Synthesis and removal of trichloroethylidene derivatives of carbohydrates

Two hexoses and two pentoses have been reacted with two equivalents of trichloroacetaldehyde using boron trifluoride etherate as promoter in dioxane at reflux. The major products are exo-1,2-O-trichloroethylidenealdofuranoses in every case in varying yields, identical to the products previously obtained in lower yields using sulfuric acid or hydrochloric acid as promoter in trichloroacetaldehyde as solvent. Careful fractionation of the remainders of the product mixtures showed that the other isomer in the acetal ring was also produced in most cases but small amounts of the two 1,2-O-trichloroethylidenealdopyranose isomers were also produced. In addition, very small amounts of other products were identified that were derived from boron trifluoride etherate and the solvent. A mechanism involving rate-determining acetal ring closure was proposed to explain the formation of furanose derivatives, rather than pyranose derivatives. Methods for removal of the trichloroethylidene acetals were investigated for substituted derivatives and it was found that a process involving reduction with tributyltin hydride was effective.