Hydrogen bonding of fluorinated saccharides in solution: F acting as H-bond acceptor in a bifurcated H-bond of 4-fluorinated levoglucosans

4-Fluorinated levoglucosans were synthesised to test if OH center dot center dot center dot F H-bonds are feasible even when the O center dot center dot center 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 KHF2 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 (Pr3Si)-Pr-i-protected derivative of 8. Intramolecular bifurcated H-bonds from HO-C(2) to F-C(4) and O-C(5) of the 4-fluorinated levoglucosans in CDCl3 solution are evidenced by the H-1-NMR scalar couplings (h1)J(F,OH) and (3)J(H,OH). The OH center dot center dot center dot F H-bond over an O center dot center dot center dot F distance of ca. 3.0 angstrom is thus formed in apolar solvents, at least when favoured by the simultaneous formation of an OH center dot center dot center dot O H-bond.