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Synlett 2007(4): 0615-0618  
DOI: 10.1055/s-2007-970748
LETTER
© Georg Thieme Verlag Stuttgart · New York

Yb(OTf)3-Catalyzed [4+2] Cycloaddition of Allenyltrimethylsilylthioketenes with Arylaldimines

Shigenobu Aoyagi*, Satoru Ohata, Kazuaki Shimada, Yuji Takikawa
Department of Chemical Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 0208551, Japan
Fax: +81(19)6216324; e-Mail: aoyagi@iwate-u.ac.jp;
Further Information

Publication History

Received 6 December 2006
Publication Date:
21 February 2007 (online)

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Abstract

Yb(OTf)3-catalyzed [4+2] cycloaddition of an in situ generated allenyltrimethylsilylthioketene with arylaldimines afforded unsaturated δ-thiolactams, one of which was converted into a known 4-azafluorenone, a synthetic precursor of onychine, in three steps.

Key words

allenyltrimethylsilylthioketene - arylaldimine - [4+2] cycloaddition - Yb(OTf)3 - onychine

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A CHCl3 (30 mL) solution of 1a (900 mg, 4.95 mmol), N-(p-methoxybenzyl)-2-bromobenzylidenamine (3f; 100 mg, 3.29 mmol) and Yb(OTf)3 (408 mg, 0.66 mmol) was heated to reflux for 14 h. The resulting reaction mixture was subjected to column chromatography on silica gel (hexane-EtOAc, 10:1). 4f: yield: 1040 mg (65%); yellow plates; mp 138.8-139.2 °C. MS: m/z (%) = 485 (6) [M+], 470 (47) [M+ - Me], 412 (16) [M+ - TMS], 121 (100) [PMB]. IR (KBr): 2943, 1514, 1514, 1475, 1253, 1244, 875, 844 cm-1. 1H NMR (400 MHz, CDCl3): δ = 0.45 (s, 9 H), 1.93 (s, 3 H), 3.78 (s, 3 H), 3.99 (d, J = 14.5 Hz, 1 H), 5.34 (s, 1 H), 5.61 (s, 1 H), 5.67 (s, 1 H), 8.08 (d, J = 14.5 Hz, 1 H), 6.81 (d, J = 8.6 Hz, 2 H), 7.10-7.15 (m, 1 H), 7.21-7.25 (m, 2 H), 7.28 (d, J = 8.6 Hz, 2 H), 7.52 (d, J = 7.9 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 3.2 (q), 19.0 (q), 54.0 (t), 55.2 (q), 62.6 (d), 113.9 (d), 117.5 (dd), 123.1 (s), 127.5 (d), 127.8 (d), 128.1 (s), 129.5 (d), 129.7 (d), 133.6 (d), 136.6 (s), 141.4 (s), 141.9 (s), 142.7 (s), 159.1 (s), 195.6 (s). Anal. Calcd for C24H28BrNOSSi: C, 59.25; H, 5.80; N, 2.88. Found: C, 59.14; H, 5.89; N, 2.90.

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To a CHCl3 (30 mL) solution of 4f (500 mg, 1.03 mmol) was added N-tosyl-3-phenyloxaziridine (340 mg, 1.23 mmol) portionwise at 0 °C and then the solution was stirred at r.t. for 30 min. The reaction was quenched with an excess amount of aq Na2SO3 solution and the organic layer was separated and dried over anhyd Na2SO4. After evaporation of the solvent, the residue was subjected to column chromatography on silica gel (hexane-EtOAc = 7:1). 6f: yield: 325 mg (67%); colorless oil. MS: m/z (%) = 469 (19) [M+], 454 (19) [M+ - Me], 121 (100) [PMB]. IR (neat): 2951, 1625, 1512, 1247, 1036, 845 cm-1. 1H NMR (400 MHz, CDCl3): δ = 0.39 (s, 9 H), 1.99 (s, 3 H), 3.47 (d, J = 14.6 Hz, 1 H), 3.78 (s, 3 H), 5.22 (d, J = 14.6 Hz, 1 H), 5.28 (s, 1 H), 5.55 (s, 1 H), 5.59 (s, 1 H), 6.81 (d, J = 6.6 Hz, 2 H), 7.10-7.20 (m, 1 H), 7.22 (d, J = 6.6 Hz, 2 H), 7.24-7.29 (m, 2 m), 7.53-7.55 (m, 1 H). 13C NMR (100 MHz, CDCl3): δ = 2.0 (q), 18.6 (q), 47.0(dd), 55.1 (q), 61.1 (d), 113.6 (d), 115.7 (dd), 122.8 (s), 127.6 (d), 127.8 (d), 129.3 (s), 129.8 (d), 133.3 (d), 134.0 (s), 139.3 (s), 142.9 (s), 150.4 (s), 158.8 (s), 166.6 (s). Anal. Calcd for C24H28BrNO2Si: C, 61.27; H, 6.00; N, 2.98. Found: C, 61.09; H, 6.07; N, 2.97.

