Stereoselective synthesis of 2-acyl-3,4-dihydro-1,4-benzothiazines

2-Benzylidene-4-methyl-3-oxo-2H-1,4-benzothiazine 2 undergoes 1,2-addition with MeMgI and allylic magnesium halides to give 2-acyl-4-methyl-3-phenyl-3,4-dihydro-2H-1,4-benzothiazines 3a-d. Lithiation of 3a and subsequent reaction with MeI and PhCH2Br leads to compounds 3e, 3f, 3h and 3i. In contrast, the reaction of lithiated 3a with benzaldehyde and 2,6-dichlorobenzaldehyde furnished compounds 3k and 3l respectively. © 1994.