Completing an SAR study, a series of (RS)-6-substituted-7- or 9-(1,2,3,5-tetrahydro-4,1-benzoxazepine-3-yl)-7H or 9H-purines has been prepared under microwave-assisted conditions. Their antiproliferative activities on MCF-7 and MDA-MB-231 cancerous cell lines are presented, being the majority of the IC(50) values below 1μM. The most active compound (RS)-2,6-dichloro-9-[1-(p-nitrobenzenesulfonyl)-1,2,3,5-tetrahydro-4,1-benzoxazepin-3-yl]-9H-purine (14) presents an IC(50) of 0.166μM against the human cancerous cell line MDA-MB-231. Compound 14 was the most selective against the human breast adenocarcinoma MCF-7 and MDA-MB-231 cancer cell lines (Therapeutic Indexes, TIs=5.1 and 11.0, respectively) in relation to the normal one MCF-10A. (RS)-14 was resolved into its enantiomers. Both enantiomers are equally potent, but more potent than the corresponding racemic mixture. (R)-14 induces apoptosis against MCF-7 up to 52.50% of cell population after 48h, being more potent than the clinical-used drug paclitaxel (43%). (RS)-14 induces no acute toxicity in mice after two weeks of treatment.