New reactive asymmetrical dicarboxylic acid ester compounds of the formula ##STR1## wherein R.sub.1 is an alkyl radical containing up to 3 carbon atoms or is a digoxin or digitoxin residue, R.sub.2 and R.sub.3, which may be the same or different, are alkoxy radicals containing up to 3 carbon atoms or together represent an oxygen atom, X is a cyanomethoxy, succinimide-N-oxy, N-methyl-pyridiniumoxy, 2,4-dinitrophenoxy, 2,4,5-trichlorophenoxy, pentachlorophenoxy, phenylthio, p-nitrophenoxy, p-nitrophenylthio, piperidyl-N-oxy, phthalimido-N-oxy or benztriazol-N-oxy radical and n is 2, 3, 4 or 5. are useful for the preparation of reagents for the investigation of cardiac glycosides (Digitalis glycosides).