Scope and applications of "active and latent" thioglycosyl donors.: Part 4

The relative reactivity of various thioglycosyl donors having ethyl, phenyl, or parasubstituted phenyl groups with electron donating (N-Ac) or electron withdrawing (NO2) substituents were compared using 1,2:3,4-di-O-isopropylidene-alpha-D-galactopyranose (3) as standard glycosyl acceptor. The reactivity order was found to decrease from ethyl > phenyl > p-acetamidophenyl > p-nitrophenyl. In the latter situation, when the thioglycosyl donor was also equipped with "disarming" ester protecting groups, they were found to be inert or inactive toward common thiophilic promoters. Alternatively, it was possible to selectively activate the "armed" perbenzylated p-nitrophenyl 1-thio-beta-D-galactopyranoside (21) in the presence of the corresponding "disarmed" perbenzoylated p-nitrophenyl 2,3,4-tri-O-benzoyl-1-thio-beta-D-galactopyranoside (15) which served as the glycosyl acceptor. When both "armed" perbenzylated thioglycosides 7 and 25 were used as thioglycosyl donor and thioglycosyl acceptor, respectively, the milder thiophilic promotor methyl triflate was required for chemoselective activation. These results further demonstrate the potential of "armed and disarmed" "active and latent" thioglycosides in blockwise oligosaccharide syntheses.