How would one carboxylate CBG into its acid form, CBGA?
The substrate is a resorcinol derivative: a strongly activated aromatic ring, which upon reaction with a base delocalizes its negative charge on the ring, making it reactive towards acids (Eg. CO2).

Here (entries 130-132) good conversion is claimed (85%) in a high pressure vessel with magnesium methoxide and dry ice, or a lower conversion (40%) is reported at ambient pressure with magnesium methyl carbonate