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3 For a recently reported example of a gem-dimethylcyclo-propane natural product, see: Gao S.
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15b There is also a more recent report on the use of RuO2/NaIO4 for the oxidation of trimethylsulfonium salts: Forrester J.
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18
Preparation of Triisopropylsulfoxonium Tetrafluoroborate
A 250 mL round-bottomed flask with stirrer bar was charged with triisopropylsulfonium tetrafluoroborate
[14]
(6.20 g, 25.0 mmol, 1.0 equiv), and then MeCN (36 mL), CCl4 (36 mL), and H2O (54 mL) were added via syringe. The resulting biphasic solution was stirred vigorously and ruthenium(III) chloride (2.07 g, 10.0 mmol, 0.40 equiv) added in a single portion. The mixture was stirred for 10 min and then NaIO4 (40.10 g, 187.5 mmol, 7.5 equiv) was added in 5 portions over ca. 5 min to the brown-coloured solution. The flask was loosely stoppered with a cork and the mixture vigorously stirred overnight at r.t. (14 h). The resulting grey-brown heterogeneous suspension was filtered through a Celite® pad (3 cm × 70 mm Ø) and washed with H2O (400 mL). The yellow filtrate was stirred vigorously and MeOH (50 mL) added to quench the residual RuO4. The green suspension was concentrated under reduced pressure to remove the water present (60 °C) and the grey solid residue suspended in acetone (400 mL). The reaction mixture was stirred for a further 10 min and filtered to remove the residual inorganic solids. Concentration of the filtrate under reduced pressure afforded a yellow-orange powder that was recrystallised from MeOH-Et2O. The crude solid was dissolved in 10 mL of boiling MeOH and Et2O added until the solution remained turbid. The mixture was then re-heated until homogeneous and allowed to cool, to afford the title compound as colourless plates (5.06 g, 76%); mp 108-109 °C. IR (acetone): νmax = 3427, 1699, 1642, 1462, 1369, 1235, 1198, 1049 cm-1. 1H NMR (400 MHz, acetone-d
6): δ = 1.79 (d, J = 7.0 Hz, 18 H, CH3), 4.73 (sept, J = 7.0 Hz, 3 H, CH). 13C NMR (100 MHz, acetone-d
6): δ = 15.7 (CH3), 53.0 (CH). 19F NMR (254 MHz, acetone-d
6): δ = -151.6. 11B NMR (87 MHz, acetone-d
6): δ = -1.9. ESI-MS: m/z (%) = 177 (100) [M+]. HRMS-FAB: m/z calcd for C9H21OS: 177.1308 (0.4 ppm error); found: 177.1308 [M+]. Anal. Calcd for C9H21BF4OS: C, 40.93; H, 8.01. Found: C, 40.76; H, 7.85.
19 All known products were characterised by NMR spectroscopy and comparison of key data with those published; novel products were fully characterised.
20
Representative Procedure for Cyclopropanation of α,β-Unsaturated Carbonyl Compounds (Table 1, Entry 1)
A 25 mL round-bottomed flask with stirrer bar was charged with NaH (60% dispersion in mineral oil, 23 mg, 0.57 mmol, 1.2 equiv), sealed with a rubber septum and purged with argon. The flask was maintained under argon and anhyd DMF (4 mL) was added. The vigorously stirred suspension was cooled to 0 °C, the septum briefly removed and triisopropylsulfoxonium tetrafluoroborate (152 mg, 0.57 mmol, 1.2 equiv) added in a single portion. The mixture was stirred for 5 min before the addition of a solution of (E)-chalcone (100 mg, 0.48 mmol) in DMF (1 mL) dropwise by cannula. The cooling bath was removed and the brown-coloured solution allowed to stir at r.t. until the reaction was deemed to be complete by TLC (5 h). The reaction was quenched by the addition of sat. aq NH4Cl (5 mL), diluted with H2O (20 mL) and extracted with Et2O (3 × 15 mL). The combined organic extracts were dried (Na2SO4) and the solvent removed in vacuo. The residue was purified by column chromatography (PE-Et2O, 19:1 ) to afford trans-(2,2-dimethyl-3-phenylcyclopropyl)-phenylmethanone (6) as a cream-coloured solid (109 mg, 91%); mp 63-64 °C. R
f
= 0.29 (PE-Et2O, 19:1). 1H NMR (400 MHz, CDCl3): δ = 1.12 (s, 3 H, CH3), 1.27 (s, 3 H, CH3), 2.91 (d, J = 6.0 Hz, 1 H, CH), 3.12 (d, J = 6.0 Hz, 1 H, CH), 7.20-7.24 (m, 3 H, ArH), 7.28-7.32 (m, 2 H, ArH), 7.49-7.53 (m, 2 H, ArH), 7.57-7.61 (m, 1 H, ArH),7.99-8.02 (m, 2 H, ArH). Data consistent with those reported in the literature.
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