Hydrochloric Acid → 7-Methyl-1,3-dihydro-2,1-benzoxaborol-1-ol

Butyllithium (2.5 M, 2.91 mL, 7.3 mmol) was added dropwise to a −78° C. solution of aryl halide ((Example 5: step a) 723 mg, 2.91 mmol) in Et2O (12 mL). The solution was stirred at −78° C. for 2 h and trimethylborate (3.3 mL, 29.1 mmol) was added in one portion. The solution was warmed to rt over 15 min and stirred for 1 h at rt (appearance of gelatin-like ppt). EtOAc (80 mL) and HCl (0.1 N, 30 mL) were added and the biphasic solution was stirred for 15 min. The layers were separated and the organic layer was washed with HCl (0.1N, 2×10 mL), water (10 mL), brine (30 mL), and was dried over sodium sulfate. Concentration of the solution in vacuo yielded an oily solid which further solidified upon trituration with hexanes. The crude title compound (contaminated with butylboronate products) was used without further purification, existing as a mixture of the cyclic half-ester and the free boronic acid. 1H-NMR (CDCl3): δ 7.1–7.4 (m, 3H), 5.23 (m, 2H), 2.57 (s, 3H).