Oxidative Cyclization of Thiobenzanilides to Benzothiazoles Using N-Benzyl-DABCO Tribromide under Mild Conditions

Abstract

N-benzyl-DABCO tribromide, a stable, crystalline organic ammonium tribromide (OATB), have been used as an alternative electrophilic bromine source for the efficient oxidative cyclization of thiobenzanilides to the corresponding benzothiazoles under mild conditions.

Key words

benzothiazole - N-benzyl-­DABCO-tribromide - oxidative cyclization

References

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Preparation of N -Benzyl-DABCO Tribromide. To a stirred solution of DABCO (50 mmol) in dioxane (50 mL), a solution of benzyl bromide (50 mmol) in dioxane (5 mL) was added slowly for about 20 min. Then, a solution of bromine (50 mmol) in dioxane (20 mL) was cautiously added under vigorous stirring. After cooling the solution, the resulting orange crystals were filtered and dried at r.t.

21

General Procedure for the Oxidative Cyclization of Thiobenzanilides.
To a stirred solution of thiobenzanilide (1 mmol) in CH₂Cl₂-CCl₄ (50:50, 8 mL), OATB (1.1 mmol) was added. The reaction mixture was stirred for 20 min at r.t. and the solvent removed under reduced pressure. The residue was subjected to column chromatography (EtOAc-hexane, 1:5, silica gel) to obtain pure colorless product.
Selected spectroscopic data for compounds 4, 5, 8.
Compound 4: ¹H NMR: δ = 8.28 (d, J = 7.3 Hz, 2 H), 8.16 (d, J = 8.2 Hz, 1 H), 7.31 (s, 1 H), 7.15-7.22 (m, 3 H), 7.09 (d, J = 8.2 Hz, 1 H).
Compound 5: ¹H NMR: δ = 8.96 (d, J = 7.8 Hz, 1 H), 8.17 (d, J = 7.0 Hz, 2 H), 7.83 (d, J = 7.8 Hz, 1 H), 7.75 (d, J = 8.6 Hz, 1 H), 7.66 (d, J = 8.3 Hz, 1 H), 7.60 (t, J = 7.2 Hz, 1 H), 7.48 (t, J = 7.5 Hz, 1 H), 7.36-7.41 (m, 3 H).
Compound 8: ¹H NMR: δ = 8.31 (s, 2 H), 7.21-7.36 (m, 10 H).