Resume Ethyl 3,3-dimethyl-2-thioxobutanoate 4 , a thioketone bearing an α-carbonyl group, has been synthesized and isolated. It undergoes selective thiophilic addition with alkyl, vinyl and benzyl-magnesium bromides at room temperature to give, after hydrolysis, α-alkylthio esters 7 . Reaction of the thioxoester 4 with 2-buten-1-yl-magnesium bromide was investigated. At -70°C, thiophilic addition with “inversion” of the allylic chain, giving product 8 , is demonstrated, in contrast with previous observations of carbophilic addition with thiocarbonyl compounds. When the reaction mixture is warmed to 20°, the intermediate sulphur carbanion undergoes a [2.3] sigmatropic shift to afford compound 10 , bearing a linear allylic chain. A mechanism, involving 1,4-conjugate addition, is discussed.