Di-D-fructose dianhydride-enriched products by acid ion-exchange resin-promoted caramelization of D-fructose: chemical analyses.

Caramelization commonly occurs when sugars, or products containing a high proportion of sugars, are heated either dry or in concentrated aqueous solutions, alone or in the presence of certain additives. Upon thermal treatment of sugars, dehydration and self-condensation reactions occur, giving rise to volatiles (principally 2-hydroxymethylfurfural, HMF), pigments (melanoidines) and oligosaccharidic material, among which di-D-fructose dianhydrides (DFAs) and glycosylated DFA derivatives of different degree of polymerization (DP) have been identified. This study reports a methodology to produce caramel-like products with a high content of DFAs and oligosaccharides thereof from commercial D-fructose based on the use of acid ion-exchange resins as caramelization promotors. The rate of formation of these compounds as a function of D-fructose concentration, catalyst proportion, temperature, catalyst nature and particle size has been investigated. The use of sulfonic acid resins allows conducting caramelization at remarkable low temperatures (70-90 degrees C) to reach conversions into DFA derivatives up to 70-80% in 1-2 h, with relative proportions of HMF < 2%.The relative abundance of individual DFA structures can be modulated by acting on the catalyst nature and reaction conditions, which offers a unique opportunity for nutritional studies of DFA-enriched products with well-defined compositions.

The o-xylylene protecting group as an element of conformational control of remote stereochemistry in the synthesis of spiroketals.

Protection of trans-1,2-diol segments as cyclic o-xylylene ethers strongly favours diequatorial over diaxial dispositions; the possibility of using this grouping for remote control of the stereochemistry in the synthesis of spiroketals is here demonstrated by the stereoselective synthesis of tricyclic spirodisaccharides (di-D-fructose dianhydrides).

Spacer-mediated synthesis of contra-thermodynamic spiroacetals: stereoselective synthesis of C2-symmetric difructose dianhydrides.

The xylylene moiety (ortho, meta, and para) was employed as a rigid tether in the spacer-mediated synthesis of difructose dianhydrides (DFAs), a unique class of bis-spiroacetal derivatives present in food products. The synthetic methodology exploits the suitability of triflic acid to promote spirocyclization in organic solvents under irreversible reaction conditions, using anomeric isopropylidene fructose derivatives as precursors. Advantage was taken of the strong dependence of the conformational properties of DFAs on the relative configuration of the spiroketal centers. Highly stereoselective syntheses of the contra-thermodynamic difructofuranose and difructopyranose diastereomers, namely the C2-symmetric derivatives having the beta-configuration at both anomeric centers, have been accomplished by judicious choice of the xylylene positional isomer and of the linking position to the fructose building blocks. Interestingly, the rigid spacer concept has also been implemented to favor intermolecular processes leading to higher macrocyclic architectures that incorporate the bis-spiro fructodisaccharide subunit.

Rigid spacer-mediated synthesis of bis-spiroketal ring systems: stereoselective synthesis of nonsymmetrical spiro disaccharides.

The application of the "rigid spacer-mediated linkage between nonreacting centers" concept to the preparation of nonsymmetrical bis-spiroketal structures is demonstrated by the stereoselective synthesis of the bis-spiro fructodisaccharide 1, a minor component of industrial caramels. An o-xylylene bridge has been used to limit the conformational space during the intramolecular glycosylation-spirocyclization reaction of a difructopyranose precursor, thus controlling both the ring size and the stereochemistry at the spiro centers. [Structure: see text]

Carbohydrate-derived spiroketals: stereoselective synthesis of di-D-fructose dianhydrides via intramolecular aglycon delivery.

[reaction: see text] A one-pot synthesis of C(2)-symmetric di-d-fructose dianhydrides having the 1,6,9,13-tetraoxadispiro [4.2.4.2]tetradecane skeleton has been accomplished via intramolecular aglycon delivery from (6 --> 6) xylylene-tethered fructofuranose precursors. The stereochemical outcome of the glycosylation-spiroketalization process is governed by the geometrical constraints imposed by the rigid tetracyclic structure of the final compound.