TIN(IV) CATALYZED D-GALACTURONIC ACID ANOMERIZATION

A comparative kinetic study of anomerization between the four main forms (a/b-pyranosides and
a/b-furanosides) of D-galacturonic acid (GalAH2, 1) and tin(IV) complexed D-galacturonate
([Sn(CH3)2(GalA)(H2O)2], 2) in D2O, is reported. Important differences are shown by pD vs.
mutarotation rate profiles of 1 and 2. Coordination, in fact, causes: i) a spectacular increase of the rate
due to kinetic template assistance of the carboxylic tail holding the metal in close proximity to the
reactive centre and ii) a change in the dependence of the rate on the pD. Entropic and enthalpic factors
contribute to the huge activation energy decrease in 2. Anomerization rates of the complexed ligand are
determined by 2D-EXSY cross-peak integration. Analysis of up to twelve direct and reverse rate
constants in the pD range 7.0–2.5 evidences ring size memory, and at least one reaction path which
bypasses the aldehydo open chain intermediate. Solvent oxygen atom incorporation into the ligand is,
at pH 4.5, about ten fold faster when catalyzed by the metal, and proves that the metal-assisted
nucleophilic attack of water on the anomeric carbon atom cannot significantly contribute to
mutarotation rates that are thousands of times faster.