1,2- O-(R)-Trichloroethylidene-beta-D-mannofuranose (1) was obtained from the reaction Of D-mannose with chloral. Reaction of I with potassium tert-butoxide (3 M equiv) gave the thermodynamically stable 1,2,5-0-orthodichloroacetyl-beta-D-mannofuranose as the sole product whereas 1.5 M equiv of reagent gave the kinetically controlled 1,2,3- O-orthodichloroacetyl-beta-D -mannofuranose (10) as the main product. Orthoester 10 gave the 5,6-isopropylidene derivative, which was also obtained from the reaction of 5,6-O-isopropylidene-1,2-O-(R)-trichloroethylidene-beta-D-mannofuranose with potassium tert-butoxide (1.5 M equiv). These novel orthoesters are expected to prove useful as protecting groups and as building blocks in the formations of new mannofuranisidic units. (C) 2004 Elsevier Ltd. All rights reserved.