Sugar template

Fig. 2. Concept of molecular imprinting - the covalent approach. 1. Derivatization of sugar template with p-vinylphenyl boronate. 2. Polymerization in the presence of a cross-linker...

Striegler S. Carbohydrate recognition in cross-linked sugar-templated poly(acrylates). Macromolecules 2003 36 1310-1317. 

A solid-phase synthesis of furo[3,2-3]pyran derivatives utilizing highly functionalized sugar templates has been reported <2003JOC9406>. After incorporation of alkenes within the sugar template, such as compound 95, the solid support is introduced via formation of the acid amide. This immobilized system then allows a ruthenium-catalyzed ring-closing metathesis that leads to the formation of the fused oxacycles. 

The synthesis of type II branched-chain sugars should seem a more difficult task because it needs activation of a carbon atom on the sugar template and a control of the stereochemical course of the carbon-carbon bond formation. Probably because of this apparent difficulty several methods have been devised in the last decade, in particular, in the field of organo-metallic and free radical reactions. 

The radical is generated on the sugar template using mostly the tin hydride method and halo derivatives [72,87,89], thionocarbonate [90-91], or selenium derivatives [91,94, 95]. Again, in these reactions, the kinetically controlled 5-exo cyclization is always... 

In all free radical cyclizations, a radical is formed on the sugar template (see Scheme 21 top line). Trapping of these intermediate radicals located at C-3 or C-2 [74] or at C-l [79,85] with activated olefins has been reported and is a good way to obtain doubly vicinal branched-chain sugars in a single process. 

R. Nouguier, C. Lesueur, E. De Riggi, M. P. Bertrand, and A. Virgili, Stereoselective free-radical cyclisation on a sugar template. Hie sulphonyl radical as a synthetic tool for functionalized glycosides. Tetrahedron Lett. 37 3541 (1990). 

C. Lesueur, R. Nouguier, M. P. Bertrand, P. Hoffmann, and A. De Mesmaeker, Stereocontrol in radical cyclization on sugar templates. Tetrahedron 50 5369 (1994). 

Attachment of the alkene monomers to the template (Fig. 6.5), which must be reversible - readily formed and readily broken - to permit removal of the template after polymerisation, can generally be accomplished in two ways covalently or non-covalently. While the latter interactions (ionic, hydrophobic, n-n, hydrogen bonding) can easily be reversed, there is less scope for reversible covalent linkages. One of these is the formation of boronic esters - from boronic acid units of the monomers and OH groups of sugar templates. 

Asymmetric synthesis Chiral auxiliary Sugar template Chiral ligand Transition-metal catalyst... 

Since the mid-1980s, Kunz and co-workers started an exploration of asymmetric synthesis using sugar template-based chiral auxiliaries. Their early success on this topic is particularly shown by their demonstration of the utility of six-membered pento- and hexopyranose derivatives such as per-O-pivaloylated /3-D-galactosylamine [42,43,44,45,46,47,48,49,50]. The collective studies of the Kunz group have been thoroughly reviewed [51,52]. 

R = Ph, R = Et) was carried out by hydrolysis in the presence of LiOOH. As a result, (R)-3-phenylvaleric acid 54 was obtained in a highly enantioenriched forra In addition, the sugar template 55 was recovered almost quantitatively. 

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