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Molecules 2005, 10(10), 1272-1278; https://doi.org/10.3390/10101272

Article
Towards a Synthesis of Naphthalene Derived Natural Products
Microbial Natural Product Research Group, Department of Chemistry, Australian National University, Australia
*
Author to whom correspondence should be addressed.
Received: 3 December 2004 / Accepted: 28 April 2005 / Published: 31 October 2005

Abstract

:
Dieckmann-type cyclization reactions have been employed in the synthesis of the alkyl substituted naphthoquinone 11 and the naphthalenes 10 and 12. Various conditions for the benzylic oxidation of these compounds have been investigated with a view towards the synthesis of some naphthalene based natural products.
Keywords:
Naphthalenes; benzylic oxidation; naphtho-γ-pyrone

Introduction

A large number of naphthalene derived natural products are based on a substituted naphthalene skeleton as represented by 1. These include: the naphthyl ketone derivative guieranone A (2) [1], heterocyclic derivatives like the linear naphtho-γ-pyrone (NGP) rubrofusarin (3) [2,3], and naphthoquinones such as the crinoid derived pigment 4 [4]. Given that 3 is the only linear NGP based natural product to have been synthesized, [5,6,7] and that many structurally related NGPs possessing biological properties have been isolated, we are interested in developing a general synthesis of NGPs and as a corollary this synthesis should be extendable to related naphthalene derived natural products represented here by 2 and 4.
Molecules 10 01272 i001
Synthetic methods to access these substituted naphthalene based compounds are limited. While the 2-acetyl naphthalene 1 could serve as a starting material in the synthesis of the target compounds, the synthesis of substituted 2-acyl naphthalenes is not trivial. Traditional substitution of naphthalenes using Friedel-Crafts type reactions generally leads to peri-substituted products making the regioselective synthesis of compounds such as 1 from naphthalene precursors difficult. Shibata and co-workers circumvented this by cyclising the β-diketoester 5 at 200-220°C under high vacuum, [5] to give 1, albeit in a low 9% yield (misreported as 18%) while some years later Rideout and colleagues increased the yield in the synthesis of 1 to 18% by cyclising 5 in refluxing 1-methylnaphthalene [8]. Interestingly concurrent attempts by Shibata to cyclize 5 employing polyphosphoric acid gave the alkyl naphthalenic ester 6, representing initial attack at the distal ketone carbonyl. This paper outlines recent work directed at developing an improved synthesis of substituted 1,3,6,8-tetraoxygenated-2- acyl-naphthalenes with a view towards the synthesis of naphthalene based natural products. Molecules 10 01272 i002

Results and Discussion

We envisaged a possible route to the desired 2-acyl-naphthalenes via a benzylic oxidation of an alkyl-substituted naphthalene. In this direction the ketones 8 and 9 were obtained in excellent yields through acylating the phenylacetic acid derivative 7 [9] with hexanoic or pentanoic anhydride, respectively, in toluene containing catalytic perchloric acid (Scheme 1).
Scheme 1. (a) HClO4 (70%) (cat), PhMe, rt; (b) hexanoic anhydride, 92%; (c) pentanoic anhydride, 96%; (d) NaOEt, N2; (e) NaH (3.1 equiv.), DMF, N2, 10min; (f) MEM-Cl, 27% (unoptimized); (g) NBS, AIBN, CCl4, reflux, 30h, 34% (unoptimized).
Scheme 1. (a) HClO4 (70%) (cat), PhMe, rt; (b) hexanoic anhydride, 92%; (c) pentanoic anhydride, 96%; (d) NaOEt, N2; (e) NaH (3.1 equiv.), DMF, N2, 10min; (f) MEM-Cl, 27% (unoptimized); (g) NBS, AIBN, CCl4, reflux, 30h, 34% (unoptimized).
Molecules 10 01272 g001
Base induced cyclization of 8 in refluxing sodium ethoxide gave the desired (metastable) 1,3,6,8- tetraoxygenated-2-alkyl-naphthalene 10 in 25% yield, which readily oxidized to afford the major isolated product from the reaction, the naphthoquinone 11 (52% to 77% yield). The propensity for 10 to oxidize to 11 suggested it would be unwise to attempt benzylic oxidations on 10 without first protecting the phenol groups.
The MEM protected alkyl-naphthalene 12 was therefore synthesized by treating 9 with 3.1 equivalents of sodium hydride in DMF followed by the addition of MEM-Cl. The first equivalent of NaH promotes cyclization to the naphthalene, and the remaining NaH prevents protonation of the phenol groups, trapping the naphthalene as its di-sodium salt.
It was anticipated that benzylic oxidations on both the naphthoquinone 11 and the naphthalene 12 would lead ultimately to the 2-acyl-naphthoquinones and the 2-acyl-naphthalenes respectively, thereby providing access to some of our target compounds. We had initially thought that 12 could be brominated in the benzylic position however, reactions with NBS returned only the ring-brominated product 13. To date conditions suitable for effecting the desired benzylic oxidations have not been found with the majority of the conditions attempted resulting in decomposition of the starting material (Table 1). Future work will center on replacing the MEM group with alternative protecting groups in order to ascertain if the steric interactions are responsible for the lack of reactivity in the benzylic position.
Table 1. Attempted Benzylic Oxidations
Table 1. Attempted Benzylic Oxidations
SubstrateOxidantProductsRef
12NBS, AIBN (cat)13
12K2O8S2 + CuSO4 (cat)Decomposition[10]
12Br2Deprotected
11KMnO4/CuSO4Decomposition[11]
11Br2Decomposition
11(NH4)2Ce(NO3)6 (CAN)Decomposition

