4-ANPP, also known as 4-anilino-N-phenethylpiperidine (4-ANPP), 4-aminophenyl-1-phenethylpiperidine, or despropionyl fentanyl,^[3] is a direct precursor to fentanyl and acetylfentanyl. It is commonly found as a contaminant in samples of drugs containing fentanyl, which may include samples represented by the supplier as heroin or other opioids.^[4] It is not psychoactive and is present only as a result of improper chemical purification.

4-ANPP is useful in the synthesis of pharmaceuticals, primarily fentanyl and related analogs. Paul Janssen (founder of Janssen Pharmaceutica) first synthesized fentanyl in 1960 using a similar method, with Benzylfentanyl as an intermediate.^[5] The following synthesis, developed by an individual under the pseudonym of Siegfried, involves the reductive amination of N-phenethyl-4-piperidinone (NPP) with aniline to make to 4-ANPP. This product is reacted with propionyl chloride or acetyl chloride to form either fentanyl or acetylfentanyl.

• 1-Boc-4-AP