Interesting. What is the ED50 of those compounds (& which test was used)?
Also, what is the year for the ref.?
The year is 1988. ED50 of analogue with benzylamine and N-phenethyl is 0.63mg/kg, with N-methyl 1.25mg/kg. Analogue with phenethylamine and N-phenethyl is 2.0mg/kg. In the same paper they give the ED50 of morphine as 3.15mg/kg. They used the tail-withdrawal test in rats. (When I say N-phenethyl I mean that it is substituted in the 1 position of the piperidine ring, just to clarify)
Now we 're on the topic, in literature, I never have seen a potency evalutation of 3,3-dimethyl fentanyl. However, I estimate that the potency would be intermediate between (cis)-3-methyl-fentanyl & (trans)-3-methyl fentanyl. In JMC (1985) vol 28 p 1947 they test 1,3,3-trimethyl-4-phenyl-4-(propionyloxy)piperidine (7-10 x demerol) on mice in a hot plate test & the ED50 of 0,61 mg/kg lies between 0,18 mg/kg (beta-prodine) and 0.92 mg/kg (alpha-prodine).
Unfortunately the SARs of the prodines and the fentanyls is not always the same, for example the 2,5-dimethylprodine is more potent than the unsubstituted, while as I said above it is not for the fentanyls. But I guess we won't find out if this is one of them cases until someone makes it =).