To a mixture of (4-amino-3-cyano-pyrazolo[3,4-d]pyrimidin-1-yl)-acetic acid tert-butyl ester (80 g, 0.291 mol) and EtOH (800 mL) in an ice water bath was added acetic acid (67 mL) and then ethylenediamine (195 mL). The resulting mixture was refluxed for 90 min, cooled to room temperature and filtered. The solid was suspended in water (1 L), filtered and dried in vacuo to give of tert-butyl 2-(4-amino-3-(4,5-dihydro-1H-imidazol-2-yl)-1H-pyrazolo[3,4-d]pyrimidin-1-yl)acetate (84.6 g, 90% yield). MS (ES) m/z 318.1 (M+H+).
