CARBOHYDRATE-BASED CROWN-ETHERS CONTAININ G 1,4-LINKED D-GLUCOPYRANOSE MOIETIES

The chiral crown ethers 7, 8, 9, and 13 containing a 1,4-bridged alpha -D-glucopyranose moiety were synthesized from methyl 2,3,6-tri-O-benzy l-alpha-D-glucopyranoside (1) and methyl 2,3-di-O-allyl-6-O-benzyl-alp ha-D-glucopyranoside (2), respectively, by two subsequent etherificati on reactions followed by an intramolecular transglycosylation. To buil d up the hexaethylene glycol chain in 4-position of 1 and 2, bis(2-chl oroethyl) ether (generating 3 and 3, respectively) and tetraethylene g lycol (yielding 5 and 6, respectively) were used. The cyclizations of 5 and 6 in acetonitrile to the crowns 7 and 8 were catalysed by trimet hylsilyl trifluoromethanesulfonate assisted by a ''template'' effect o f potassium tetrafluoroborate (yield 45-57%). A high alpha-stereoselec tivity was found for the intramolecular glycosylations even if a benzo ylated precursor such as 12 was cyclized (catalyst: BF3.Et2O; yield 25 -26%). Compound 12 was prepared from 5 by exchange of the benzyl group s for benzoyl functions (10-12). Finally, the crown ether 8 was deally lated to generate the crown 9.