Photochemical Ring Opening of Tricyclic Hemiketals: Diastereoselective Construction of Functionalized Medium-Sized Carbocycles and a Diquinane

Abstract

A facile stereoselective construction of functionalized medium-sized carbocycles and a diquinane from bicyclo[3.n.1]alkanones is reported. The key step is the hypervalent iodine mediated photochemical ring opening of tricyclic hemiketals, formed by dihydroxylation of methylene-bicyclo[3.3.1]nonan-9-one and methylene-bicyclo[3.2.1]octan-8-one, to afford eight- and seven-membered carbocyclic rings, respectively. Subsequent transformation of the cyclooctane into a diquinane was achieved using LDA. The reaction was also extended to the synthesis of a fused bicylic system.

medium-sized carbocycles - diquinane - radical reaction - bicyclic ketones - Suarez cleavage

References

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All new compounds were thoroughly characterized by IR, ¹H NMR (including COSY and NOESY wherever required), ¹³C NMR and HRMS.

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Typical Experimental Procedure for Photochemical Ring Cleavage: To a solution of protected hemi-ketal 6 (295 mg, 0.72 mmol) in anhyd cyclohexane (85 mL) under argon, were added PhI(OAc)₂ (396 mg, 1.29 mmol) and I₂ (312 mg, 1.29 mmol) at once and the resulting reaction mixture was irradiated with one 100 W tungsten filament lamp (internal temperature 43 °C) for 1 h, after which the reaction mixture was poured into a 1:1 mixture of Et₂O and aq NaHSO₃ solution. The organic layer was separated, washed with NaHSO₃ solution, dried over anhyd MgSO₄, filtered and concentrated. Column chromatography over silica gel using petroleum ether and EtOAc (9:1) afforded the two regiomers, 10 and 11, in a ratio of 5:1 in an overall yield of 61%.

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Spectral data for selected compounds: Compound 10: Mp 142-143 °C. ¹H NMR (300 MHz, CDCl₃): δ = 4.87 (ddd, J = 9.5, 3.5, 1.0 Hz, 1 H), 3.61 (d, J = 11.0 Hz, 1 H), 3.38 (d, J = 11.0 Hz, 1 H), 3.05 (dt, J = 5.0, 3.5 Hz, 1 H), 2.91 (ddt, J = 11.5, 5.5, 2.5 Hz, 1 H), 2.83 (dd, J = 17.5, 3.5 Hz, 1 H), 2.73-2.58 (m, 2 H), 2.25 (dd, J = 17.5, 5.0 Hz, 1 H), 2.23-2.13 (m, 2 H), 1.89-1.77 (m, 2 H), 1.62-1.40 (m, 11 H, with t-butyl group superimposed), 0.86 (s, 9 H), 0.048 (s, 3 H), 0.044 (s, 3 H). ¹³C NMR (75 MHz): δ = 178.09, 170.86, 88.69, 81.24, 67.18, 47.46, 39.38, 36.18, 36.12, 35.73, 32.49, 28.06, 28.00, 27.87, 25.76, 18.13, -5.36, -5.50. HRMS: m/z [MH⁺] calcd for C₂₂H₄₀O₅SiI: 539.168980; found: 539.168978. Compound 11: Viscous liquid. ¹H NMR (300 MHz, CDCl₃): δ = 4.47 (dddd, J = 12.0, 11.0, 3.5, 2.0 Hz, 1 H), 3.52 (d, J = 10.5 Hz, 1 H), 3.43 (d, J = 10.5 Hz, 1 H), 3.18 (td, J = 10.5, 5.0 Hz, 1 H), 3.08 (ddd, J = 10.5, 7.5, 1.5 Hz, 1 H), 2.83 (ddd, J = 16.0, 10.0, 2.0 Hz, 1 H), 2.72-2.46 (m, 2 H), 2.37 (dd, J = 15.0, 12.0 Hz, 1 H), 2.20-1.90 (m, 2 H), 1.79-1.58 (m, 3 H), 1.57-1.37 (m, 10 H,with t-butyl group superimposed), 0.89 (s, 9 H), 0.082 (s, 3 H), 0.068 (s, 3 H). ¹³C NMR (50 MHz, CDCl₃): δ = 177.01, 170.16, 88.28, 81.48, 69.35, 45.35, 42.25, 41.33, 36.69, 34.15, 29.32, 28.03, 27.06, 25.78, 24.92, 18.17, -5.21. HRMS: m/z [MH⁺] calcd for C₂₂H₄₀O₅SiI: 539.168980; found: 539.167411. Compound 12: Mp 101 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.88 (d, J = 11.5 Hz, 1 H), 3.82 (d, J = 11.5 Hz, 1 H), 2.73 (ddd, J = 10.5, 9.5, 5.0 Hz, 1 H), 2.62 (dd, J = 14.5, 5.0 Hz, 1 H), 2.46 (ddd, J = 10.5, 8.0, 6.0 Hz, 1 H), 2.20 (dd, J = 14.5, 9.5 Hz, 1 H), 2.17-2.10 (m, 1 H), 2.02-1.86 (m, 2 H), 1.82-1.60 (m, 4 H), 1.50-1.40 (m, 10 H with t-butyl group superimposed), 0.89 (s, 9 H), 0.09 (s, 3 H), 0.08 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 178.42, 170.87, 94.04, 80.80, 61.79, 59.50, 49.20, 40.95, 40.27, 35.06, 28.03, 26.72, 26.26, 25.79, 22.62, 18.22, -5.37,
-5.49. MS (DCI, CH₄): m/z (%) = 355 (100) [MH⁺ - 56], 297 (35), 157 (44). HRMS: m/z [MH⁺ -56, McLafferty] calcd for C ₁₈H₃₁O₅Si: 355.194078; found: 355.186503. Compound 19: Viscous liquid. ¹H NMR (300 MHz, CDCl₃): δ = 5.28 (dd, J = 3.5, 2.0 Hz, 1 H), 3.65 (d, J = 11.5 Hz, 1 H), 3.38 (d, J = 11.5 Hz, 1 H), 3.30 (dt, J = 8.5, 2.0 Hz, 1 H), 2.98 (dtd, J = 13.5, 5.0, 2.5 Hz, 1 H), 2.70-2.59 (m, 2 H), 2.29 (dd, J = 17.0, 8.5 Hz, 1 H), 2.07 (dd, J = 15.5, 5.0 Hz, 1 H), 2.02 (dd, J = 14.0, 2.5 Hz, 1 H), 1.66 (dd, J =1 5.5, 5.0 Hz, 1 H), 1.68-1.60 (m, 3 H), 1.48 (s, 9 H), 1.47-1.10 (m, 5 H), 1.05 (s, 3 H), 0.86 (s, 9 H), 0.04 (s, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 178.90, 170.72, 90.02, 81.42, 65.39, 53.46, 46.00, 44.79, 43.47, 40.88, 38.89, 38.58, 38.44, 32.81, 28.05, 27.33, 27.28, 25.79, 20.78, 20.53, 18.20, -5.26, -5.42. MS: m/z = 665 (20) [MH⁺], 607 (22), 492 (100), 423 (55), 365 (46), 245 (25).

11 Markgraf JH. Staley SW. Allen JR. Synth. Commun.  1989,  19:  1471