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Nagel M.
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4b In addition, several grams of 4,8-dimethylcyclohepta[b]furan-2(2H)-one(12b) were prepared analogously as described from 2,6-dimethylphenyl prop-2-ynoate(9b). Selected spectroscopic data of 9b (colorless solid): 1H NMR (300 MHz, CDCl3): 7.06 (br s, 3 H); 3.03 (s, 1 H), 2.18 (s, 6 H). 13C NMR (75 MHz, CDCl3): 150.3 (s, C=O), 147.2 (s, arom. C-O), 129.9 [s, C(2′,6′)], 128.7 [d, C(3′,5′)], 126.5 [d, C(4)], 76.5 [d, C(3)], 74.0 [s, C(2)], 16.1 (q, o-CH3). Data of 12b: 1H NMR (300 MHz, CDCl3): 6.93 (d, J = 8.9 Hz, 1 H); 6.84 (d, J = 11.4 Hz, 1 H); 6.67 (dd, J = 8.9, 9.9 Hz, 1 H); 5.57 (s, 1 H); 2.40 (s, 3 H); 2.28 (s, 3 H). 13C NMR (75 MHz, CDCl3): 168.0, 152.4, 151.7, 137.5 (4 s), 134.6, 133.4, 127.8 (3 d), 125.0 (s), 95.5 (d), 23.2, 18.6 (2 q).
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Lloyd D.
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5f For a recent synthesis of substituted azulene derivatives, see e.g.: Kane JL.
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5g For a historical flashback, see: Hansen H.-J.
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6a Some polymethylazulenes were investigated earlier in a broad and systematic study of their photoelectron (PE) spectra, but no details of their syntheses were given. cf.: Heibronner E.
Hoshi T.
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7h See, e.g.: Nozoe T.
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7i An example, starting with the naturally occurring 4-isopropyltropolone (γ-thujaplicine, 5b), is illustrated in Scheme 1: Yokota M.
Yanagisawa T.
Kosakai K.
Wakabayashi S.
Tomiyama T.
Yasunami M.
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7j This route was also applied for the synthesis of the antiulcer drug eugalen sodium (KT1-32, sodium 3-ethyl-7-isopropylazulene-1-sulfonate): Yanagisawa T.
Kosakai K.
Tomiyama T.
Yasunami M.
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8a
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8b See also: Briquet AAS.
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10c Similar thermal acetylene-vinylidene rearrangements are involved in the pyrolytic cycloisomerization of, e.g., α-alkynones: Karpf M.
Dreiding AS.
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Brown RFC.
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10g Cf. also: Ondruschka B.
Zimmermann G.
Remmler M.
Ziegler U.
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[3]
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11a
Kuehne ME.
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Leonard NJ.
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Takeda K.
Minato H.
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13 Typical procedure for the synthesis of 2-isopropyl-4,8-dimethylazulene (vetivazulene or elemazulene, 16): In a thick-walled 15 mL Pyrex tube, equipped with a srew cap and a magnetic stirrer, 300 mg to 500 mg of 4,8-dimethylcyclohepta[b]furan-2(2H)-one (12b) were dissolved in 10 mL of either a mixture of anhyd toluene and t-BuOH (ca. 3:1 to 1:1, to enhance the solubility of 12b) or pure t-BuOH. Then, 1.5-2.5 mL of enamine 15 (prepared according to ref.
[14]
) were added, and the tube was sealed and heated in an oil bath to 120-130 °C for 12-16 h with stirring. During this time, the color of the reaction mixture changed from orange to reddish brown or dark violet. The formation of the violet azulene 16 was directly monitored by TLC analyses of aliquot parts of the mixture (alox plates, eluant hexane). Samples were taken from the tube after cooling to r.t. (a small pressure relief was observed, due to the evolution of equimolar amounts of CO2 during the reaction).
