Synthesis of butyl-alpha and beta-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 investi gated over various dealuminated HY zeolites with Si/Al ratios from 2.5 to 1 00. In this way, butyl-alpha- and beta-D-glucofuranosides and -pyranosides are readily synthesized at temperatures from 363 to 383 K and with a butano l/glucose ratio from 5 to 40. From the systematic study of the glycosylatio n reaction, a kinetic scheme involving both consecutive and competitive ste ps has been proposed. Butyl-D-glucofuranosides and butyl-D-glucopyranosides are primary products, butyl-D-glucofuranosides being then quantitatively c onverted into their pyranoside form. The use of microporous catalysts, asso ciated with their shape selectivity properties, reduces the amount of oligo mers and increases the amount of BD-glucopyranosides in a significant manne r. The beta/alpha anomeric ratios observed are then discussed in terms of s tereoelectronic effects which are, according to the principle of microrever sibility, of the same nature as those reported for the reverse reaction, i. e., hydrolysis of alkyl-D-glucopyranosides. (C) 1999 Academic Press.