Main categories:

According to the synthesis stage, intermediates can be classified into three categories: initial intermediates (primary), key intermediates (secondary), and final intermediates (tertiary). Initial intermediates are directly synthesized from basic chemical raw materials; they have a simple structure and require a purity of ≥95%, such as phenylacetic acid used in the synthesis of penicillin-based drugs. Key intermediates are synthesized through 2–5 reaction steps and contain specific functional groups; they demand a purity of ≥98%, for example, 7-ACA, which is used in the synthesis of cephalosporin drugs. Final intermediates closely resemble the structure of the active pharmaceutical ingredient (API) and can be converted into the API with only 1–2 reaction steps; they require a purity of ≥99.5% and an impurity content of ≤0.1%, such as the ritonavir intermediate used in the synthesis of anti-HIV drugs.

By chemical structure, intermediates can be categorized into heterocyclic intermediates, amino acid-based intermediates, steroid intermediates, and others. Heterocyclic intermediates contain heterocyclic rings in their molecular structures and serve as the “active scaffolds” for many drugs—for example, 2-aminopyridine is used in the synthesis of antihistamine drugs. Amino acid-based intermediates feature both amino and carboxyl groups and are crucial building blocks for synthesizing peptide drugs and protein-based therapeutics—for instance, L-proline is employed in the synthesis of angiotensin inhibitors. Steroid intermediates possess a steroidal core structure that is complex and challenging to synthesize; for example, diosgenin is used in the synthesis of steroid hormone drugs.

By application area, these intermediates can be categorized into antibiotic intermediates, antiviral intermediates, antitumor intermediates, and others. Antibiotic intermediates are suitable for the synthesis of antibiotics, such as 6-APA and 7-ACA. Antiviral intermediates contain key functional groups that inhibit viral replication—for example, acyclovir intermediates are used in the synthesis of antiherpetic drugs. Antitumor intermediates carry functional groups that target tumor cells—for instance, cisplatin intermediates are used in the synthesis of the chemotherapy drug cisplatin.