(S)-N-tert-Butoxycarbonyl-2-ethylpyrrolidine is an intermediate in the synthesis of (2S)-Ethylpyrrolidine Hydrochloride (E818080), which is used as a reagent in the synthesis of N,N'-substituted-1,3-diamino-2-hydroxypropanes for treating Alzheimer's disease. Also used as a reagent in the synthesis of cyclobutyl substituted pyrrolopyridine and pyrrolopyrimidine derivatives as JAK kinase inhibitors for treatment of autoimmune disease, cancer, inflammatory disease, and other diseases.