Acid-promoted synthesis of per-O-sulfated fucooligosaccharides related to fucoidan fragments

The synthesis of per-O-sulfated derivatives of di-, tetra-, hexa-, octa-, dodeca-, and hexadecafucosides related to natural fucoidans of different types has been performed with the use of previously reported acid-promoted protocol for per-O-sulfation of polyols by SO₃ complexes.² During the treatment of (1→3)-linked oligofucosides under these conditions with the promotion by TfOH, the unusual rearrangement of the reducing pyranose residue into furanose one was observed. To avoid the formation of rearrangement by-products, the use of a series of strong acids as promoters of sulfation of large oligofucosides was studied and the improved protocol was developed based on the use of TFA instead of TfOH. The efficiency of the new method was demonstrated by the syntheses of per-O-sulfated derivatives of dodeca- and hexadecafucosides. The described method of O-sulfation opens access to the preparation of the oligosaccharides related to fucoidan fragments and their per-O-sulfated derivatives interesting for elucidation of the relationship between their structure and biological activity.