Work-up: The reaction mixture was poured into about 50 mL of hexane and washed several times with water (to hydrolyze the excess of non-reacted enamine), successively with 5% HCl solution, and sat. NaHCO3 solution. After evaporation of the volatile components in vacuo the dark-colored oily residue was filtered through a pad of aluminium oxide (or SiO2) with hexane as eluant. The violet filtrate was collected, dried on MgSO4 and concentrated. After purification by column chromatography on aluminium oxide with hexane as eluant, azulene 16 was obtained as blue-violet oil (GC purity > 95%) in typical yields of 70-80% (cf. also the similar general procedure in ref.
[20d]
). Selected spectroscopic data of 16: 1H NMR (300 MHz, CDCl3): 7.34 [t, J = 10.2 Hz, H-C(6)]; 7.25 [s, H-C(1,3)]; 7.07 [d, J = 10.2 Hz, H-C(5,7)]; 3.30 [sept, J = 6.9 Hz, 1 H, iPr-C(2)]; 2.88 [s, H3C-C(4,8)]; 1.43 (d, J = 6.9 Hz, 6 H, iPr-C(2)). 1H NMR (300 MHz, C6D6): 7.33 (s, 2 H); 7.11 (t, J = 10.0 Hz, 1 H), 6.83 (d, J = 10.2 Hz, 2 H); 3.29 (sept, J = 6.9 Hz, 1 H); 2.65 (s, 6 H); 1.45 (d, J = 6.9 Hz, 6 H).13C NMR (75 MHz, CDCl3): 158.0 [C(2)], 144.6 [C(4,8)], 137.7 [C(3a,8a)], 133.5 [C(6)], 125.5 [C(5,7)], 113.5 [C(1,3)], 30.1 [d, iPr-C(2)], 24.5 [Me-C(4,8)], 24.0 [q, iPr-C(2)]. EI-MS: 199.0 (22), 198.0 (92, M+·), 182.9 (100, M - 15), 168.0 (55, M - 30), 165.0 (47, M - 43).
14
Carlson R.
Nilsson A.
Strömqvist M.
Acta Chem. Scand., Ser. B
1983,
37:
7
15 Preparations of azulenes 17 and 35 were performed analogously to the procedure described in ref.
[13]
Selected spectroscopic data of 2-isopropyl-4,6,8-trimethylazulene (17) (reddish-violet oil): 1H NMR (300 MHz, CDCl3): 7.17 (s, 2 H); 7.00 (s, 2 H), 3.26 (sept, J = 6.9 Hz, 1 H); 2.83 (s, 6 H); 2.28 (s, 3 H); 1.41 (d, J = 6.9 Hz, 6 H).1H NMR (300 MHz, C6D6): 7.32 (s, 2 H); 6.82 (s, 2 H); 3.30 (sept, 1 H); 2.67 (s, 6 H); 2.30 (s, 3 H); 1.47 (d, J = 6.9 Hz, 6 H). 13C NMR (75 MHz, CDCl3): 156.5, 144.1, 143.5 136.2 (4 s), 127.2 (d), 113.4 (s), 29.9 (d), 28.4, 24.8, 23.9 (3 q). EI-MS: 212.0 (87, M+·), 197.0 (100, M - 15). Data of 2-isopropyl-4,5,7,8-tetramethylazulene (35) (dark-blue solid): 1H NMR (300 MHz, CDCl3): 7.50 (s, 1 H); 7.14 (s, 2 H), 3.25 (sept, J = 6.9 Hz, 1 H); 2.72 (s, 6 H); 2.51 (s, 6 H); 1.41 (d, J = 6.9 Hz, 6 H).1H NMR (300 MHz, C6D6): 7.37 (s, 3 H); 3.34 (sept, 1 H); 2.58 (s, 6 H); 2.30 (s, 6 H); 1.50 (d, J = 6.9 Hz, 6 H). 13C NMR (75 MHz, CDCl3): 157.2, 142.6 (2 s), 139.7 (d), 138.6, 130.2 (2 s), 111.7, 30.0 (2 d), 26.8, 23.9, 21.1 (3 q). EI-MS: 226.0 (100, M+·), 211.0 (65, M - 15).
