Methanesulfonic Acid → 5-Fluoro-2,2-dimethylindoline

Participants

Reaction Conditions

Reaction Steps

A solution of N-Boc-4-fluoroaniline (9.02 g, 42.7 mmol) in THF (112 mL) was cooled to −60° C. using a cryocool instrument. The solution was treated with 1.7 M t-BuLi in pentane (63 mL, 106.7 mmol) dropwise. After the first equivalent of base was consumed, a yellow solution formed. The reaction was allowed to warm to −20° C. and was stirred at that temperature for 2.5 hours. The reaction was then treated with a solution of methallyl bromide (5.67 g, 42.7 mmol) in THF (35 mL) dropwise and stirred for an additional 1.5 hours at −20° C. The reaction was then quenched by addition of water. After coming to room temperature, the reaction was treated with ethyl acetate and extracted with water and brine, dried over MgSO4 and filtered. The solvent was then removed and the residue was purified on silica gel using a gradient of 0–25% ethyl acetate in hexane to afford 11.3 g (80% yield) of [4-Fluoro-2-(2-methyl-allyl)-phenyl]-carbamic acid tert-butyl ester as a white solid; 1H NMR (CDCl3, 400 MHz) δ 1.50 (s, 9H), 1.72 (s, 3H), 3.28 (s, 2H), 4.71 (s, 1H), 4.92 (s, 1H), 6.32–6.50 (m, 1H), 6.86 (dd, 1H, J1=3.0, J2=9.1), 6.93 (ddd, 1H, J1=3.0, J2=8.5, J3=11.5), 7.65–7.82 (m, 1H); HPLC-MS calcd. for C15H20FNO2 (M+H+−tBu) 210.1. found 210.3. Step B: A sample of [4-Fluoro-2-(2-methyl-allyl)-phenyl]-carbamic acid tert-butyl ester (1.10 g, 4.14 mmol) was treated with anisole (5 mL), dichloromethane (5 mL) and trifluoroacetic acid (5 mL) and stirred for 4 hours. The solvent was removed and the reaction was transferred to a microwave reaction vial using methanesulfonic acid (3 mL). The reaction was heated to 170° C. for 10 minutes. The reaction was cooled to room temperature and quenched into excess stirring 1 M NaOH. The aqueous phase was extracted twice with ethyl acetate and the combined organics were dried over MgSO4 and filtered. The resulting oil was purified on silica gel using a gradient of 0–70% t-butyl ethyl ether and hexane to afford 450 mg (66% yield) of 2,2-dimethyl-5-fluoroindoline; 1H NMR (CDCl3, 400 MHz) δ 1.08 (s, 6H), 2.58 (s, 2H), 6.24 (dd, 1H, J1=4.4, J2=8.4), 6.43–6.48 (m, 1H), 6.53–6.56 (m, 1H); HPLC-MS calcd. for C10H12FN (M+H+) 166.1. found 166.4.

Provenance & Evidence