Preparation of diastereomeric PG 12 (Fig. ). Isopropylidene PG isomers (diastereomers and enantiomers, i.e., S,R, S,S, R,S, and R,R) (see above) were dissolved in 1 mL of trimethyl borate, 50 mg of boric acid was added, and the mixture was heated at 100°C for 2.5 h. After the solvent was evaporated under a stream of nitrogen, the reaction mixture was dissolved in 6 mL of chloroform/methanol (2:1, vol/vol), then washed with 1 mL of distilled water, and the chloroform layer was dried over anhydrous sodium sulfate. The solvent was evaporated, and the crude PGs were analyzed by chiral HPLC.