Regioselective synthesis of isotopically labeled Δ9-tetrahydrocannabinolic acid A (THCA-A-D3) by reaction of Δ9-tetrahydrocannabinol-D3 with magnesium methyl carbonate.

For the reliable quantification of Δ9-tetrahydrocannabinolic acid A (THCA-A), the biogenetic precursor of Δ9-tetrahydrocannabinol (THC), in biological matrices by LC-MS/MS and GC-MS(/MS), an isotopically labeled internal standard was synthesized starting from Δ9-tetrahydrocannabinol-D(3) (THC-D(3)). Synthesis strategy was based on a method reported by Mechoulam et al. in 1969 using magnesium methyl carbonate (MMC) as carboxylation reagent for the synthesis of cannabinoid acids. Preliminary experiments with THC to optimize yield of the product (THCA-A) resulted in the synthesis of the positional isomer tetrahydrocannabinolic acid B (THCA-B) as a byproduct. Using the optimized conditions for the desired isomer, THCA-A-D(3) was prepared and isolated with a yield of approx. 10% after two synthesis cycles. Isotope purity was estimated to be >99% by relative abundance of the molecular ions. The synthesized compound proved to be suitable as an internal standard for quantification of THCA-A in serum and hair samples of cannabis consumers.