Tricyclic furanoid dichloroacetyl orthoesters of D-mannose from 1,2-O-trichloroethylidene-beta-D-mannofuranose.

1,2-O-(R)-Trichloroethylidene-beta-D-mannofuranose (1) was obtained from the reaction of D-mannose with chloral. Reaction of 1 with potassium tert-butoxide (3 Mequiv) gave the thermodynamically stable 1,2,5-O-orthodichloroacetyl-beta-D-mannofuranose as the sole product whereas 1.5 Mequiv 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 Mequiv). These novel orthoesters are expected to prove useful as protecting groups and as building blocks in the formations of new mannofuranisidic units.