It's ALMOST the case that their are amide & non-amide downers, Non-amide downers in effect act like inhalational anesthetics. They tend to be toxic, lead to a hangover and are toxic in overdose. About the only example that straddles that boarder is clomethiazole (and bromethiazole). These latter compounds are not well explored. We KNOW that they are very toxic if mixed with alcohol but they bind at a novel site. Not the benzodiazepine site, not the barbiturate site bur not more generally as per the manner of the non-selective agents that act more like sevoflurane than any decent hypnotic.
I have looked and looked but because clomethiazole was discovered in 1931, retrospect occurred in the 1950s, a period in which we still did not have a good handle on such agewnts.
What I CAN tell you is that all of the agents I picture induce CYP2D6 and so when taken with codeine, dihydrocodeine, hydrocodone, oxycodone and tramadol undergo O-demethylation and thus are much stronger. I don't think doctors caught on until the 1970s when they were all abruptly BANNED.
Oh, their IS ONE exception:
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The above was commonly used in the UK until the mid 1980s when modern alternatives like diazepam, nitrazepam and temazepam became commonplace. I have no other homologues to compare it with so HOW it binds I really do not know.... but if YOU know, pleas post. It's simple to make and finding some interesting analogues would pose a real challange.