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.[1]. 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.[2].
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.from wiki.