noun which may take the genitive case particle 'no'
semi-synthetic
Used in chemistry and pharmacology to describe compounds derived from natural products through chemical modification.
この抗生物質は半合成ペニシリンです。
This antibiotic is a semi-synthetic penicillin.
天然 means 'natural', the opposite of synthetic; 半合成 lies between natural and fully synthetic.
Compound of 半 (half) and 合成 (synthesis), literally 'half-synthesis'.