Synthesis of butyl-α and β-d-glucopyranosides in the presence of dealuminated H-Y faujasites:: Kinetic study, mechanism, stereoelectronic effects, and microreversibility principle

The glycosylation reaction between D-glucose and n-butanol has been investigated over various dealuminated HY zeolites with Si/Al ratios from 2.5 to 100. In this way, butyl-alpha- and beta-D-glucofuranosides and -pyranosides are readily synthesized at temperatures from 363 to 383 K and with a butanol/glucose ratio from 5 to 40. From the systematic study of the glycosylation reaction, a kinetic scheme involving both consecutive and competitive steps has been proposed. Butyl-D-glucofuranosides and butyl-D-glucopyranosides are primary products, butyl-D-glucofuranosides being then quantitatively converted into their pyranoside form. The use of microporous catalysts, associated with their shape selectivity properties, reduces the amount of oligomers and increases the amount of BD-glucopyranosides in a significant manner. The beta/alpha anomeric ratios observed are then discussed in terms of stereoelectronic effects which are, according to the principle of microreversibility, of the same nature as those reported for the reverse reaction, i.e., hydrolysis of alkyl-D-glucopyranosides. (C) 1999 Academic Press.