By the reaction of prednisolone and dexamethasone with 2,2-dimethoxypropane, the synthesis of the 17.alpha.,21-acetonides of these corticosteroids was examined. The utility of the 17.alpha.,21-acetonide function as a protecting group for the labile cortical dihydroxyacetone function is demonstrated. It is particularly valuable for the direct introduction into intact corticoids of C-21 substituents. The synthesis of 21.alpha.-methyl and 21-trifluoroacetyl analogs of prednisolone and dexamethasone are described. Condensation of the 17.alpha.,21-acetonides with formaldehyde yields the 21-methylene derivatives which serve as intermediates for the synthesis of the 21.beta.-nitroethyl derivative and the .gamma.-lactone.