The present invention provides an efficient process for the synthesis of (2R,3R,4S,5R)-2-(6-amino-9H-purin-9-yl)-5-((((1r,3S)-3-(2-(5-(tert-butyl)-1H-benzo[d]imidazol-2-yl)ethyl)cyclobutyl)(isopropyl)amino)methyl)tetrahydrofuran-3,4-diol and hydrates thereof and methods for treating disorders in which DOT1-mediated protein methylation plays a part, such as cancer and neurological disorders, by administering these compounds and pharmaceutical compositions to subjects in need thereof. The present invention also provides novel crystalline forms of (2R,3R,4S,5R)-2-(6-amino-9H-purin-9-yl)-5-((((1r,3S)-3-(2-(5-(tert-butyl)-1H-benzo[d]imidazol-2-yl)ethyl)cyclobutyl)(isopropyl)amino)methyl)tetrahydrofuran-3,4-diol and hydrates thereof (Form A, Form B, and Form C), characterized by a unique X-ray diffraction pattern and Differential Scanning Calorimetry profile, as well as a unique crystalline structure.