The crystal structures of 2-oxo-2-phenyl-N-[(R)-1-phenylethyl]acetamide, C16H15NO2, (I), and N,N-dimethyl-2-(1-naphthyl)-2-oxoacetamide, C14H13NO2, (II), were determined in an attempt to understand the reason for the lack of Yang photocyclization in their respective crystals. In the case of (I), the long distance between the O atom of the carbonyl group and the gamma-H atom, and between the C atom of the carbonyl group and the gamma-C atom, preclude Yang photocyclization. For (II), the deviation of the gamma-H atom from the plane of the carbonyl group and interactions between the naphthalene rings are regarded as possible reasons for the chemical inertia. The two independent molecules of (I) differ in their conformation. N-H...O hydrogen bonds link molecules of (I) into chains extended along the b axis.