Highly selective ring-opening of 1,3,3-trihalogenoepoxycyclohexanes by boron trihalides; methodology for the determination of the regioselectivity in the cyclisation of 2,2,6,6-tetrahalogenocyclohexanols
Reaction of 1,3,3-trihalogeno-7-oxabicyclo[4.1.0]heptanes with boron trihalides (F, CI, Br) resulted in the regio- and stereo-selective formation in high yield of the corresponding tetrahalohydrins. In these reactions the hypervalent halogenoborate species are responsible for the selective halogenation of the epoxides by cis opening. A methodology is given to predict the regioselectivity in the epoxidic cyclisation of 2,2,6,6-tetrahalogenocyclohexanols. These processes allowed the preparation of all the isomers of 1,3,3-trihalogeno-7-oxabicyclo[4.1.0] heptanes or 2,2,6,6-tetrahalogenocyclohexanols (X = Cl, F), very useful intermediates.