 g of narciclasine was hydrogenated in acetic acid in the presence of 5% Pd/C (8 mol %) for 20 h only starting material was recovered. Repeating the hydrogenation with narciclasine peracetate employing 1 g and using 10% Pd/C (25.8 mol%) the trans and cis products were isolated in 30 and 62% yield, respectively, following chromatography on silica gel. The eluent solvent system 1:1 CH2Cl2-CH3CH2OH gave good results (65% trans following chromatography) when narciclasine tetraacetate (200 mg) was hydrogenated in the presence of 10% Pd/C (10 mol%). The peracetylated isomers 1b, 3b, and 4b were separated by column chromatography on silica gel. The structure of the synthetic trans isomer 1b was established by detailed spectral data comparison with an authentic sample.1b,3

Table I
Effect of Solvent on Hydrogenation of Narciclasine Acetate (2b) with 5% Pd/C (20 mol%) at 1 atm, 25 °C for 2 h

Once a method for the hydrogenation of 2b to 1b was developed, a similar procedure was employed for the conversion of 7-deoxynarciclasine (2c) to 7-deoxy-trans-dihydronarciclasine (1c). The cis diol was first protected as the acetonide 2e in good yield (92%). The phenol group was next protected as its silyl ether (2f) to avoid the potential problems which had already been encountered when proceeding with hydrogenation of narciclasine acetonide (2d). Hydrogenation of the olefin was then attempted using the conditions most successful in the reduction of peracetate 2b to the trans-dihydroperacetate 1b. Hydrogenation of 2f (0.05 g) with 10% Pd/C (10 mol%) at 1 atm in 1:1 CH2Cl2:CH3CH2OH gave a reasonably separable mixture of the trans:cis:iso product in 56:26:10% yields, respectively. However, yields suffered upon scaleup and for example the trans was reduced to 27% when a 2 g-scale was used. The natural trans-isomer (1d) was obtained by cleavage of the acetonide group using formic acid (60%) followed by silyl ether deprotection with TBAF to yield 1c which was identical with an authentic sample of 7-deoxy-trans-dihydronarciclasine.3a

The scale-up preparation of trans-dihydronarciclasines 1a and 1c utilizing the preceeding hydrogenation improvements increased the availability of both cancer cell growth inhibitors for conversion to the respective phosphate prodrugs9 and further preclinical development.

Experimental Section

General Experimental Procedures

Narciclasine (2a) was isolated from the bulbs of Narcissus imcomparabilus as previously described20 and 7-deoxynarciclasine (2c) from the bulbs of Hymenocallis littoralis.3a All solvents were redistilled, 5% and 10% Pd/C as well as 4-dimethylaminopyridine were purchased from Sigma-Aldrich Co., while acetic anhydride, pyridine, tert-butyldimethylsilyl chloride and imidazole were purchased from Lancaster Chemical Company. Reaction progress was ascertained by thin-layer chromatography using Analtech silica gel GHLF Uniplates visualized under long- and short-wave UV irradiation and developed using an ethanolic solution of phosphomolybdic acid reagent (Sigma-Aldrich Co.). Column chromatography was performed with silica gel 60 (230–400 mesh) from E. Merck. All reaction products were colorless solids unless otherwise noted. All solvent extracts of aqueous phases were dried over anhydrous magnesium sulfate.

All melting points were determined with an Electrothermal digital melting point apparatus model IA9200 and are uncorrected. Optical rotation values were recorded using a Perkin Elmer 241 polarimeter. High- and low-resolution FAB spectra were obtained from a Kratos MS-50 mass spectrometer (Midwest Center for Mass Spectrometry, University of Nebraska, Lincoln, NE) using a glycerol-triglycerol matrix with a JEOL LCMate magnetic sector instrument either in the FAB mode, with a glycol matrix, or by APCI with a polyethylene glycol reference. The 1H and 13C spectra were recorded with Varian Gemini 300 or 400 MHz instruments.

