Benzeneselenenyl derivatives, PhSe-X (X = Cl, Br, OCOCH₃, and SCN) react readily with α-diazo ketones, RCOC(N₂)R¹(R¹= H or alkyl), with loss of nitrogen, furnishing αα-adducts of the type RCOCR¹(X)SePh. The α-chloro and α-bromo adducts can be converted into α-methoxy adducts in methanol–sodium hydrogen carbonate. The utility of these adducts in synthesis is illustrated by their conversion (where structural considerations permit)via selenoxide fragmentation into α- heterosubstituted αβ-unsaturated ketones. Treatment of the series RCOCR¹(X)SePh (R¹= alkyl; X = Cl, Br, OCOCH₃, and OCH₃) with hydrogen peroxide–pyridine produces α-chloro-, α-bromo-, α-acetoxy-, and α-methoxy-αβ–unsaturated ketones, whereas treatment of the series RCOCR¹(X)SePh (R¹= alkyl; X = Cl and Br) with lithium carbonate in dimethylformamide produces α-phenylseleno-αβ-unsaturated ketones. Several α-substituted cyclopentenones, cyclohexenones, and cycloheptenones have been synthesised in this way and acyclic examples are illustrated by the synthesis of 3-chloro-, 3-bromo-, 3-acetoxy-, 3-methoxy-, and 3-phenylselenobut-3-en-2-one from 3-diazobutan-2-one.