Organoboranes Mediated Radical Cyclizations

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.

Key words

radicals - boron - hydroborations - lactones - cyclizations

References

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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.