Experimental

General

1H- and 13C-NMR spectra were recorded at 300 and 75 MHz, respectively, on either a Varian Gemini 300 or on a Varian Inova 300 spectrometer. 1H spectra were referenced to residual protonated solvent (7.26 ppm) while 13C spectra were referenced to the central peak of the CDCl3 triplet (77.0 ppm). HMQC and HMBC spectra recorded on a Varian Inova 300 spectrometer were routinely measured to assist in spectra assignment and structural identification. Results are reported in the form δ(ppm), integration, mult, J, assignment (for 1H) or δ(ppm), assignment (for 13C). IR spectra were recorded on a Shimadzu FTIR 8400 Series spectrometer as thin films on NaCl discs. ESI-MS spectra were recorded on a FISONS VG QUATTRO II mass spectrometer, operating at a cone voltage between 20 and 70 V, with positive ion detection. EI-MS and HREI-MS were recorded on a VG autospec mass spectrometer, operating at 70 eV using positive ion detection. UV spectra were recorded on a Varian Cary 4G UV-Vis spectrophotometer in a 1cm cell. Melting points (mp) are uncorrected and were recorded on an Electrothermal digital melting point apparatus (Electrothermal Eng. Ltd.). Most reagents were obtained from Aldrich Chemical Company and used as supplied.

