A process of preparation of Vilsmeier-Haack reagent is provided where di(trichloromethyl) carbonate reacts with N,N-dimethylformamide to form a Vilsmeier reagent, which can be used efficiently for chlorination of sucrose-6-acetate or sucrose-6-benzoate and other sucrose acylates. This process has application in the process for preparation of 1-6-Dichloro-1-6-DIDEOXY-.beta.-Fructofuranasyl-4-chloro-4-deoxy-galactop- yranoside.