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

✍ Scribed by Bruno Bernet; Andrea Vasella

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: German
• Weight: 338 KB
• Volume: 90
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.200790196

✦ Synopsis

Abstract

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 HOC(2) to OC(5). They were synthesised from the ^i^Pr~3~Si‐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 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.