General procedure to prepare the ketones \8 and 9

To a stirred solution of methyl 3,5-dimethoxyphenylacetate (7) [9] (1.0 mmol) in toluene (1.5 mL) was added the appropriate anhydride (2.0 mmol) along with perchloric acid (70%, 2 drops initially followed by 1 drop after 18 h). After stirring for 72 h the solution was concentrated in vacuo and subjected to flash chromatography (an efficient method to hydrolyze excess anhydride) on a silica column (Pet. Sp/EtOAc 25:75) to give one main fraction containing the product and the appropriate carboxylic acid, which was concentrated in vacuo and then extracted with ether, aqueous NaHCO3 (5%; 3×30 mL) and then H2O (30 mL). The organic phase was dried (MgSO4) and concentrated in vacuo to return 8 (96%), or 9 (92%) as orange oils.
3,5-Dimethoxy-2-hexanoyl-phenylacetic acid methyl ester (8): TLC: Rf 0.65 [EtOAc/Pet. Sp, 30:70 Molecules 10 01272 i003 (v/v)]. IR: ν (cm-1): 2935 s, 2858 m, 1738 s, 1682 m, 1605 s, 1583 m, 1455 m, 1428 m, 1318 s, 1293 w, 1257 w, 1205 m, 1156 s, 1086 m, 1058 m, 1012 m, 954 w, 835 m. EIMS m/z: 308 (M+⋅, 26%), 277 (M-OCH3, 17%), 237 (M-(CH2)4CH3, 100%), 209 (M-C(O)(CH2)4CH3, 88%). HREI-MS m/z: found 308.1624 (C17H24O5 requires 308.1624). UV (MeOH) λmax (log ε): 201 (3.46), 266 (2.93). 1H-NMR (CDCl3): 6.39 (1H, d, 2.2 Hz, 4-H), 6.37 (1H, d, 2.2Hz, 6-H), 3.81 (3H, s, 3-OCH3)*, 3.80 (3H, s, 5-OCH3)*, 3.67 (3H, s, -CO2CH3), 3.62 (2H, s, -CH2CO2CH3), 2.81 (2H, t, 7.2 Hz, 2'-H), 1.64 (2H, m, 3'-H), 1.31 (4H, m, 4'&5'-H), 0.89 (3H, t, 6.9 Hz, 6'-H). 13C APT-NMR (CDCl3): 206.8 (C-1'), 171.6 (-CO2CH3), 161.2 (C-3)*, 158.8 (C-5)*, 134.3 (C-1), 123.9 (C-2), 107.8 (C-6), 97.4 (C-4), 55.5 (C-3-OCH3), 55.3 (C-5-OCH3), 51.9 (-CO2CH3), 44.3 ( C-2'), 38.7 (-CH2CO2CH3), 31.4 (C-4'), 23.7 (C-3'), 22.4 (C-5'), 13.9 (C-6'). Shifts with identical superscripts (*,) within a data set are interchangeable.
3,5-Dimethoxy-2-pentanoyl-phenylacetic acid methyl ester (9): TLC: Rf 0.55 [EtOAc/ Pet. Sp, 30:70 Molecules 10 01272 i004 (v/v)]. IR: ν (cm-1): 2957 s, 2871 m, 1740 s, 1686 m, 1603 s, 1460 m, 1433 m, 1319 s, 1294 w, 1263 w, 1205m, 1157 s, 1099 m, 1082 m, 1013 m, 948 w, 835 m. EIMS m/z: 294 (M+⋅, 15%), 293 (52%), 275 (14%), 261 (35%), 250 (24%), 234 (100%), 206 (71%), 202 (33%), 176 (20%), 162 (28%), 148 (21%). HREI-MS m/z: found 294.1467 (C16H22O5 requires 294.1467). UV (MeOH) λmax (log ε): 202 (3.45), 263 (2.90). 1H-NMR (CDCl3): 6.39 (1H, d, 2.2 Hz, 4-H), 6.36 (1H, d, 2.2Hz, 6-H), 3.80 (3H, s, 3-OCH3)*, 3.79 (3H, s, 5-OCH3)*, 3.67 (3H, s, -CO2CH3), 3.62 (2H, s, -CH2CO2CH3), 2.82 (2H, t, 7.3 Hz, 2'-H), 1.61 (2H, m, 3'-H), 1.36 (2H, m, 4'-H), 0.91 (3H, t, 7.3 Hz, 5'-H). 13C APT-NMR (CDCl3): 206.5 (C-1'), 171.4 (-CO2CH3), 161.1 (C-3)*, 158.6 (C-5)*, 134.2 (C-1), 123.7 (C-2), 107.8 (C-6), 97.2 (C-4), 55.4 (C-3-OCH3), 55.3 (C-5-OCH3)∞, 51.7 (-CO2CH3), 43.9 ( C-2'), 38.5 (-CH2CO2CH3), 26.0 (C-3'), 22.2 (C-4'), 13.7 (C-5'). Shifts with identical superscripts (*,) within a data set are interchangeable.
Cyclization of 3,5-dimethoxy-2-hexanoyl-phenylacetic acid methyl ester (8) with ethoxide, to give the naphthol (10) and the naphthoquinone (11) by auto-oxidation.