16a The well-established Ziegler-Hafner procedure, starting from the corresponding trimethylpyrylium salt and sodium cyclopentadienide, delivers optimized yields of 20 in the order of 45-65%: Hafner K.
Kaiser H.
Liebigs Ann. Chem.
1958,
618:
140
16b cf. also:
Org. Synth. Coll. Vol. V
J. Wiley and Sons;
New York:
1964.
p.1088-1091
16c For NMR data, see also: Braun S.
Kinkeldei J.
Tetrahedron
1977,
33:
1827-1832 ; and references therein.
16d Cf.: Matsubara Y.
Takekuma S.
Yokoi K.
Yamamoto H.
Nozoe T.
Bull. Chem. Soc. Jpn.
1987,
60:
1415
16e
Collins MJ.
Sternhell S.
Tansey CW.
Aust. J. Chem.
1990,
43:
1541
16f
Fallahpour R.-A.
Hansen H.-J.
Helv. Chim. Acta
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78:
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17 Hafner"s synthesis (cf. ref.16a) leads to badly separable mixtures of 1,4,6,8- and 2,4,6,8-tetramethylazulene (23, and 25, respectively): Anderson AG.
Anderson RG.
Hollander GT.
J. Org. Chem.
1965,
30:
131 ; cf. also ref.5c
For further literature, including spectroscopic characterizations, see:
18a
Chen Y.
Kunz RW.
Uebelhart P.
Weber RH.
Hansen H.-J.
Helv. Chim. Acta
1992,
75:
2447
18b
Fallahpour RA.
Hansen H.-J.
Helv. Chim. Acta
1992,
75:
2210
18c
Rippert AJ.
Ph. D. Thesis
University of Zürich;
Switzerland:
1994.
19 Cf. ref.
[1g]
and references cited therein.
For the preparation of pyrrolidine enamines from aminals cf.:
20a
Mannich G.
Davidsen H.
Ber. Deutsch. Chem. Ges.
1936,
69:
2106
20b
Opitz G.
Hellmann H.
Schubert HW.
Liebigs Ann. Chem.
1959,
623:
112
20c
Igarashi M.
Tada M.
J. Heterocyclic Chem.
1995,
32:
807 ; and references therein
20d
In situ aminal thermolysis (general method): Finely powdered, dry K2CO3 (1.2-2 mol equiv) was suspended in toluene, and pyrrolidine (2 mol equiv) was added. The aldehyde (1 mol equiv) was added with stirring at 0-5 °C and the suspension stirred for 12 h at r.t. (inert gas atmosphere). After filtration (or centrifugation) the slightly yellowish ‘aminal solutions’ were used without further purification. The cyclohepta[b]furan-2(2H)-ones 12 were dissolved in TEGDME (or NMP or toluene, respectively) and heated with stirring together with about 5-7 mol equiv of the ‘aminal solution’ to 120-140 °C in a stainless steel autoclave or Schlenk flask, respectively. Within 12-36 h the mixture changed the color from yellow to reddish brown and finally to violet with a slight evolution of gas (CO2). The formation of the azulenes was monitored by TLC analyses after acidic work-up of aliquot parts of the product mixture. Finally, the mixtures were poured in diluted HCl solution (pH ca. 4-5) and the organic phase dissolved in hexane. The intensely blue-green to red-violet colored organic layers were washed several times with diluted HCl solutions and brine, and filtered through a pad of silica gel or alox. The now blue or violet organic phases were dried (NaSO4) and the solvent removed. The azulenes were subsequently purified by column chromatography on alox (basic, act. IV) or on silica gel with hexane as eluent.
21a
Hafner K.
Angew. Chem.
1958,
70:
419
21b
Hafner K.
Stephan A.
Bernhard C.
Liebigs Ann. Chem.
