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Hydrogen Bonding of Fluorinated Saccharides in Solution: F Acting as H‐Bond Acceptor in a Bifurcated H‐Bond of 4‐Fluorinated Levoglucosans
Author(s) -
Bernet Bruno,
Vasella Andrea
Publication year - 2007
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200790196
Subject(s) - chemistry , hydrogen bond , silylation , intramolecular force , acceptor , stereochemistry , medicinal chemistry , crystallography , catalysis , molecule , organic chemistry , physics , condensed matter physics
4‐Fluorinated levoglucosans were synthesised to test if OH⋅⋅⋅F H‐bonds are feasible even when the O⋅⋅⋅F distance is increased. The fluorinated 1,6‐anhydro‐ β ‐ D ‐glucopyranoses were synthesised from 1,6 : 3,4‐dianhydro‐ β ‐ 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 HOC(2) to OC(5). They were synthesised from the i Pr 3 Si‐protected derivative of 8 . Intramolecular bifurcated H‐bonds from HOC(2) to FC(4) and OC(5) of the 4‐fluorinated levoglucosans in CDCl 3 solution are evidenced by the 1 H‐NMR scalar couplings h1 J (F,OH) and 3 J (H,OH). The OH⋅⋅⋅F H‐bond over an O⋅⋅⋅F distance of ca. 3.0 Å is thus formed in apolar solvents, at least when favoured by the simultaneous formation of an OH⋅⋅⋅O H‐bond.