Semisynthesis, or partial chemical synthesis, is a type of chemical synthesis that uses chemical compounds isolated from natural sources (such as microbial cell cultures or plant material) as the starting materials to produce novel compounds with distinct chemical and medicinal properties. The novel compounds generally have a high molecular weight or a complex molecular structure, more so than those produced by total synthesis from simple starting materials. Semisynthesis is a means of preparing many medicines more cheaply than by total synthesis since fewer chemical steps are necessary.
thumb|right|400px|alt=|Semisynthesis of [[paclitaxel. Installation of the necessary side chain and acetyl group of paclitaxel by a short series of steps, starting from isolated 10-deacetylbaccatine III. While there is no strict boundary between total synthesis and semisynthesis, they differ primarily in the degree of engineered synthesis employed. Complex or fragile functional groups are often more cost-effective to extract directly from an organism than to prepare from simpler precursors, making semisynthesis the preferred approach for complex natural products.
Notable examples in drug development
Practical applications of semisynthesis include the groundbreaking isolation of the antibiotic chlortetracycline and the subsequent semisynthesis of antibiotics such as tetracycline, doxycycline, and tigecycline. Other notable examples include the early commercial production of the anti-cancer agent paclitaxel from 10-deacetylbaccatin, isolated from Taxus baccata (European yew), the semisynthesis of LSD from ergotamine (derived from fungal cultures of ergot), and the preparation of the antimalarial drug artemether from the naturally occurring compound artemisinin. As synthetic chemistry advances, transformations that were previously too costly or difficult to achieve become more feasible, influencing the economic viability of semisynthetic routes.