Synthesis of haloconduritols from an endo-cycloadduct of furan and vinylene carbonate

A method for preparing haloconduritols having a conduritol-A construction is described. A mixture of endo- and exo-cycloadduct derivatives prepared from the Diels–Alder reaction of furan and vinylene carbonate was converted into diacetate derivatives by hydrolysis (K2CO3/MeOH) followed by acetylation (Ac2O/pyridine). Boron trihalide (BBr3 or BCl3)-assisted ring-opening of the endo-diacetate in CH2Cl2 at −78°C gave (1α,2α,3β,6β)-6-halogeno-4-cyclohexene-1,2,3-triol 1,2-diacetate from which the corresponding triacetate was prepared by acetylation (AcCl). trans-Esterification of the triacetate (MeOH/HCl) afforded (1α,2α,3β,6β)-6-halogeno-4-cyclohexene-1,2,3-triol (X=Br or Cl). BF3-Assisted ring-opening of the endo-diacetate in CH2Cl2 gave (1α,2α,3β,6β)-6-chloro-4-cyclohexene-1,2,3-triol 1,2-diacetate by means of halogen exchange.