Hydrogen Bonding of Fluorinated Saccharides in Solution: F Acting as H-Bond Acceptor in a Bifurcated H-Bond of 4-Fluorinated Levoglucosans

摘要

4-Fluorinated levoglucosans were synthesised to test if OH centre dot centre dot centre dot F H-bonds are feasible even when the O centre dot centre dot centre dot F distance is increased. The fluorinated 1,6-anhydro-beta-D-glucopyranoses were synthesised from 1,6 :3,4-dianhydro-beta-D-galactopyranose (8). Treatment of 8 with KHF_2 and KF gave 43% of 4-deoxy-4-fluorolevoglucosan (9), which was transformed into the 3-O-protected derivatives 13 by silylation and 15 by silylation, acetylation, and desilylation. 4-Deoxy-4-methyllevoglucosan (19) and 4-deoxylevoglucosan (21) were prepared as reference compounds that can only form a bivalent H-bond from HO-C(2) to O - C(5). They were synthesised from the ~iPr3Si-protected derivative of 8. Intramolecular bifurcated Ff-bonds from HO-C(2) to F-C(4) and O-C(5) of the 4-fluorinated levoglucosans in CDC13 solution are evidenced by the ~1H-NMR scalar couplings ~(h1)J(F,OH) and ~3J(H,OH). The OH centre dot centre dot centre dot F H-bond over an O centre dot centre dot centre dot F distance of ca. 3.0 A is thus formed in apolar solvents, at least when favoured by the simultaneous formation of an OH centre dot centre dot centre dot O H-bond.