Synlett 2003(10): 1485-1487
DOI: 10.1055/s-2003-40835
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Organoboranes Mediated Radical Cyclizations

Barbara Becattinia, Cyril Ollivierb, Philippe Renaud*a
a University of Berne, Department of Chemistry and Biochemistry, Freiestrasse 3, 3000 Berne 9, Switzerland
b University of Fribourg, Department of Chemistry, Pérolles, 1700 Fribourg, Switzerland
Fax: +41(31)6313426; e-Mail: philippe.renaud@ioc.unibe.ch;
Further Information

Publication History

Received 17 April 2003
Publication Date:
24 July 2003 (online)

Abstract

Hydroboration of dienes with catecholborane followed by radical cyclization using pyridine-2-thione-N-methoxycarbonyl­oxy (= PTOC-OMe, a Barton carbonate) as chain transfer reagent is reported. A facile access to a bicyclic α-methylenelactone is described.

    References

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  • 1b Radicals in Organic Synthesis   Vol. 2:  Renaud P. Sibi MP. Wiley-VCH; Weinheim, Germany: 2001. 
  • 2 Renaud P. Lacôte E. Quaranta L. Tetrahedron Lett.  1998,  39:  2123 ; and references therein
  • For recent review articles, see:
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  • 5c Cadot C. Dalko PI. Cossy J. Ollivier C. Chuard R. Renaud P. J. Org. Chem.  2002,  67:  7193 
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8

Procedure A: N , N -Dimethylacetamide-catalyzed Intramolecular Conjugate Addition. To a solution of the diene (1.0 mmol) in CH2Cl2 (0.7 mL) cooled to 0 °C were added N,N-dimethylacetamide (9 mg, 0.1 mol) and catecholborane (0.21 mL, 2.0 mmol or 0.14 mL, 1.2 mmol). The reaction mixture was stirred at r.t. for 3 h then MeOH (50 µL, 1.2 mmol) was added at 0 °C and the mixture was stirred for 15 min at r.t. To a solution of PTOC-OMe (3.0 mmol), freshly prepared by stirring for 1 h in the dark the sodium salt of N-hydroxypyridine-2-thione (469 mg, 3.15 mmol) and methyl chloroformate (0.23 mL, 3.0 mmol) in benzene (10 mL), was added the B-alkylcatecholborane mixture followed by benzene (2 × 10 mL) and 1,3-dimethyl hexahydro-2-pyrimidone (DMPU 0.12 mL, 1.0 mmol). The reaction mixture was irradiated at 10 °C with a 150 W tungsten lamp for about 14 h then treated with 1 N NaOH
(7 mL). The aqueous layer was extracted with CH2Cl2 and the combined organic phases were dried over MgSO4 and concentrated in vacuo. The crude product was purified by flash chromatography.

9

The Lewis acidic character of the intermediate organoborane is presumably leading to some decomposition of the acetal moiety via an intramolecular complexation.

10

Procedure B: Rhodium-catalyzed Intramolecular Conjugate Addition. Catecholborane (0.21 mL, 1.2 mmol) was added dropwise at -78 °C to a solution of the olefin (1.0 mmol) and rhodium catalyst (2 mol%), prepared according to the procedure reported by Burgess [14] from [Rh(COD)Cl]2 (5 mg, 0.01 mmol) and PPh3 (10.5 mg, 0.02 mmol) in THF (4 mL). The reaction mixture was stirred overnight at r.t. MeOH (50 µL, 1.2 mmol) was added at 0 °C, the mixture stirred for 15 min at r.t. and the solvent evaporated under vacuum. A solution of PTOC-OMe (3.0 mmol), freshly prepared by stirring for 1 h in the dark the sodium salt of N-hydroxypyridine-2-thione (469 mg, 3.15 mmol) and methyl chloroformate (0.23 mL, 3.0 mmol) in benzene (5 mL), was added to the B-alkylcatecholborane mixture followed by benzene (5 mL) and 1,3-dimethyl hexahydro-2-pyrimidone (DMPU, 0.12 mL, 1.0 mmol). The reaction mixture was irradiated at 10 °C with a 150 W tungsten lamp overnight then treated with 1 N NaOH (7 mL). The aqueous layer was extracted with CH2Cl2 and the combined organic phases were dried over MgSO4 and concentrated in vacuo. The crude product was purified by flash chromatography.

11

3-Methyl-3-(2-pyridinylsulfanyl)hexahydro-2 H -cyclopenta[ b ]furan-2-one(12). Prepared according to procedure B from 11 (138 mg, 1.0 mmol). FC (hexane/EtOAc 2:1) afforded 12 (157 mg, 63%). Light yellow oil.
IR (film): 2966, 1761, 1577, 1558, 1453, 1416, 1374, 1200 cm-1. 1H NMR (300 MHz, CDCl3): δ = 8.53-8.51 (m, 1 H), 7.63-7.58 (m, 1 H), 7.47-7.44 (m, 1 H), 7.20-7.15 (m, 1 H), 5.05 (td, J = 1.8, 5.9 Hz, 1 H), 3.05-2.98 (m, 1 H), 2.06-1.64 (m, 6 H), 1.61 (s, 3 H). 13C NMR (75.5 MHz, CDCl3): δ = 176.8, 154.8, 149.6, 136.6, 127.3, 121.9, 83.8, 54.0, 50.9, 32.1, 27.5, 24.4, 19.9. MS (EI): m/z (%) = 250 (2) [M+ + 1], 249 (2), 220 (4), 112 (21), 111 (100), 78 (13), 67 (21). HRMS (LSIMS) for C13H15NO2S: calcd 249.0829. Found: 249.0823.