A synthesis of dexamethasone multiply labeled with 13C about the A ring is described. 16-α- Methyl-17α, 20-dihydroxypregn-1, 4, 9 (11)-trien-3, 20-dione (1) was blocked as the bismethylenedioxy derivative and degraded to the ring C, D-fragment, Des-A, B-9-keto-16α- methyl-17α, 20.20, 21-bismethylenedioxypregn-8α-propionic acid (9) Conversion to the enol lactone and condensation with 1-diethylphosphono-4-pentanone-13C ethylene ketal ...