Mécanisme de la réaction de glycosylation par réversion acide en présence d'anhydride trifluorométhane sulfonique comme agent de condensation

Abstract

The acid-catalysed mechanism of the reaction using trifluoromethanesulfonic anhydride to synthesize symmetrical 1,1′-glycosyl-glycosides such as trehalose and galacto, manno, arabino, xylo etc. analogues has been elucidated. The authors were able to prove by 19 F nmr study at low temperature that the efficiency of the system is due to the peculiar property of trifluoromethanesulfonic acid generated in situ to form a stable, insoluble hydroxonium trifluoromethanesulfonate CF 3 SO 3 − H 3 O + . The procedure satisfies the two necessary conditions for a glycosylation reaction being workable: (i) the presence in catalytic amounts of free acid which is regenerated during the condensation step; (ii) a very efficient trapping of the water liberated during the protonation step which displaces the equilibrium towards the formation of the glycoside. The α-stereoselectivity of the reaction is discussed in terms of anomerisation of the β-anomers on to the thermodynamically more stable α ones, through a cyclic oxonium intermediate.