Corticosteroid decomposition via a mixed anhydride

Oxidation of dexamethasone in an aqueous suspension by air during prolonged storage produces the 17α-formyloxy-17β-carboxylic acid 4. A pathway to 4 is proposed that involves Baeyer−Villiger-type oxidation of keto aldehyde 1 to mixed anhydride 5, followed by intramolecular formyl transfer. Synthetically, acid 3 was reacted with N,N‘-carbonyldiimidazole followed by triethylammonium formate in order to generate the transient anhydride 5 en route to an authentic sample of 4.