Figure : Structures of BU72 and analogs BU72 was the ligand in the first crystal structure of active μOR . As noted there, the electron density exhibited two unexplained features. Firstly, fitting the published structure of BU72 (1a, Figure ) required a near-planar orientation of the phenyl group, an implausibly high-energy conformation that required many extreme deviations from ideal geometry and left unexplained density around the benzylic carbon (Figure ).