derivatives were screened for their cyclooxygenase inhibitory properties relative to naproxene. Both 5‐chloromethyl naproxene (2) and 2‐(5‐((carboxyethyl)‐ 2‐methyloxynaphthyl)‐6‐methoxy‐2‐naphthyl)propanoic acid (4) were inactive in the concentration range of 0.1—10 μmole against both COX‐1 and COX‐2, indicating that bulky substituents in position 5 in naproxene are unfavourable for both COX‐1 and COX‐2 inhibition