A solution of 3,5-dimethoxy-2-hexanoyl-phenylacetic acid methyl ester (8, 95mg, 0.31 mmol) in dry EtOH (10 mL), was added dropwise, over 7 min, to a refluxing solution of sodium ethoxide (prepared from Na (0.42 g) in 10 mL of dry EtOH) under N2. After a further 10 min at reflux the reaction mixture was cooled, diluted with ether (30 mL), neutralized with HCl (1M) and then extracted with aqueous NaHCO3 (5%; 3×30 mL) giving a light pink aqueous phase and a yellow organic phase. The ethereal phase was dried (MgSO4) and concentrated in vacuo to give a crude mixture, which was purified by flash chromatography on a silica column (r = 6 mm, l = 20 cm, silica gel 0.040-0.063 mm; Pet. Sp/EtOAc 25:75), giving 3-butyl-2-hydroxy-5,7-dimethoxy-1,4-naphthoquinone (11) (47 mg, 0.16 mmol, 52%) as a bright yellow solid and 2-butyl-6,8-dimethoxy-naphthalene-1,3-diol (10) (21 mg, 0.076 mmol, 25%) as a yellow oil, which auto-oxidized to the naphthoquinone (11) over several days on exposure to air.
Butyl-6,8-dimethoxy-naphthalene-1,3-diol (10): TLC: Rf 0.48 [EtOAc/ Pet. Sp, 25:75 (v/v)]. IR: ν Molecules 10 01272 i005 (cm-1): 3402 (s, br), 2932 s, 2858 m, 2362 w, 1636 s, 1597 s, 1448 m, 1404 m, 1371 s, 1338 w, 1246 w, 1209 m, 1150 m, 1107 m, 1041 m. ESI-MS (+ve ion, cv 50V) m/z: 277 (M+H, 100%), 263 (M-CH3+H, 10%), 220 (2%), 205 (2%). UV (MeOH) λmax (log ε): 245 (4.90), 292 (3.91). 1H-NMR (CDCl3): 9.43 (1H, s, 1-OH), 6.55 (1H, s, 4-H), 6.50 (1H, d, 2.1 Hz, 5-H), 6.28 (1H, d, 2.1 Hz, 7-H), 3.98 (3H, s, 8-OCH3), 3.85 (3H, s, 6-OCH3), 2.74 (2H, t, 7.5 Hz, 1'-H), 1.58 (2H, m, 2'-H), 1.44 (2H, m, 3'-H), 0.94 (3H, t, 7.2 Hz, 4'-H). 13C APT-NMR (CDCl3): 157.5 (C-6)*, 157.0 (C-8) *, 154.6 (C-3), 152.9 (C-1), 135.9 (C-4a), 112.1 (C-2), 106.2 (C-8a), 100.8 (C-4) 98.0 (C-5), 95.4 (C-7), 56.0 (C-8-OCH3), 55.3 (C-6-OCH3), 31.5 (C-2'), 22.9 (C-1')Δ, 22.8 (C-3')Δ, 14.1 (C-4'). Shifts with identical superscripts (*,∞,Δ) are interchangeable.
Butyl-2-hydroxy-5,7-dimethoxy-1,4-naphthoquinone (11): TLC: Rf 0.28 [EtOAc/ Pet. Sp, 25:75 Molecules 10 01272 i006 (v/v)]. mp: 160∞C. IR: ν (cm-1): 3209 m, 2932 m, 2359 m, 1655 m, 1638 s, 1595 m, 1460 m, 1321 s, 1209 s, 1157 m, 1126 w, 1034 w. EIMS m/z: 290 (M+∞, 100%), 248 (M-C3H6, 43%), 233 (M-(CH2)3CH3, 13%), 219 (35%), 165 (22%). HREI-MS m/z: found 290.1163 (C16H18O5 requires 290.1154). UV (MeOH) λmax (log ε): 212 (4.68), 261 (4.51), 304 (4.29). 1H-NMR (CDCl3): 7.25 (1H, d, 2.4 Hz, 8-H), 6.75 (1H, d, 2.4 Hz, 5-H), 3.96 (3H, s, 5-OCH3)*, 3.93 (3H, s, 7-OCH3)*, 2.56 (2H, t, 7.5 Hz, 1'-H), 1.50 (2H, m, 2'-H), 1.40 (2H, m, 3'-H), 0.92 (3H, t, 7.2 Hz, 4'-H). 13C APT-NMR (CDCl3): 183.6 (C-4)*, 181.7 (C-1)*, 163.7 (C-7)∞, 161.7 (C-5)∞, 150.9 (C-2), 133.2 (C-8a), 126.2 (C-3), 114.2 (C-4a), 105.2 (C-6), 103.0 (C-8), 56.4 (C-5-OCH3)Δ, 55.8 (C-7-OCH3)Δ, 30.5 (C-2'), 23.2 (C-1'), 22.9 (C-3'), 13.8(C-4'). Shifts with identical superscripts (*,∞,Δ) within a data set are interchangeable.
Synthesis of 6,8-dimethoxy-1,3-bis-(2-methoxy-ethoxymethoxy)-2-propyl-naphthalene (12)
