SCOPE AND APPLICATIONS OF ACTIVE AND LATENT THIOGLYCOSYL DONORS - PART-4

The relative reactivity of various thioglycosyl donors having ethyl, p henyl, or parasubstituted phenyl groups with electron donating (N-Ac) or electron withdrawing (NO2) substituents were compared using ,2:3,4- di-O-isopropylidene-alpha-D-galactopyranose (3) as standard glycosyl a cceptor. The reactivity order was found to decrease from ethyl > pheny l > p-acetamidophenyl > p-nitrophenyl. In the latter situation, when t he thioglycosyl donor was also equipped with ''disarming'' ester prote cting groups, they were found to be inert or inactive toward common th iophilic promoters. Alternatively, it was possible to selectively acti vate the ''armed'' perbenzylated p-nitrophenyl 1-thio-beta-D-galactopy ranoside (21) in the presence of the corresponding ''disarmed'' perben zoylated p-nitrophenyl ,3,4-tri-O-benzoyl-1-thio-beta-D-galactopyranos ide (15) which served as the glycosyl acceptor. When both ''armed'' pe rbenzylated thioglycosides 7 and 25 were used as thioglycosyl donor an d thioglycosyl acceptor, respectively, the milder thiophilic promotor methyl triflate was required for chemoselective activation. These resu lts further demonstrate the potential of ''armed and disarmed'' ''acti ve and latent'' thioglycosides in blockwise oligosaccharide syntheses.