N-Desmethyl-N-cyclopentyl Tadalafil-D4 is a labelled analogue of N-Desmethyl-N-cyclopentyl Tadalafil (B199000). N-Desmethyl-N-cyclopentyl Tadalafil is used as a reagent to synthesize tetracyclic cGMP-specific phosphodiesterase inhibitors, compounds that have potential to treat cardiovascular disorders and erectile dysfunction.