To a stirred solution of 3,5-dimethoxy-2-pentanoyl-phenylacetic acid methyl ester (9, 12.5 mg, 43.1 μmol) in dry DMF (1 mL) under N2, was added NaH (50% dispersion in mineral oil; 3.2 mg, 130 μmol). After stirring for 10 min MEM-Cl (35 μL, 38 mg, 0.30 mmol) was added and the solution left stirring overnight before being extracted with ether and water. The ethereal phase was dried (MgSO4) and concentrated in vacuo to give a crude orange oil, which was purified by flash chromatography (r = 3 mm, l = 5 cm, silica gel 0.040-0.063 mm; Pet. Sp/EtOAc 70:30), to give 6,8-dimethoxy-1,3-bis-(2- methoxy-ethoxymethoxy)-2-propyl-naphthalene (12) (5.1 mg, 12 μmol, 27%, unoptimized) as an orange oil.
6,8-Dimethoxy-1,3-bis-(2-methoxy-ethoxymethoxy)-2-propyl-naphthalene (12): TLC: Rf 0.22 [EtOAc/ Molecules 10 01272 i007Pet. Sp, 30:70 (v/v)]. IR: ν (cm-1): 2932 s, 1622 s, 1580m, 1458 m, 1389 m, 1337 m, 1252 w, 1204 m, 1155 s, 1119 m, 1097 m, 1036 s, 847 w. ESI-MS (+ve ion, cv 50V) m/z: 461 (M+Na, 100%), 239 (M+H, 11%), 373 ([M-MEM]+Na), 28%), 221 (22%). HREI-MS m/z: found 438.2258 (C23H34O8 requires 438.2254). UV (MeOH) λmax (log ε): 216 (4.25), 242 (4.46), 288 (3.74). 1H-NMR (CDCl3): 7.14 (1H, s, 4-H), 6.65 (1H, d, 2.2 Hz, 5-H), 6.37 (1H, d, 2.2 Hz, 7-H), 5.38 (2H, s, 1'''-H), 5.12 (2H, s, 1'-H), 4.00-3.57 (8H, m, {3'-H, 4'-H, 3'''-H, 4'''-H}), 3.90 (3H, s, 8-OCH3)*, 3.86 (3H, s, 6-OCH3)*, 3.40 (3H, s, 6'-H), 3.39 (3H, s, 6'''-H), 2.80 (2H, t, 7.5 Hz, 1''-H), 1.60 (2H, m, 2''-H), 0.97 (3H, t, 7.3 Hz, 3''-H). 13C APT-NMR (CDCl3): 157.6 (C-8), 156.4 (C-6), 154.9 (C-3), 151.5 (C-1), 136.6 (C-4a), 123.4 (C-2), 111.3 (C-8a), 105.2 (C-4), 100.2 (C-1')∗, 98.5 (C-5), 97.0 (C-7), 93.3 (C-1''')∗, 71.7 (C-4')Δ, 71.6 (C-4''')Δ, 69.2 (C-3'), 67.6 (C-3'''), 59.1 (C-6')Ψ, 59.0 (C-6''')Ψ, 55.8 (C-8-OCH3)Ω, 55.2 (C-6-OCH3)Ω, 26.5 (C-2''), 23.2 (C-1''), 14.5 (C-3''). Shifts with identical superscripts (*,∞,Δ,Ψ,Ω) within a data set are interchangeable.
NBS Bromination of 12: Synthesis of 5-Bromo-6,8-dimethoxy-1,3-bis-(2-methoxy-ethoxymethoxy)-2- propyl-naphthalene (13)
To a solution of 6,8-dimethoxy-1,3-bis-(2-methoxy-ethoxymethoxy)-2-propyl-naphthalene (12) (10 mg, 0.023 mmol) in CCl4 (4 mL) was added N-bromosuccinimide (5.1 mg, 0.031 mmol) and a catalytic amount of AIBN. The solution was refluxed until all the starting material had been consumed (TLC), and was then extracted with ether (30 mL) and aqueous NaHCO3 (5%; 30 mL). The ethereal phase was concentrated in vacuo and the resulting crude oil was purified on a short flash chromatography column (r = 2.5 mm, l = 5 cm, silica gel 0.040-0.063 mm; 2% MeOH in DCM), to give the ring-brominated naphthalene (13), (4 mg, 8 μmol, 34%).
5-Bromo-6,8-dimethoxy-1,3-bis-(2-methoxy-ethoxymethoxy)-2-propyl-naphthalene (13): TLC: Rf 0.32 Molecules 10 01272 i008 [MeOH/ DCM, 2:98 (v/v)]. ESI-EIMS (+ve ion, cv 20 V) m/z: 541 (M+Na, 81Br, 100%), 539 (M+Na, 79Br, 100%), 519 (M+H, 81Br, 55%), 517 (M+H, 79Br, 55%). HREI-MS m/z: found 516.1356 (C23H33O879Br1 requires 516.1359). 1H-NMR (CDCl3): 7.65 (1H, s, 7-H), 6.55 (1H, s, 4-H), 5.43 (2H, s, 1'''-H), 5.10 (2H, s, 1'-H), 4.00 (3H, s, 8-OCH3)*, 3.97 (3H, s, 6-OCH3)*, 3.87 (4H, m, (3'-H, 3'''-H)), 3.61 (4H, m, (4'-H, 4'''-H)), 3.40 (6H, s (coincident), 6'-H & 6'''-H), 2.82 (2H, t, 7.8 Hz, 1''-H), 1.58 (2H, m, 2''-H), 0.98 (3H, t, 7.4 Hz, 3''-H). Shifts with identical superscripts (*,) within a data set are interchangeable.

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