 Hans Erlenmeyer extended the concept to biological systems in 1932.Classical isosteres are defined as being atoms, ions and molecules that had identical outer shells of electrons, This definition has now been broadened to include groups that produce compounds that can sometimes have similar biological activities.Some evidence for the validity of this notion was the observation that some pairs, such as benzene, thiophene, furan, and even pyridine, exhibited similarities in many physical and chemical properties.