1961,
650:
42
21c
Hafner K.
Stephan A.
Bernhard C.
Liebigs Ann. Chem.
1961,
650:
62
22 For azulene formation from 12e by cycloaddition with itself or with other cyclohepta[b]furan-2(2H)-ones such as 12a or 12c, see ref.
[1f]
23 Data of selected azulenes: For 14, 33 and 34, see ref.
[4]
. NMR data (standard conditions: 300/75.5 MHz, in CDCl3/TMS): 1,4,6,8-Tetramethylazulene(23): (violet-blue cyrstals) 1H NMR: 7.44 [d, 3
J {H-C(3)} = 4 Hz, H-C(2)]; 7.24 [d, 3
J {H-C(2)} = 4 Hz, H-C(3)]; 6.86 [br s, H-C(5,7)]; 3.02 [s, CH3-C(8)]; 2.81 [s, CH3-C(4)]; 2.56 [br s, 6 H, CH3-C(1), CH3-C(6)]. 13C NMR: 147.1 [C(8)], 145.7 [C(6)]; 144.9 [C(4)]; 136.7 [C(3a)]; 136.5 [C(2)]; 132.9 [C(8a)]; 127.7 [C(7)]; 126.8 [C(1)]; 125,6 [C(5)]; 114.6 [C(3)]; 28.4 [CH3-C(8)]; 27.7 [CH3-C(6)]; 25.3 [CH3-C(4)]; 19.7 [CH3-C(1)]. EI-MS (GC-MS): 184 (100, M+·), 169 (85, [M - CH3]+·). 2,4,6,8-Tetramethylazulene (25): (blue-violet crystals) 1H NMR: 7.12 [s, H-C(1,3)]; 7.02 [s, H-C(5,7)]; 2.82 [s, H3C-C(4,8)]; 2.61, 2.598 [2 s, H3C-C(6), H3C-C(2)]. 13C NMR: 145.0, 144.0, 143.0, 136.5 [4 q, C(2,3a/8a,4/8,6)]; 127.2 [d, H-C(5/7)]; 116.3 [d, H-C(5/7)]; 28.4 [q, H3
C-C(6)]; 24.8 [q, H3
C-C(4/8)]; 16.4 [q, H3
C-C(2)]. EI-MS (GC-MS): 184 (100, M+·), 169 (65, [M - CH3]+·). 2-Ethyl-4,6,8-trimethylazulene (28): 1H NMR (taken from the 1:1 mixture with known 27): 7.12 [
s
, H-C(1,3)]; 6.99 [(H-C(5,7)]; 3.03 [q, J = 7.4 Hz,
Me-CH
2-C(2)]; 2.81 [s, CH3-C(4,8)]; 2.60 [s, CH3-C(6)]; 1.32 [t, J = 7.4 Hz, CH
3-CH2-C(2)]. EI-MS (GC-MS): 198 (100, M
+
), 183 (75, [M - CH3]+
). 1-Ethyl-4,6,8-trimethylazulene (30): 1H NMR: 7.52 [d, 3
J = 4 Hz, H-C(2)]; 7.28 [d, J = 4 Hz, H-C(3)]; 6.86 [br s, H-C(5,7)]; 3.25 (q, J = 7.4 Hz, H
2CCH3); 2.98, 2.80, 2.53 (3 s, 3 CH3); 1.36 (t, J = 7.4 Hz, H2CCH
3). 13C NMR: 146.7, 145.6, 144.9, 136.8 (4s, arom C); 134,6 [d, H-C(2)]; 133.8 132.0 (2 s, arom. C); 128.1, 125.9, 115.1 (3 d, H-C); 28.4, 27.5, 25.5 (3 q, CH3); 25.3 (t, CH
