[soundwave106]

The United States has a "broad ban" in place called the Federal Analogue Act -- https://en.wikipedia.org/wiki/Federal_Analogue_Act

The definition is "not 100% exact" in its nature, but dealers have been prosecuted and sent to prison for violating this.

It's also possible to apply broad bans to Markush structures (https://en.wikipedia.org/wiki/Structural_scheduling_of_synth...), which is how I understand some places attempt to regulate synthetic cannabinoids and other broad classes of chemicals.

(Keep in mind these laws do not apply to fentanyl -- in the United States, fentanyl is schedule II and is available as prescription medicine. Illicit fentanyl is more "illegal drug market" than "research chemicals"; cocaine is in the same boat in the US as well, a schedule II drug that's available on the illicit market. The above might apply to any fentanyl analogues though.)