2,3,4,7-O-Tetraacetoxy-narciclasine (2b)

To a stirred solution of narciclasine (2a, 1.00 g, 3.25 mmol) in pyridine (3 mL under nitrogen) was added acetic anhydride (6 mL). After stirring for 16 h at rt, ice (50 mL) was added and the mixture was extracted with CH2Cl2 (3 × 20 mL). The combined extract was dried, filtered, and evaporated in vacuo to afford the title compound (2b) as a light brown powder (1.4 g, 90% yield; Caution: this compound can cause a contact dermatitis; [α]D24 +224 (c 1.03, CHCl3); mp 230–231 °C; {lit.18 mp 220–231 °C, [α]D24 +229 (c 0.33, CHCl3)}. 1H NMR (CDCl3, 300 MHz) δ 6.94 (1H, s, H-10), 6.51 (1H, bs, 5-NH), 6.16 (1H, m, H-1), 6.12 and 6.11 (each 1H, d, J = 1Hz, OCH2O), 5.44 (1H, m, H-3), 5.35 (1H, m, H-2), 5.23 (1H, dd, J = 2 Hz, 8 Hz, H-4), 4.60 (1H, d, J = 8 Hz, H-4a), 2.34 (3H, s, OAc), 2.14 (6H, s, 2 OAc), 2.10 (3H, s, OAc); 13C NMR (CDCl3, 75 MHz) 170.3 (C=O), 169.5 (C=O), 169.3 (C=O), 168.9 (C=O), 162.3 (C=O), 152.4 (C), 141.5 (C), 131.4 (C), 133.7 (C), 131.6 (C), 118.1 (CH), 114.7 (C), 103.1 (CH2), 101.9 (CH), 71.3 (CH), 71.3 (CH), 68.3 (CH), 50.2 (CH), 20.93 (CH3), 20.92 (CH3), 20.88 (CH3), 20.78 (CH3); HRMS (FAB) m/z 498.1003 [(M + Na+)] (calcd. for C22H21NO11Na , 498.1003 [(M + Na+)]).

2,3,4,7-Tetraacetoxy-trans-dihydronarciclasine (1b)

Method 1

To a solution of narciclasine tetraacetate (2b) (0.97 g, 2.04 mmol) in glacial acetic acid (120 mL) was added 5% Pd/C catalyst (0.56 g, 26 mol%). The mixture was stirred under an atmosphere of hydrogen at rt for 3 h and the solution filtered through fluted filter paper. The filtrate was dried, again filtered and evaporated in vacuo. The residue was separated by column chromatography (CC) on silica gel eluting with 99.5:0.5 CH2Cl2-CH3OH to afford the product (1b) as a powder (0.290 g, 30%) along with the cis-dihydro-peracetate (3b) as a solid (0.60 g, 62%). Analysis of 1b by comparison of NMR data found it to be identical with an authentic sample.1b

Method 2

To a solution of narciclasine tetracetate (2b, 0.200 g, 0.42 mmol) in 1:1 CH2Cl2- CH3CH2OH was added 10% Pd/C catalyst (0.004 g, 0.042 mmol). The mixture was stirred under 1 atm of hydrogen at room temperature for 4 h, filtered through a pad of silica, and the solvent removed in vacuo. The residue was separated by CC (flash silica; eluant 45:55 hexanes-EtOAc) to afford the trans-dihydro-peracetate (1b) as a solid (0.131 g, 65%), along with the cis-dihydro-peracetate (3b) as a solid (0.050 g, 25%).

trans-Dihydronarciclasine (1a)

Trans-dihydronarciclasine-2,3,4,7-tetraacetate (1b, 0.512 g, 1.07 mmol) was dissolved in CH3OH-H2O (9:1, 20 mL) with CH2Cl2 (12 mL) added to increase solubility. Potassium carbonate (9 mg, 0.06 mmol) was added and the mixture stirred at rt for three days when TLC (4:96 CH2Cl2:CH3OH) showed complete deprotection.

