We report the convergent synthesis of two novel tetrasaccharide and two novel pentasaccharide fragments of the
Le(a)Le(x) TACA: the tetrasaccharides contain neither the
galactose at the Le(a) nonreducing end nor the
fucose at the Le(x) reducing end; the pentasaccharides only lack the
galactose residue at the Le(a) nonreducing end. Two of the analogues were prepared as hexyl
glycosides to be used in NMR experiments and as soluble inhibitors in binding studies and two as 6-aminohexyl
glycosides to be conjugated to
carrier proteins. Our strategy relied on stepwise extensions using excess
monosaccharide glycosyl donors (trichloroacetimidates and
thioglycosides) in sequential glycosylation reactions. The protecting groups were chosen to limit the number of deprotection steps required to obtain the final derivatives. While this strategy ensured that all glycosylation reactions proceeded in very good yields (70-84%), deprotection of the
oligosaccharide intermediates was challenging. Global deprotection using Birch
metal dissolving conditions did not remove the tert-butyldiphenylsilyl group, which indeed was incompatible with such reaction conditions. Attempts to remove the TBDPS with
tetrabutylammonium fluoride was unsuccessful and led to a
complex mixture of compounds that could not be separated. The desired hexyl and aminohexyl tetrasaccharides were finally obtained after four- and five-step deprotection sequences, respectively. Deprotection of the pentasaccharide intermediate to give the hexyl and aminohexyl analogues also led to unexpected results. Indeed, during Zemplén deacylation, a
chloroacetamide chlorine atom was displaced by methoxide
ions leading to the corresponding methoxyacetamide. Once the
chloroacetamide was fully reduced to an
acetamide the pentasaccharides were obtained in four and five steps, respectively.