Enzymatic transglycosylation using p-nitrophenyl alpha-D-rhamnopyranoside as the glycosyl donor and 6 equiv of ethyl 1-thio-alpha-D-rhamnopyranoside as the glycosyl acceptor yielded a D-rhamnooligosaccharide derivative. The reaction was catalyzed by jack bean alpha-mannosidase in a 1: 1 (v/v) mixture of 0.1 M sodium citrate buffer (pH 4.5)-MeCN at 25degreesC. The enzyme exhibited high catalytic activity for the reaction, to afford in 32.1% isolated yield (based on donor substrate) ethyl alpha-D -rhamnopyranosyl-(1--> 2)- 1-thio- alpha-D-rhamnopyranoside, which is a derivative of the common oligosaccharide unit of the antigenic lipopolysaccharides from Pseudomonas. (C) 2004 Elsevier Ltd. All rights reserved.