I'm not totally sure on the chemistry, but isomerization of cannabinoids into other cannabinoids is already happening at very large scale, as demonstrated by the widely available d8 isomerized from cbd. Idk if isomerization is a type of synthesizing, or if synthesizing would more refer to starting with precursors and raw materials then making cannabinoids- but the reason no one is doing that is because the plant biosynthesizes cannabinoids a lot more efficient than anyone has discovered how to do yet. So, what people are doing instead, is isomerizing one to another(largely for the legal loophole in order to sell a psychoactive form of THC in the open) -or- like mentioned by others, isolating more rare cannabinoids, like thcv, cbg, cbdv, thcp. This usually involve working with a breeder who can help increase the level of cannabinoids a plant produces(cbd heavy, thcv heavy, etc), and then extracting that plant and isolating the major cannabinoid.
