Hi... Thanks to all who has suggested me the different path to find the epimerization reaction. But here they (authors of different articles) will tel C-1 to C-2 interconversion as well as C-2 and C-3 bond cleavage, along with hydride shift, at glucose leads to the formation of Mannose. But am unable to get the correct picture of the mechanism. If any one knows/worked on this problem, kindly help me to complete this chemical reaction mechanism.
Zuzana Hricoviniova
Dear colleague, you are asking about “epimerization of aldoses catalyzed by Mo(VI) ions” that is known as “Bílik reaction”. I have been dealing with this topic for years (in continuation of my father’s work – Prof. V. Bílik). This epimerization reaction is very stereospecific and it proceeds via the formation of molybdate complexes of aldoses. E.g. D-glucose in acidic aqueous solution in the presence of Mo(VI) salts create a catalytically active molybdate complexes. Aldose in its acyclic form is complexed (by its four adjacent hydroxyls 1-OH, 2-OH, 3-OH, 4-OH) in the teradentate binuclear complex. In the transition state the isomerization of carbon skeleton occurs with simultaneous cleavage of C2-C-3 bond and formation of new C1-C-3 bond. The process results in the formation of the molybdate complex of the epimeric aldose and produces equilibrium epimeric mixture of D-glucose and D-mannose (3:1) under conventional conditions (Bílik, V. Chem. Zvesti 1972, 26, 183–186; 187–189). Petruš L., Petrušová M., Hricovíniová Z. The Bílik Reaction, Topics in Current Chemistry, Springer-Verlag 2001, p.15-41. Transformation was studied also under microwave conditions (2.5:1) (Hricovíniová, Z. Carbohydrate Research 341 (2006) 2131–2134). This stereospecific rearrangement was studied in detail also with 2-C-branched aldoses, which gave rise 2-ketoses. This transformation was studied by NMR with isotopically labeled sugars and the mechanism is published in Hricovíniová-Bíliková, Z.; Hricovíni, M.; Petrušová, M.; Serianni, A. S.; Petruš, L. Carbohydrate Research 319 (1999) 38–46., Hricovíniová, Z. Tetrahedron: Asymmetry 19 (2008) 204–208. It is perhaps of interest that epimerization at C-3 has also been described Hricovíniová, Z.; Hricovíni Carbohydrate Research 370 (2013) 1–8. I understand it must be hard to get this without schemes but I'm afraid I can't draw any structures of complexes or transition states here.
Dear zuzana, Thank you for your valuable information.
Whether hydride shift is mandatory in epimerization of glucose to mannose
- Badges
- Science topic
- Similar topics
- Chemistry
- Organic Chemistry