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Electrochemical and chemical reductions are complementary methods for the selective hydrogenation of 2-substituted-6-methoxycarbonyl-4H-1,3-thiazine-4-ones leading to 2,3-dihydro; 5,6-dihydro- and 2,3,5,6-tetrahydro-4H-1,3-thiazine-4-ones without ring opening and ketone function alteration. Selective hydrogenation of the imine bond (C[dbnd]N) or the alkene bond (C[dbnd]C) depends on the nature of the heteroatomic substituent [(CH3)2N, C2H2O, C6H5CH2S] at the 2 position. La reduction par voie electrochimique (a potentiel controˇle et sur cathode de mercure) et par voie chimique (borohydrures, hydrure de tributyletain, hydrure de triethylsilane, aluminium amalgame) des 6-methoxycarbonyl-4H-1,3-thiazine-4-ones substituees en position 2 par un groupement aromatique ou heteroatomique permet l'obtention selective des 2,3-dihydro; 5,6-dihydro- et 2,3,5,6-tetrahydro-4H-1,3-thiazine-4-ones correspondantes, sans ouverture du cycle thiazinique ni alteration de la fontion cetonique. L'orientation de la reduc...

2-Phenyl-4-dimethylamino-1-thia-3-azabutadiene and methylacrylate gave 5,6-dihydro-4H-1,3-thiazine by 4 + 2 cyclocondensation reaction at 55°C. The heterodienes configuration was established by X-Ray spectra. The Diels-Alder character of this reversible reaction was indicated by kinetic and thermodynamic parameters determined by 1H N.M.R.