A mild and efficient approach to the 6H-oxazolo[3, 2-f]pyrimidine-5, 7-dione scaffold via unexpected rearrangement of 2, 3-dihydropyrimido[6, 1-b][1, 5, 3]dioxazepine-7, 9(5H, 8H)-diones: synthesis, crystallographic studies, and cytotoxic activity screening. Issue 7 (17th February 2016)