Cyclization of Substituted Phenyl N -(2-Hydroxybenzyl)carbamates in Aprotic Solvents. Synthesis of 4 H -1,3-Benzoxazin-2(3 H )-ones
The kinetics of cyclization of substituted phenyl N -(2-hydroxybenzyl)carbamates and their N -methyl analogs, prepared by the reaction of 2-(aminomethyl)phenols with substituted phenyl chloroformates, was studied in dioxane or toluene at the temperatures 110-180 °C. Electron-withdrawing substituents in the leaving phenoxy group strongly accelerate the rate of cyclization (ρ = 2.45 ± 0.15) while the substituents in the other ring have virtually no effect. The cyclization was catalyzed with triethylamine in toluene but not in dioxane. On the basis of these results, the most convenient method for preparation of substituted 4 H -1,3-benzoxazin-2(3 H )-ones was a one-hour reflux of substituted 4-nitrophenyl N -(2-hydroxybenzyl)carbamates in dioxane. Based on the influence of substituents, solvents (dioxane and toluene) and triethylamine, the reaction mechanism and structure of the transition state were proposed.