The reaction solution was concentrated under reduced pressure and the residue separated by CC on silica gel (96:4 CH2Cl2-CH3OH) to give (1a) as an amorphous solid (0.134 g, 40%); mp 260 °C (dec), 285 °C (melts) {lit.2 mp 290–291 °C} [α]D22 +2.8 (c 0.97, THF), {Lit.2 [α]D25 +4.7 (c 0.27, THF)} 1H-NMR (DMSO-d6, 300 MHz) δ 13.0 (s, 1H), 7.56 (s, 1H), 6.46 (s, 1H), 6.04 and 6.01 (each 1H, d, J = 1.2 Hz, OCH2O), 4.96-4.84 (m, 3H), 3.38 (nm, 1H), 3.70 (nm, 2H), 3.34-3.26 (m, 1H), 2.84 (td, 1H, J = 3.9, 12.3 Hz, H-10b), 2.07 (m, 1H), 1.65-1.57 (m, 1H). 13C NMR (DMSO-d6, 75 MHz) 174.4, 156.8, 150.1, 143.2, 136.6, 111.6, 106.5, 101.2, 76.3, 74.2, 73.2, 59.8, 3.5, 32.8. HRMS(APCI+) m/z 310.0925 [M+H]+ (calcd. for C14H16NO7, 310.0927 [M+H]+).

3,4-Isopropylidene-7-deoxynarciclasine (2e)

7-Deoxynarciclasine (2c, 0.205 g, 0.704 mmol)3a,9 and TsOH (0.133 g, 0.704 mmol) were dissolved in N,N-dimethylformamide (10 mL) and 2',2'-dimethoxypropane (0.864 ml, 7.04 mmol) added. The resulting solution was stirred for 16 h, poured into water (50 mL) and extracted with ethyl acetate (4 × 30 mL). The combined organic extract was dried, filtered and concentrated in vacuo to yield a pale yellow solid which was separated by CC (flash silica; eluant 3:7 hexanes-EtOAc) to afford product 2e as a solid (0.215 g, 92%); recrystallized from methanol as needles; mp 251–253 °C; [α]D25 −32.6 (c 0.61, CH3OH); {lit.21 [α]D25 −34.3 (c 0.76, CH3OH)}; 1H NMR, (CDCl3, 300 MHz) δ 7.59 (1H, s, H-7), 7.02 (1H, s, H-10), 6.28 (2H, bs, H-1, NH), 6.04 (2H, s, -OCH2O-), 4.35–4.45 (1H, m, H-2), 4.10–4.20 (3H, m, H-3,4,4a), 3.04 (1H, d, J = 4.5 Hz, -OH), 1.53 (3H, s, -CH3), 1.39 (3H, s, -CH3); 13C NMR (CDCl3, 75 MHz) 162.4, 151.8, 148.6, 128.3, 127.5, 124.0, 120.8, 111.4, 107.6, 101.9, 101.4, 79.5, 78.9, 72.8, 55.9, 27.0, 24.7; HRMS, (APCI+) m/z 332.1147 (calcd. for C17H18NO6, 332.1134).

2-[tert-Butyldimethylsilyl]oxy-3,4-isopropylidene-7-deoxy-narciclasine (2f)

To 3,4-isopropylidene-7-deoxynarciclasine (2e, 0.024 g, 0.0725 mmol) in N,N-dimethylformamide (3 mL) was added tert-butyldimethylsilyl chloride (TBDMSCl, 0.016 g, 0.109 mmol) and imidazole (0.007 g, 0.109 mmol). The resulting solution was stirred for 5 h and the dimethylformamide was removed in vacuo to afford a pale yellow oil. The residue was separated by CC (flash silica; eluant 3:2 hexanes-EtOAc) to provide the silyl ether as a solid (0.028 g, 87%): mp 269 °C; [α]D27 +20.2; (c 0.45, CH2Cl2). 1H NMR (CDCl3, 300 MHz) δ 7.60 (H, s, H-7), 7.04 (1H, s, H-10), 6.15–6.25 (2H, m, H-1, NH), 6.03 (2H, s, OCH2O-), 4.30–4.35 (1H, m, H-2), 4.00–4.15 (3H, m, H-3, 4, 4a), 1.50 (3H, s, -CH3), 1.36 (3H, s, -CH3), 0.96 (9H, s, -C(CH3)3), 0.17 (6H, s, Si(CH3)2); 13C NMR (CDCl3, 75 MHz) 162.3, 151.7, 148.4, 128.4, 126.8, 126.1, 120.8, 110.8, 107.6, 101.8, 101.4, 79.4, 79.1, 73.4, 55.6, 27.1, 25.8, 24.8, 18.1, −4.5, −5.0; HRMS (APCI+) m/z 446.1997 (Calcd. for C23H32NO6Si, 446.1999): anal. C, 61.96%, H, 7.32%, N, 3.12%, calcd. for C23H31NO6Si: C, 62.00; H, 7.01; N, 3.14.

