NMR Spectroscopic Isotope Tracking Reveals Cascade Steps in Carbohydrate Conversion by Tin‐Beta

Abstract

Figure 1. Structures of the products in the Sn-Beta-catalyzed conversion of commoncarbohydrates, which include ML, THM,D PM, MVG, and assorted 3DE and 3DL. The Rg roupo f3 DE and 3DL may be H, CH 2 OH, or CH(OH)-CH 2 OH for tetroses, pentoses,orh exoses,r espectively.[a] S.

Effects of Alkali‐Metal Ions and Counter Ions in Sn‐Beta‐Catalyzed Carbohydrate Conversion

“…The 13 Cl abel in the product was primarily found at the C1 and C2 positions of all dehydration products, and at the C3 and C2 positions of methyl lactate, as the C1 position of xylose primarily gets converted into C3 of methyl lactate ( Figure 5B). [31] All major products showed ac omparable trend in the distribution of isotopici somers with increasing amountso fK 2 CO 3 .S cramblingi ncreased with K 2 CO 3 concentrations for the positions derived from C1 andC 2o fx ylose up to K 2 CO 3 concentrationst hat provided the optimal methyl lactate yield (0.25 mm K 2 CO 3 ,0 .17 K/Sn). These observations show that the epimerization of carbohydrates by a1 ,2-carbon shift [21] also occurs under high-temperature conditions that lead to significant retro-aldol cleavage and dehydration reactions.…”

Probing the Lewis Acid Catalyzed Acyclic Pathway of Carbohydrate Conversion in Methanol by In Situ NMR

“…The diastereomeric excess as determined from the HSQC integrals was less than 2.3 % both for HA-MEG and MMHB, while values above 45 % have been found for the C3 positions in C5 carbohydrates converted by heterogeneous Sn-containing zeolite. [19] The lack of stereoselectivity in the formation of MMHB, indicated that both the ketonization of C2 and the hydride shift converting HA-MEG to MMHB have little stereoselectivity. This finding contrasted the significant stereoselectivity in larger sugars, [1d,10,19] where higher populations of cyclic intermediates may affect the stereochemistry of the aforementioned reactions due to different reactivities of axial and equatorial hydrogen atoms.…”

Mechanism and malleability of glucose dehydration to HMF: entry points and water-induced diversions

“…13 C-assisted flux analysis using product labeling from 13 C glucose substrate is widely employed in metabolic research, 21,22 while being considerably less widely established in chemocatalytic investigations. 23 Both 1,2 and 1,5 H-shift convert glucose to ketoses (fructose and sorbose, respectively), which are onpathway intermediates towards HMF. In contrast, a 1,2 C-shift converts glucose to mannose as an off-pathway intermediate.…”