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Synlett 2009(8): 1281-1284  
DOI: 10.1055/s-0029-1216728
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Cyclobutylcarbinyl Radical Fragmentation Reactions of Tetronate [2+2] Photocycloaddition Products

Jörg P. Hehn, Michael Kemmler, Thorsten Bach*
Lehrstuhl für Organische Chemie I, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany
Fax: +49(89)28913315; e-Mail: thorsten.bach@ch.tum.de;
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Publication History

Received 18 February 2009
Publication Date:
17 April 2009 (online)

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Abstract

Cyclobutanes derived from [2+2] photocycloaddition ­reactions of tetronic acid esters were subjected to free-radical fragmentation reactions. The product formation was strongly dependent on the relative stabilities of the intermediate radicals, thus leading to either an oxepane or a methylenetetrahydrofuran in good yields.

Key words

cycloadditions - free radicals - photochemistry - ring expansion - ring opening

13

Free-Radical Fragmentation of Iodide 3
A solution of iodide 3 (29.6 mg, 100 µmol) in toluene (1.0 mL) was treated at 0 ˚C with Bu3SnH (53.9 µL, 200 µmol) and Et3B (1 M in hexane, 30.0 µL, 30.0 µmol). The reaction was stirred at 0 ˚C for 15 min and directly purified by flash chromatography (9 g Al2O3 Brockmann grade III + 1 g KF, pentane-Et2O = 1:0 to 5:1) to give 10.3 mg of methylene-tetrahydrofuran 4 (61%) as a colorless oil. ¹H NMR (360 MHz, CDCl3): δ = 1.63-1.74 (m, 2 H), 2.01 (dddd, 1 H, J = 14.2, 8.8, 7.0, 5.6 Hz), 2.11-2.20 (m, 1 H), 2.35-2.49 (m, 2 H), 2.63-2.73 (m, 1 H), 3.69 (s, 3 H), 3.80 (virt t, 1 H, J = ca. 1.8 Hz), 3.99 (dt, 1 H, J = 8.3, 6.6 Hz), 4.10-4.16 (m, 1 H), 4.25 (virt t, 1 H, J = ca. 1.8 Hz). ¹³C NMR (62.9 MHz, CDCl3): δ = 28.0 (t), 31.0 (t), 32.1 (t), 40.2 (d), 51.6 (q), 68.7 (t), 79.2 (t), 166.6 (s), 173.6 (s). HRMS (EI): m/z [M+] calcd for C9H14O3: 170.0943; found: 170.0941.

18

The photocycloaddition of tetronate 10 was conducted at λ = 254 nm as previously described.6b 2,10-Dioxa-5 ( tert -butyldimethylsilyloxy)methyl-tricyclo[6.3.0.0. ¹,6 ]undecan-9-one (11)
¹H NMR (250 MHz, CDCl3): δ = 0.07 (s, 6 H), 0.90 (s, 9 H), 1.65-1.85 (m, 2 H), 2.05 (dd, 1 H, J = 12.8, 10.5 Hz), 2.17 (ddd, 1 H, J = 12.9, 4.8, 0.8 Hz), 2.90 (dd, 1 H, J = 10.4, 4.6 Hz), 3.62 (d, 1 H, J = 10.8 Hz), 3.69 (d, 1 H, J = 10.8 Hz), 3.95 (ddd, 1 H, J = 11.2, 9.2, 5.8 Hz), 4.16 (d, 1 H, J = 9.8 Hz), 4.17-4.25 (m, 1 H), 4.58 (d, 1 H, J = 9.9 Hz). ¹³C NMR (62.9 MHz, CDCl3): δ = -5.7 (q), -5.7 (q), 18.1 (s), 25.7 (q), 26.3 (t), 34.2 (t), 38.6 (d), 51.5 (s), 63.9 (t), 67.6 (t), 87.4 (s), 177.9 (s). HRMS (EI): m/z [M+ - C4H9] calcd for C11H17O4Si: 241.0896; found: 241.0897.

21

Free-Radical Fragmentation of Iodide 12
A solution of iodide 12 (22.8 mg, 77.5 µmol) in toluene (0.8 mL) was treated at 0 ˚C with (TMS)3SiH (36.1 µL, 116 µmol) and Et3B (1 M in hexane, 19.4 µL, 19.4 µmol). Oxygen was bubbled through the solution for 2 min, and the reaction mixture was stirred in an oxygen atmosphere for 90 min. The reaction mixture was directly purified by flash chromatography (silica 60, pentane-Et2O = 4:1 to 1:2) to give 12.5 mg of oxepane 14 (96%) as a colorless liquid. ¹H NMR (250 MHz, CDCl3): δ = 2.32-2.54 (m, 3 H), 2.66-2.77 (m, 2 H), 3.33 (ddd, 1 H, J = 11.9, 11.8 3.6 Hz), 4.15 (ddd, 1 H, J = 12.1, 5.7, 2.3 Hz), 4.26 (dd, 1 H, J = 10.1, 1.8 Hz), 4.36 (ddd, 1 H, J = 6.4, 6.1, 1.9 Hz), 4.44 (dd, 1 H, J = 10.1, 5.9 Hz), 4.89 (br s, 1 H), 4.97 (br s, 1 H). ¹³C NMR (62.9 MHz, CDCl3): δ = 33.7 (t), 40.5 (t), 44.7 (d), 72.2 (t), 72.7 (t), 80.1 (d), 114.5 (t), 144.4 (s), 177.2 (s). HRMS (EI):
m/z [M+] calcd for C9H12O3: 168.0786; found: 168.0786.