Synthesis of 2<i>H</i>-1,2-Benzothiazine 1,1-Dioxides via Heteroannulation Reactions of 2-Iodobenzenesulfonamide with Ketone Enolates under S<sub>RN</sub>1 Conditions<sup>§</sup>
2005-11-11T00:00:00Z (GMT) by
2-Iodobenzenesulfonamide (<b>1a</b>) underwent photostimulated S<sub>RN</sub>1 reactions in liquid NH<sub>3</sub> with the potassium enolates derived from acetone, pinacolone, butanone, and 3-methyl-2-butanone to give fair to good yields of 2<i>H</i>-1,2-benzothiazine 1,1-dioxides<b> 2</b>. Reductive dehalogenation of <b>1a </b>was found to predominate in photoinduced reactions of <b>1a</b> with 3-pentanone, 2-methyl-3-pentanone, and 2,4-dimethyl-3-pentanone, the extent of reduction being proportional to the number of β-hydrogen atoms present in the ketone enolate. Isotopic labeling studies with 2,4-dimethyl-3-pentanone-<i>d</i><sub>14</sub> (<b>24</b>) confirmed the major role of the β-hydrogens in the reduction process. Reactions of<b> 1a</b> with cyclopentanone, cyclohexanone, and cyclooctanone enolates afforded new tricyclic benzothiazine derivatives <b>26</b>−<b>29</b>. Attempts to extend the heteroannulation reaction to the preparation of 2<i>H</i>-1,2-benzothiazin-3(4<i>H</i>)-one 1,1-dioxides <b>3</b> (R = H, Ph) through reactions of <b>1a</b> with <i>tert</i>-butyl acetate and ethyl phenylacetate enolates resulted only in hydrodehalogenation of <b>1a</b>. However, oxazoline anion<b> 30</b>, a synthetic equivalent of ethyl phenylacetate, was successfully employed in an alternative S<sub>RN</sub>1-based synthesis of benzothiazine <b>3</b> (R = Ph).