Topics in Catalysis 13 (2000) 335–338 335 Regioselectivity in acid- or base-catalysed acetalation of sucrose: selection of [OH-2, OH-3] or [OH-4, OH-6] diols Stanislaw Porwanski, Piotr Salanski, Nathalie Panaud, G´ erard Descotes, Alain Bouchu and Yves Queneau ∗ Unit´ e Mixte de Sucrochimie CNRS – B´ eghin-Say (UMR 143), c/o Eridania B´ eghin-Say, CEI, 27 bvd du 11 Novembre 1918, BP 2132, 69603 Villeurbanne Cedex, France Regioselective monoacetalation of sucrose can be achieved without prior protection. Under acidic catalysis, and notably with the use of lanthanide-exchanged cation resins as heterogeneous catalysts, sucrose monoacetals of α,β-unsaturated carbonyl compounds involving OH-4 and OH-6 are obtained. On the other hand, the reaction of unprotected sucrose with a α-chloromethyl ketone in the presence of base provided a α-hydroxymethyl 5-membered ring acetal involving OH-2 and OH-3 as the major product, illustrating the pre-eminent reactivity of OH-2 in sucrose and its consequences on the product distribution for reactions under kinetic control. Keywords: sucrose, acetals, lanthanide, ytterbium, resin Methods for the selective functionalisation of carbohy- drates are useful for both, the direct transformation of un- protected sugars to low added value derivatives, and the subtle protection–deprotection strategies in multistep syn- theses of complex molecules. In the case of sucrose, which can be considered as a promising raw material for the chem- ical industry because of its exceptional availability, the pres- ence of eight hydroxyl groups provides an interesting field of experimentation aiming at the control of the regioselec- tivity and/or the degree of substitution [1]. Among the nu- merous chemical reactions involving the alcohol function, the acetalation reaction is extensively used in the case of carbohydrates, mostly for protection purposes. We review in this presentation our recent work on the study of the acetalation of sucrose. We discuss the additional difficulty, compared to simpler carbohydrates, due to the sensitivity of sucrose to acidic conditions when acidic catalysis is re- quired. We show also how it is possible to obtain acetals with different regioselectivities depending on the thermo- dynamic or kinetic control of the process using basic or acidic conditions. The fragility of the glycosidic bond of sucrose under acidic conditions, because of its fructofuranosidic nature, is a strict limitation for acid-catalysed acetalations. Only very reactive carbonyl compounds are reactive enough un- der acidic conditions which preserve the disaccharidic in- tegrity of the molecule. Notably, the transacetalation reac- tion of α,β-unsaturated dialkylacetals has been shown by Gelas and co-workers then by our group, to occur under mild conditions [2,3]. The reaction is very selective of the [OH-4, OH-6] diol because it yields the unique and most stable bicyclic (trans-decalin structure) linkage in the mol- ecule. Therefore, both the regiochemistry and the degree of substitution are controlled. If the reaction is continued, then the second acetalation will occur at the [OH-2, OH-1 ′ ] diol, ∗ To whom correspondence should be addressed. showing how much these two hydroxy groups are reactive among the remaining six [4]. We concentrated on the reaction of sucrose with cit- ral dimethylacetal (CDMA, (E,Z)-3,7-dimethyl-octa-2,6- dienal), chosen for its interest in the fragrance industry (scheme 1). In this case, we used a number of acidic cata- lysts, in homogeneous or heterogeneous conditions. The results are given in table 1. Good yields of sucrose acetals were obtained, always as a mixture of the E and Z isomers (citral is a mixture of geranial (E) and neral (Z )). The unsaturation which provides better reactivity is also responsible for this isomerisation under acidic conditions. Starting directly from citral, sucrose acetals were obtained in low yield, but with a difference in the E/Z ratio (6 : 1 instead of 2 : 1), allowing full characterisation of the prod- ucts. Polyvinyl pyridinium p-toluenesulfonate was used as an example of a heterogeneous acidic catalyst, in view of the good results obtained with the homogeneous pyridinium p-toluenesulfonate. Sucrose acetals were obtained in mod- erate yield but with the conveniency provided by heteroge- neous catalysis at the work-up and isolation stages. Then we tested some lanthanide-cation-exchanged resins, interesting heterogeneous catalysts described by Yu et al. [5], which have never been used for the acetalation of carbohy- drates. These catalysts were shown to be extremely mild (table 2), since the reaction of sucrose with CDMA in DMF required heating at 80 ◦ C compared to the room tempera- ture reactions in the presence of homogeneous sulfonic acid pyridinium salts [6]. The catalysis is due to the oxophilicity of the lanthanide cations which activates the C–O bonds in the starting acetal or carbonyl compound. When we widened the scope of the reaction to other carbonyl compounds and other sugars, we found that these lanthanide-exchanged resins permitted a differentiation be- tween aldehyde- (fast) and ketone- (slow) dialkylacetal and between glucose (fast) and galactose (slow). Ytterbium- and erbium-resins were used with success for the prepara-  J.C. Baltzer AG, Science Publishers