(S)-t-Butyl 1-((S)-2-((t-Butoxycarbonyl)amino)propanoyl)pyrrolidine-2-carboxylate-d3 is an intermediate in synthesizing Enalapril-d3 Hydrochloride (E555258), which is the hydrochloride salt of Enalapril-d3 (E555253), a labeled version of Enalapril (E555403), an antihypertensive that is also an angiotensin-converting enzyme (ACE) inhibitor.