Hydrogen‐Bonding Network Anchors the Cyclic Form of Sugar Arylhydrazones

Abstract

The “classical” challenge, raised by Emil Fischer as to why one monosaccharide arylhydrazone adopts a cyclic structure but another an acyclic structure, is answered here. The present comprehensive analysis of hexose and hexosamine arylhydrazones, based on 2D NMR spectroscopy and theoretical modeling, has established that the chain of hydrogen bonds needed for conformational selection can only be completed for d‐glucosamine derivatives. Thus, d‐glucosamine 4‐nitrophenylhydrazone exclusively adopts its cyclic form, but any configurational changes imply the formation of acyclic structures. In conclusion, three criteria dominate structure selection, namely 1) an amino function at the C‐2 position, 2) the “all‐equatorial” substitution mode of the pyranoid ring, and 3) an electron‐withdrawing group on the arylhydrazone are all needed to get the cyclic form only.