Carbohydrate‐Based Crown Ethers

Chiral 12‐crown‐4 ethers ( 13–16 ) and 18‐crown‐6 ethers ( 21–24 ) containing two and three ethylene‐bridged D ‐glucopyranose moieties, respectively, have been synthesized. 2‐ O ‐(Acetoxyethyl)‐3,4,6‐tri‐ O ‐benzoyl‐α‐ D ‐glucopyranosyl fluoride ( 5 ), generated by treatment of 2,3,4‐tri‐ O ‐benzoyl‐1,2‐ O ‐ethylene‐β‐ D ‐glucose ( 4 ) with HF/nitromethane/acetic anhydride was the key building block. It was used as a glycosyl donor and/or – after modification – as an acceptor component in head‐to‐tail‐type glycosylations. Thus, the openchain precursors 9/11, 10/12, 17/18 and 19/20 of the crown ethers were prepared from reaction of 5 with 7, 11 and 12 , respectively, in BF 3 ‐catalyzed glycosylations. The stereoselectivity of the glycosylations could be influenced by the use of different solvents. α‐Selective syntheses of the crown ethers 14 (from the thioglycoside 11 ) and 23 (from the thioglycoside 20 ) were achieved in dichloromethane using NIS/trifluoromethanesulfonic acid for activation. In acetonitrile, the thioglycoside 11 yielded predominantly the 12‐crown‐4 ether 13 , while thioglycoside 18 formed a mixture of the α‐and β‐anomers 21 and 22 under the same activation conditions.