fentanyl1-phenethyl-4-(N-phenylpropionamido)piperidine1-phenethyl-4-N-propionylanilinopiperidineN-(1-phenethyl-4-piperidinyl)-N-phenylpropionamideN-(1-phenethyl-4-piperidyl)propionanilideN-(1-phenethyl-piperidin-4-yl)-N-phenyl-propionamideN-(1-phenethylpiperidin-4-yl)-N-phenylpropionamideN-phenethyl-4-(N-propionylanilino)piperidineN-phenyl-N-(1-(2-phenylethyl)-4-piperidinyl)propanamidephentanyl