wherein R2 has the same meanings as given above and R4 is a lower alkyl group, to form a cyclic 17??,21-ortho ester of 6??-methylprednisolone of the general formula: ##STR10## wherein R2 and R4 have the same meanings as given above, and then this 17??,21-ortho ester is subjected to a ring-opening reaction to form a 17??-acyloxy-11??,21-dihydroxy-6??-methyl-1,4-pregnadiene-3,20-dione compound of th