Synthetic cannabinoids are a group of compounds that act on the CB1 receptor and are used illicitly as substitutes for cannabis. Given the rapid and extensive metabolism of synthetic cannabinoids, urinary biomarkers are essential if proof of drug intake is to be obtained in forensic laboratories. To identify good biomarker candidates, the metabolism of synthetic cannabinoids must be studied and reference standards need to be acquired. Studies on the metabolism of synthetic cannabinoids containing a terminally fluorinated pentyl side chain have shown that hydroxylation can occur at the four position of the side chain. This makes the 4-hydroxy-5-fluoropentyl side-chain metabolite a good urinary biomarker for proving intake of the corresponding parent drug, as this compound cannot be formed from its nonfluorinated analogue. Here, a concise synthetic route to the 4-hydroxy-5-fluoropentyl side-chain metabolites of the synthetic cannabinoids STS-135, MAM-2201, AM-2201, and XLR-11 is reported.