Experimental
Synthesis of 79: Alcohol 24 (1.01 g, 2.32 mmol) and PhI(OAc)2 (3.74 g, 11.6 mmol, 5 equiv) were dissolved in CH2Cl2 (23 mL, 0.1 M) at room temperature under Ar. After cooling to –30°C, Br2 (954 μL, 18.6 mmol, 8 equiv) was added. The temperature was maintained between –30 and –36 °C with sunlamp irradiation (75 W, 18 inches above surface of cooling bath) for 10 hours. Irradiation was then halted, the reaction was diluted with ice-cold CH2Cl2 (200 mL), and the crude mixture was shaken with 10% aq. Na2S2O3 (100 mL) until colorless. The aqueous layer was back-extracted twice with CH2Cl2 (2 × 100 mL), dried over MgSO4, filtered, and concentrated in vacuo at 5 °C to about 40 mL. At 0 °C, imidazole (0.79 g, 11.7 mmol, 5 equiv) and TMSCl (1.5 mL, 11.7 mmol, 5 equiv) were added. After 15 minutes, the reaction was diluted with EtOAc (200 mL), washed with water (20 mL), then sat. aq. NaHCO3 (20 mL), then sat. aq. NaCl (10 mL), dried over MgSO4, filtered, and concentrated in vacuo. Purification by flash column chromatography (1:2 EtOAc:hexanes) afforded dibromide 79 (877 mg, 57%) as a white foam. IR (neat) νmax = 3354, 2957, 2882, 1686, 1424, 1254, 1173, 1115, 1083, 1042, 873, 846, 732, 723 cm-1.
Synthesis of 80: Dibromide 79 (2.3 g, 3.4 mmol) was dissolved in THF (70 mL, 0.05 M) and freshly flame-dried LiCl (719 mg, 17 mmol, 5 equiv) was added, followed by dry DBU (1.0 mL, 6.8 mmol, 2 equiv). The reaction mixture was stirred for 24 hours, at which point it was diluted with EtOAc (100 mL), washed with twice with water (100 mL), and once with brine (100 mL). The aqueous layer was extracted twice with CH2Cl2; these portions were added to the organic phase, which was dried with MgSO4, filtered, and concentrated in vacuo. Flash Column Chromatography (1:3 EtOAc:hexanes) afforded SI-14 bromocylopropane 80 (1.7 g, 85%) as a colorless foam. IR (neat) νmax = 2957, 2882, 1738, 1684, 1425, 1252, 1117, 1032, 1012, 880, 844 cm-1.
Definitions:
Sunlap irradiation: Refers to the light that comes from an unltraviolet lamp. Ultraviolet light is used for many different reasons in chemistry, including photochemical cycloaddition reactions. In this reaction, the ultraviolet light is used to trigger homolysis of the bromine-bromine bond which serves as the initiator of the radical halogenization.
Flash column chromatography: Similar to typical column chromatography; however, gas is used to force the eluent through the stationary phase.
Under Ar: refers to a piece of machinery that is used when chemicals are unstable under normal atmospheric air. These reactions are carried out in a glove box, which is a tank whose air is removed and replaced with a special kind of gas. In this experiment, that gas is argon. These hoods have holes covered by a flexible rubber material; chemists manipulate the chemicals inside the hoods though these pieces of rubber.