The title diastereomeric compounds, C18H19ClN4O2, are the products formed when the (2S,3S)- and (2R,3S)-3-(5-amino-6-chloropyrimidinyl)amino-2-(2-benzyloxy)ethyloxolanes are treated with triethyl orthoformate in the presence of 4-toluenesulfonic acid. The crystal structure determination unambiguously shows the cis and trans orientations, respectively, of the 2-benzyloxyethyl and the 6-chloropurinyl substituents of the oxolanyl ring.