Executing and Rationalizing the Synthesis of a Difluorinated Analogue of a Ring-Expanded Calystegine B2

A difluorinated analogue of a ring-expanded calystegine B2 and some N-protected species were prepared via microwave-mediated transannular ring-opening of an epoxyketone. The diastereofacial selectivity of the epoxidation reaction, which delivers the key intermediate, and the regioselectivity of the transannular reactions were analyzed by density functional theory (DFT) methods. The epoxidation stereoselectivity arises from simple steric control, whereas the ring-closure reactions are subject to thermodynamic control.