2CH3); 17.1 (q, CH3). EI-MS (GC-MS): 198 (35, M+·), 183 (100, [M - CH3]+·). 1-Isopropyl-4,6,8-trimethylazulene (32): (blue oil) 1H NMR: 7.69 [d, J = 4.2 Hz, H-C(2)]; 7.33 [d, J = 4.2 Hz, H-C(3)]; 6.98, 6.88 [2 s, H-C(5), H-C(7)]; 3.91 [sept, J = 6.7 Hz, H-C(CH3)2]; 3.03, 2.82, 2.54 (3 s, 3 CH3); 1.38 [d, J = 6.7 Hz, H-C(CH
3)2]. 13C NMR: 146.2, 145.3, 144.8, 139.2, 136.7 (5 s), 131.8 (d), 130.9 (s); 128.5, 126.0, 115.5 (3 d, arom C-H); 28.5 [d, H-C(CH3)2]; 28.3, 28.1, 26.0, 25.9, 25.6 (5 q, CH3). EI-MS (GC-MS): 212 (25, M+·), (100, [M - CH3]+·). 4,5,7,8-Tetramethylazulene (36) (blue oil): 1H NMR: 7.70 [t, J = 4.0 Hz, H-C(2)]; 7.66 [s, H-C(6)]; 7.39 [d, J = 4.0 Hz, H-C(1,3)]; 2.83 (s, 2 CH3); 2.60 (s, 2 CH3). 13C NMR: 144.8 [s, C(3a,8a)]; 141,3 [d, H-C(6)]; 138.4 [s, C(4,8)]; 133.3 [d, H-C(2)]; 130,2 [s, C(5,7)]; 114.2 [d, H-C(1,3)]; 26.8, 21.4 (2 q). EI-MS (GC-MS): 184 (100, M+·), 169 (85, [M - CH3]+·). 1,4,5,7,8-Pentamethylazulene (37): 1H NMR: 7.60 [d, J = 4.1 Hz, H-C(2)]; 7.37 [d, J = 4.1 Hz, H-C(3)]; 2.79, 2.60, 2.54 (3 s, CH3); 2.33 (s, 2 CH3). 13C NMR: 146.6, 143.8 [2 q, C(3a), C(8a)]; 140.8 [d, H-C(6)]; 140.0 (q, arom C); 138.1 [d, H-C(2)]; 136.5, 134.7, 130.0, 128.9 (4 q, arom C), 114.1 [d, H-C(3)]; 27.2, 26.4, 23.1, 21.4, 20.5 (5 q, CH3). 1-Isopropyl-4,5,7,8-tetramethylazulene (38): (dark blue oil) 1H NMR: 7.72 [d, J = 4.4 Hz, H-C(2)]; 7.44 [s, H-C(6)]; 7.27 [d, J = 4.4 Hz, H-C(3)]; 3.80 [sept, J = 6.9 Hz, H-C(CH3)2]; 2.83, 2.76 (2 s, CH3); 2.51 (s, 2 CH3); 1.37 [d, J = 6.7 Hz, H-C(CH
3)2]. EI-MS (GC-MS): 226 (30, M+·), 211 (100, [M - CH3]+·). 4,5,6,7,8-Pentamethylazulene (40)
[1f]
(blue crystals): 1H NMR: 7.56 [t, J = 4.3 Hz, H-C(2)]; 7.25 [d, J ˜ 4 Hz, H-C(1,3)]; 2.84 (br s, 6 H, 2 CH3); 2.52 (s, 3 H, CH3); 2.47 (s, 6 H, 2 CH3). 13C NMR: 145.3, 144.4, 137.6 (3 s); 132.1 [d, C(2)]; 130.5 (s); 113.9 [d, C(1,3)]; 24.5, 22.7, 22.5 (3 q). 1,4,5,6,7,8-Hexamethylazulene (39): (blue-violet oil) 1H NMR: 7.20 [d, J = 3.9 Hz, H-C(2)]; 6.99 [d, J = 3.9 Hz, H-C(3)]; 2.75, 2.70, 2.68, 2.37 (4 s, 4 CH3); 2.33 (br s, 2 CH3).
