Piperidine is condensed with o-flouroaniline to produce the flouro substituted product, ocfentanil which is slightly more potent than its parent fentanyl. (if that helps?)
I think a valid target should be ohmefentany, it is easier to synth than carfentanyl but comparable in potency;
Ohmefentanyl (beta-hydroxy-3-methylfentanyl) is an extremely potent analgesic drug which selectively binds to the µ-opioid receptor.
Ohmefentanyl is one of the most potent u-receptor agonists known, comparable to super-potent opioids such as carfentanil and etorphine which are used for tranquilizing large animals such as elephants in veterinary medicine. In mouse studies, the most active isomer 3R,4S,?S-ohmefentanyl was 28 times more powerful as a painkiller than fentanyl, the chemical from which it is derived, and 6300 times more effective than morphine.. Ohmefentanyl has three stereogenic centers and so has eight stereoisomers, which are named F9201-F9208. Researchers are studying the different pharmaceutical properties of these isomers..The 4"-fluoro analogue of the 3R,4S,?S isomer of ohmefentanyl is the most potent opioid yet discovered, possessing an analgesic potency approximately 18,000-fold greater than morphine.