abstract |
unit; A represents an oxygen atom, a sulfur atom, an —NH group or an —NR′ group; and R′1, R′2, R′3 and R′4 each represent a hydrogen atom or a substituent not including a mesomeric-effect electron-withdrawing group conjugated to the aromatic ring, a substituent from R′1, R′2, R′3 and R′4 representing the covalent bond with the benzylic carbon. |