7-Deoxy-2-[tert-butyldimethylsilyl]oxy-3,4-isopropylidene--trans-dihydronarciclasine (1d)

To a solution of 2-(tert-butyldimethylsilyl]oxy-3,4-isopropylidene-7-deoxynarciclasine (2f, 0.050 g, 0.112 mmol), in 1:1 CH2Cl2-CH3CH2OH (8 mL) was added 10% Pd/C (1.2 mg, 0.0112 mmol). The resulting mixture was stirred under 1 atm. of hydrogen for 4 h and then passed through a short column of silica gel, eluting with EtOAc. Removal of solvent in vacuo afforded a solid which was separated by CC (gravity, silica gel; eluant 7:3 hexanes-EtOAc) to yield a solid (0.028 g, 56%): mp 181.5–182.5 °C; [α]D −23.8 (c 0.52, CH2Cl2); 1H NMR (CDCl3, 300 MHz) δ 7.59 (1H, s, H-7), 6.76 (1H, s, H-10), 6.02 (2H, s, -OCH2O-), 6.01 (1H, s, NH), 4.41 (1H, d, J = 2 Hz, H-2), 4.12–4.20 (1H, m, H-4), 4.04–4.12 (1H, m, H-4), 3.42 (1H, dd, J = 13 Hz, 9, H-4a), 3.11 (1H, dt, J = 13 Hz, 2, H-10b), 2.20–2.25 (1H, m, H-1), 1.70–1.84 (1H, m, H-2), 1.45 (3H, s, -CH3), 1.40 (3H, s, -CH3), 0.90 (9H, s, -C(CH3)3), 0.14 (6H, s, -Si(CH3)2);13C NMR (CDCl3, 75 MHz) 165.3, 151.2, 146.6, 136.4, 123.1, 109.8, 108.4, 104.2, 101.6, 77.6, 77.4, 67.0, 57.8, 31.9, 31.6, 28.2, 26.4, 25.6, 17.8, −4.9, −5.0; HRMS(APCI+) m/z 448.2169 (calcd. for C23H34NO6Si, 448.2155). anal. C 61.74%, H 7.78%, N 3.11%, calcd. for C23H33NO6Si, C 61.72%, H 7.43%, N 3.13%.

Continued elution of the column led to 7-deoxy-2-[tert-butyldimethylsilyl]oxy-3,4-isopropylidene-cis-dihydro-narciclasine (3c) as a solid (0.013 g, 26%): mp 237.5–238.5 °C; [α]D +82.2 (c 0.51, CH2Cl2); 1H NMR (CDCl3, 300 MHz) 7.49 (1H, s, H-7), 6.68 (1H, s, H-10), 6.08 (1H, s, NH), 6.02 and 6.00 (each 1H, d, J = 1.2 Hz, OCH2O), 4.10–4.20 (2H, m, H-4, 4a), 3.94–4.02 (1H, m, H-3), 3.79 (1H, ddd, J = 12 Hz, 7, 5, H-2), 2.94–3.08 (1H, m, H-10b), 1.84 (1H, q, J = 13 Hz, H-1), 1.55–1.65 (1H, m, H-1), 1.55 (3H, s, -CH3), 1.41 (3H, s, -CH3), 0.85 (9H, s, -C(CH3)3), 0.11 (3H, s, -SiCH3), 0.07 (3H, s, -SiCH3); 13C NMR (CDCl3, 75 MHz) 165.9, 151.3, 147.2, 137.8, 121.7, 108.6, 107.9, 107.0, 101.6, 79.9, 77.1, 72.6, 51.8, 35.8, 35.4, 28.4, 26.4, 25.7, 17.9, −4.4, −4.8. HRMS(APCI+) m/z 448.2168 (calcd. for C23H34NO6Si 448.2155). anal. C 61.72% H 7.81%, N 3.11%, calcd. for C23H33NO6Si, C 61.72%, H 7.43% N 3.13%.

