Ring-Opening of Epoxides by Pendant Silanols

We present a new ring-opening reaction of epoxides by pendant silanols, catalyzed by either Ph3C+BF4– or BINOL-phosphoric acid. In all cases examined, the reaction is perfectly regioselective and diastereoselective. Silanol epoxides derived from trans-allylic alcohols, cis-allylic alcohols, trans-homoallylic alcohols, and cis-homoallylic alcohols were all compatible and gave products from either endo- or exo-ring opening. With silanol epoxides derived from 4-alkenyl silanols, an unusual rearrangement to tetrahydrofuran products was observed, which is likely the result of tandem nucleophilic attacks. The utility of this methodology was demonstrated in a short preparation of protected D-arabitol.

Facile synthesis of vicinal halohydrins via organocatalytic halogen nucleophile-induced regioselective opening of epoxides

Background: The calix[4]pyrrole is reported as a novel organocatalyst for regioselective ring opening of epoxides under mild reaction conditions. Methods: The reaction involves elemental halogen as a nucleophile to afford vicinal halohydrins in good to excellent yield (75-95%). Results : The reactivity of the halide anion in the reaction is governed by different factors, including solvent polarity, temperature and non-covalent interactions of the functional group present on calix[4]pyrrole moiety with halide ions. Conclusion: An efficient methodology has been developed for the regioselective synthesis of halohydrins in good to excellent yields.