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A protocol has been developed for the synthesis of six‐ and seven‐membered ring lactams fused to coumarins through a hydroaminocarbonylation reaction catalyzed by palladium. The catalytic system (PdBr2/Xantphos) provided a strategy for the synthesis of seven‐membered ring lactams with good isolated yield and regioselectivity. Control of selectivity by the substrate in the synthesis of six‐membered ring lactams was observed. Control reactions showed the importance of the hydrogen in the cyclocarbonylation reaction, and NMR data supported the role of the hydride palladium species as a catalytic precursor. A preliminary screening of the cytotoxic activity of coumarin‐fused lactams against six cancer cell lines was evaluated. The biological tests indicated that both the size of the heterocycle and presence of a substituent on the aromatic ring affected the inhibition of cell growth. [ABSTRACT FROM AUTHOR]