(2S)-Ethylpyrrolidine Hydrochloride 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.