equimolecular mixture of (3-13C) 3-bromo-2-chloro-5-methylbenzoquinone (6a) and (2-13C) 2-bromo-3-chloro-5-methylbenzoquinone (7a) which was easily separated by HPLC under the hydroquinone form. The labelledpositions were assigned on the basis of 1H-NMR and the two intermediates were separately elaborated to the corresponding (5-13C) and (6-13C) ubiquinones-3.