Among presently used pharmaceuticals, about 60% were developed from natural products with unique chemical diversity and biological activities. Hence, the discovery of new bioactive compounds from natural products is still important for further drug development. In addition, breakthroughs in synthetic biology have also begun to produce many useful compounds through manipulations of the biosynthetic genes for secondary metabolites. Theoretically, this approach can also be exploited to generate new unnatural compounds by intermixing the genes from different biosynthetic pathways and/or engineering the secondary metabolite enzyme(s) with expanded substrate and product specificities. Δ⁹-Tetrahydrocannabinol (Δ⁹-THC), the heat-decarboxylated tetrahydrocannabinolic acid (Δ⁹-THCA) produced by Cannabis sativa, is the most important therapeutic cannabinoid due to its useful pharmacological features, such as analgesic, anti-emetic, anti-inflammatory, and anti-epileptic activities. In the structures of cannabinoids, the resorcinyl alkyl chain is a critical pharmacophore, and the therapeutic effects of Δ⁹-THC can be enhanced by converting the pentyl (C₅) moiety at C-3 to other acyl moieties. Thus, the expansion of unnatural cannabinoids with different C-3 alkyl moiety analogs might establish an excellent platform for the further development of therapeutically beneficial cannabinoids. This article reviews the structure-based dual engineering of both enzymes responsible for the formation of the resorcinyl core of Δ⁹-THC and describes the effect of C-6 alkyl-length extension of olivetolic acid, along with related analogs, on the antibacterial activities against Bacillus subtilis and Staphylococcus aureus.