Cannabis Cannabinoid Res. 2022 Mar 15. doi: 10.1089/can.2021.0216. Online ahead of print.
There appears to be consensus among Cannabis biologists that delta-9-tetrahydrocannabinolic acid A (THCA-A) is the exclusive product of the enzyme THCA synthase. This then leaves an open question for formation of the THCA-A structural isomer, delta-9-tetrahydrocannabinolic acid B (THCA-B), discovered as a minor product in Cannabis in 1969. With no reasonable biochemical pathway to explain the presence of THCA-B in Cannabis, a synthetic route was next considered. Using established literature precedent, a photochemical mechanism has been proposed for the conversion of THCA-A to cannabidiolic acid (CBDA), followed by conversion of CBDA to THCA-B employing bond-breaking and bond-forming reactions.
Source: ncbi 2