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A DMF solution (10 mL) of 6f (300 mg, 64 mmol), tri(o-tolyl)phosphine (19 mg, 6 mmol), Pd(OAc)2 (7 mg, 3 mmol) and Et3N (323 mg, 320 mmol) was stirred at 90 °C for 12 h under a N2 atmosphere. After removal of DMF by distillation under reduced pressure, the residue was subjected to column chromatography on silica gel (hexane-EtOAc, 7:1). 7f: yield: 144 mg (58%); pale yellow plates; mp 142.7-144.0 °C. MS: m/z (%) = 389 (42) [M+], 374 (23) [M+ - Me], 121 (100) [PMB]. IR (KBr): 2951, 1619, 1579, 1247, 850 cm-1. 1H NMR (400 MHz, CDCl3): δ = 0.39 (s, 9 H), 2.40 (s, 3 H), 3.62 (s, 2 H), 3.73 (s, 3 H), 5.66 (br s, 2 H), 6.80 (d, J = 8.2 Hz, 2 H), 7.09 (d, J = 8.2 Hz, 2 H), 7.21 (t, J = 7.6 Hz, 1 H), 7.28 (t, J = 7.4 Hz, 1 H), 7.51 (d, J = 7.4 Hz, 1 H), 7.58 (d, J = 7.9 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 1.7 (q), 19.7 (q), 34.4 (t), 46.6 (t), 55.1 (q), 114.1 (d), 122.4 (s), 122.5 (d), 124.5 (d), 125.1 (d), 126.95 (d), 127.01 (d), 127.4 (d), 128.4 (s), 136.4 (s), 144.0 (s), 146.1 (s), 153.6 (s), 158.5 (s), 166.4 (s). Anal. Calcd for C24H27NO2Si: C, 74.00; H, 6.99; N, 3.60. Found: C, 73.83; H, 7.15; N, 3.65.

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A CHCl3 (30 mL) solution of 7f (120 mg, 31 mmol) and p-toluenesulfonic acid monohydrate (205 mg, 108 mmol) was heated to reflux for 5 h. The reaction was quenched with an excess amount of aq NaHCO3 solution and the organic layer was separated and dried over anhyd Na2SO4. After evaporation of the solvent, the residue was subjected to column chromatography on silica gel (hexane-EtOAc = 7:1). 8f: yield: 86 mg (88%); pale yellow solid; mp 162.2-163.8 °C (lit. [7g] 164-165 °C). MS: m/z (%) = 317 (10) [M+], 121 (100) [PMB]. IR (KBr): 2956, 1651, 1526, 1516, 1252, 1174, 1032, 842 cm-1. 1H NMR (400 MHz, CDCl3): δ = 2.32 (s, 3 H), 3.62 (s, 2 H), 3.73 (s, 3 H), 5.71 (br s, 2 H), 6.48 (s, 1 H), 6.82 (d, J = 8.6 Hz, 2 H), 7.12 (d, J = 8.6 Hz, 2 H), 7.25-7.35 (m, 2 H), 7.54 (d, J = 7.1 Hz, 1 H), 7.62 (d, J = 7.6 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ = 19.0 (q), 33.6 (t), 46.8 (t), 55.2 (q), 114.2 (d), 116.6 (d), 121.9 (s), 122.5 (d), 125.2 (d), 127.07 (d), 127.13 (d), 127.4 (d), 128.2 (s), 136.3 (s), 144.4 (s), 145.6 (s), 147.5 (s), 158.6 (s), 163.7 (s).