Chiral template method

We do not address the chiral template method in this article. 

Preparation of chiral mesoporous materials has become a great interest for material scientists. Normally chiral property is introduced into chiral mesoporous material via an organic chiral templating component. But, by using a sonochemical method, Gabashvili et al. [36] have prepared mesoporous chiral titania using a chiral inorganic precursor and a non-chiral dodecylamine as a template. Size of the pores was 5.5 nm. 

Synthetic methods include the use of silanes bearing a chiral group for silylating the surface of the porous sol-gel silica, the use of such silanes as monomers or co-monomers in the polycondensation, the physical entrapment of chiral molecules, the imprinting of SG materials with chiral templates and the creation of chiral pores, and the induction of chirality in the matrix skeleton itself 48... 

Aldonolactones are useful starting materials for the synthesis of modified sugars. They have also been used as chiral templates in synthesis of natural products. Some of them are inexpensive, commercially available products or they may be obtained readily from the respective monosaccharides. The purpose of this chapter is to survey the main reactions of aldonolactones. Previous reviews on the subject include articles on gulono-1,4-lactones (1) and D-ribonolactone (2). Methods of synthesis, conformational analysis, and biological properties are not discussed in this chapter. 

For the total synthesis of aminodeoxy sugars from nonsugar precursors, it is logical to turn to amino acids as primary sources of chiral templates. The challenge is to find innovative and stereocontrolled methods of chain extension reactions that lead to higher-carbon aminodeoxy sugars. 

Cyclopolymerization of bifunctional monomers is an effective method of chirality induction. Optically active vinyl homopolymers and copolymers have been synthesized by using optically active distyrenic monomers (41) based on a readily removable chiral template moiety. Free-radical copolymerization of 41a with styrene and removal of the chiral template moiety from the obtained copolymer led to polystyrene, which showed optical activity ([Oc]365 -0.5-3.5°) (Scheme 11.6) [84], The optical activity was explained in terms of chiral (S,S)-diad units generated in the polymer chain through cyclopolymerization of 41a [85], Several different bifunctional monomers have been synthesized and used for this type of copolymerization [86-90]. 

While technically simpler than the covalent approach, the self-assembly approach creates more heterogeneous sites and also requires templates with specific functional groups.8 Since sol-gel chemistry is aqueous based, H-bonding interactions are significantly weaker compared to the conventional organic polymerization methods. Often, hydrophobic effects and ionic interactions are utilized. A number of other examples of the noncovalent approach to imprinting in sol-gel-derived materials are provided in recent reviews.5 17 In the sections below, the focus will be on some of the newer aspects of small molecule imprinting in silica that involve the use of chiral templates... 

The rhodium containing hybrid materials prepared with the BINOL bicarbamate moieties are less active and enantioselective than the previous hybrid catalyst. The observed enantioselectivity was attributed to supramolecular effect of the chiral tridimensional network owing to the weakness of the interaction of the transition metal and the chiral ligand. The control of the texture and morphology of these solids by templating methods firstly reported by Macquarrie39 and Mann40 with suitable surfactants would improve the catalytic performance of this new class of chiral materials. 

The aqueous rate-enhanced Claisen rearrangement of glycoorganic compounds was shown to proceed in excellent yields [57]. The water solubility of the reactants was induced by grafting a free sugar onto the allyl vinyl ether moiety moreover, the sugar functioned as a chiral template and gave highly crystalline diastereomers which were easily separable to yield pure enantiomers after enzymic hydrolysis. This method allows the preparation of either enantiomerically pure (R)- or (S)-l,3-diol (Scheme 1). 

The hnal approaches rely on the transformation of a readily available, natural amino acid where the side chain is manipnlated to provide an nnnatnral analogne (ronte h) or the stereogenic center of the natnral amino acid is nsed to control the introdnction of a second side chain (route i). This last method is closely related to the use of chiral templates and auxiliaries. 

A wide variety of methods are available for the large-scale synthesis of unnatural amino acids. In a few cases, chiral templates or auxiliaries can be used, but usually cost becomes an issue with this approach. Resolutions can be performed by either a chemical or biological agent. These methods are not cost effective because the maximum yield of the reaction is 50%. Methods in which the undesired isomer can be recycled in situ offer a much better solution. Biological and chemical... 

In this article we provide a broad overview of the application of radical methods in carbohydrate chemistry, including typical examples classified by the type of bond formed. The factors controlling the stereoselectivity of inter- and intramolecular C-C bond formation are now well understood and have been exploited in the synthesis of C-glycosides [2]. Intramolecular C C bond formation using carbohydrate-based chiral templates also provides a powerful route to branched-chain sugars [3] and carbocycles [4]. Finally, we include synthetically useful processes involving key carbon-heteroatom and C-H bond formation. 

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