Sd. Cao et al., SCOPE AND APPLICATIONS OF ACTIVE AND LATENT THIOGLYCOSYL DONORS - PART-4, Journal of carbohydrate chemistry, 17(4-5), 1998, pp. 609-631
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.