The third minor component isolated was 7-deoxy-2-[tert-butyldimethylsilyl]oxy-3,4-isopropylidene-iso-dihydro-narciclasine (4c) as a solid (0.005 g, 10%): mp 246.5–248.0 °C; [α]D −47.3 (c 0.92, CH2Cl2); 1H NMR (CDCl3, 300 MHz) δ 8.87 (1H, bs, NH), 7.81 (1H, s, H-7), 7.00 (1H, s, H-10), 6.09 , (2H, s, -OCH2O-), 4.92 (1H, d, J = 4.5 Hz, H-4), 4.12–4.18 (2H, m, H-2, 3), 2.86 (1H, dd, J = 16 Hz, 4, H-1), 2.62 (1H, dd, J =16 Hz, 5, H-1), 1.44 (3H, s, -CH3), 1.27 (3H, s, -CH3), 0.85 (9H, s, -C(CH3)3), 0.12 (3H, s, -SiCH3), 0.08 (3H, s, -SiCH3); 13C NMR (CDCl3, 75 MHz) 162.3, 152.3, 147.5, 134.6, 130.6, 121.6, 110.5, 107.4, 105.9, 101.8, 101.1, 77.4, 72.1, 68.4, 28.9, 27.8, 26.2, 25.7, 17.9, −4.7, −4.8; HRMS(APCI+) m/z 446.1999 (calcd. for C23H32NO6Si 446.1998): anal. C 61.89%, H 7.35%, N 3.09%, calcd. for C23H31NO6Si C 62.00%, H 7.01%, N 3.14%.

7-Deoxy-trans-dihydronarciclasine (1c)

1d (0.02 g, 0.045 mmol) was dissolved in tetrahydrofuran (2 mL) and formic acid (2 mL, 60%) was added at rt. The reaction was heated to 60 °C for 3 h. TLC (85:15 EtOAc-hexanes) showed complete conversion to a slower moving product. The solution was concentrated to a white residue which was separated by silica gel flash CC (90:10 CH2Cl2-CH3OH) to yield a white solid (13.1 mg, 71.4%) mp 230 °C; 1H NMR (DMSO-d6, 300 MHz) showed the silyl ether still present, which was confirmed by HRMS (APCI+) m/z = 408.1845 (calcd. for C20H30NO6Si 408.1842). Then the silyl ether intermediate, 7-deoxy-2-[tert-butyldimethylsilyl]oxy-trans-dihydro-narciclasine (0.037 g, 0.09 mmol), was dissolved in THF (5 mL) and tetrabutylammonium fluoride (TBAF, 0.01 mL, 0.01 mmol) was added and the mixture stirred at rt under argon. After 6 h, TLC (1:9 CH3OH-CH2Cl2) showed incomplete conversion of starting material to product. Additional TBAF (0.1 mL, 0.1 mmol) was added, the reaction continued for 24 h, more TBAF (0.1 mL, 0.1 mmol) was added, and stirring continued for 5 days. Ethyl acetate (35 mL) was added and the organic phase washed with brine (25 mL), dried, filtered and concentrated in vacuo to a yellow oil. The oil in THF was passed into a column of silica gel and chromatographed using a gradient elution (9:1 CH2Cl2-CH3OH to 7:3 CH2Cl2:CH3OH ). The product (1c) was isolated as a white solid, 13.4 mg, (50%) and was identical by 1H NMR with a natural sample of 7-deoxy-trans-dihydronarciclasine.4

figure nihms124470f1

Acknowledgments

The financial support for this investigation was provided by Outstanding Investigator Grant CA-44344-01A1-10-12, and Grants RO1-CA90441-01-03 and 5RO1-CA090441-07 from the Division of Cancer Treatment and Diagnosis, National Cancer Institute, DHHS; the Arizona Disease Control Research Commission; the Fannie E. Rippel Foundation; Robert B. Dalton Endowment Fund; and Dr. Alec D. Keith. We are also very pleased to thank for other assistance Drs. Dennis L. Doubek, Cherry L. Herald, Fiona Hogan, and Brian Orr.

References and Notes

1. Contribution 454 of the Antineoplastic Agents series. For Part 453 refer to: (a) Pettit G, Orr GR, Ducki BS. Anti-Cancer Drug Des. 2000;15:389–396. [PubMed] (b) Pettit GR, Cragg GM, Singh SB, Duke JA, Doubek DL. J. Nat. Prod. 1990;53:176–178. [PubMed]
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