One-step ring-closure procedure for 4,5-dihydro-1,3-thiazino[5,4-b]indole derivatives with Lawesson's reagent. The fifth dihydro-1,3-thiazino[b]indole isomer

We report a convenient approach for the synthesis of a new ring system: 4,5-dihydro-1,3-thiazino[5,4-b]indoles. The procedure involves the use of Lawesson's reagent in the presence of silica to achieve the one-step ring-closure reactions of 2-benzoylamino-3-hydroxymethylindole intermediates to furnish 4,5-dihydro- 2-aryl-1,3-thiazino[5,4-b] indoles. 2-Phenylimino-1,3-thiazino[5,4-b]indoles were obtained via the corresponding 3-phenylthiourea-2-carboxylic acid ester derivatives by chemoselective reduction of the ester group, followed by ring closure under acidic conditions. The structures of the novel products were elucidated by IR, H-1-NMR, and C-13-NMR spectroscopy, including 2D-HMQC, 2D-HMBC, and DEPT measurements.