Chemical templates

Chemical templates are being increasingly employed for the development of dynamic combinatorial libraries (DCL) [94-98]. These (virtual) libraries of compounds are produced from all the possible combinations of a set of basic components that can reversibly react with each other with the consequent potential to generate a large pool of compounds. Because of the dynamic equilibria established in a DCL, the stabilization of any given compound by molecular recognition will amplify its formation. Hence the addition of a template to the library usually leads to the isolation of the compound that forms the thermodynamically more stable host-guest complex (see Scheme 37). 

Site-Selective Attachment of Capture Probes onto Chemical Templates. .. 107... 

The success of these experiments provides a complete vindication of the suggestion that the coordination sphere of a metal ion represents the simplest and best understood chemical template, and that it may hold reactive groups in juxtaposition so that complicated multistep reactions may occur in a sterically highly selective manner. The specific examples cited provide a new principle of ring closure which has a counterpart in the synthesis of basketlike and cagelike structures. 

Protein is formed mainly of polymerised amino-acids. The primary structure, unlike that of synthetic polymers, is non-repetitive and, for its production, requires a chemical template stored in the structure of the DNA molecule. The sizes of proteins vary considerably (in a range of molecular weights from 6000 to 1,000,000). Proteins fulfill many roles within the cell, the most important of which is that of catalysis. Proteins which have catalytic activity are called enzymes whilst other proteins have important roles in storage, transport, protection (antibodies), as chemical messengers (hormones) and in structure 17,, 8). 

We promised to return to the earlier statement that a healthy development pipeline of natural product derived candidates implies that natural products will still have a role to play in modern day drug discovery. In fact, this is far from reality. Firstly, these late-stage clinical candidates reflect the output from research activities undertaken at least 10 years ago and certainly not the current situation. Secondly, there is a lack of truly novel chemical templates in the pipeline and thirdly, it is clear that very few pharmaceutical companies remain engaged, at least internally, in natural product drug discovery activities. 

Relatively few studies on the synthesis of mesoporous alumina have been reported to date [8]. One of the limitations of the reported synthetic strategies is that the rate of hydrolysis (and condensation) reaction of aluminum alkoxide are much faster than that of silicon alkoxide. In this study, we proposed a novel method to prepare bimodal porous aluminas with meso- and macropores with narrow pore size distribution and well-defined pore channels. The fiamewoik of the porous alumina is prepared via a chemical templating method using alkyl caiboxylates. Here, self-assemblied micelles of carboxylic acid were used as a chemical template. Mesoporous aluminas were prepared through carefiil control of the reactants pH, while the procedures are reported elsewhere [9]. 

Important trends in N2 isotherm when the PS beads are used as a physical template are shown in Table 1 and Fig. 2. In Table 1, PI is the alumina prepared without any templates, P2 is prepared without ]4iysical template (PS bead), P3 is prepared without chemical template (stearic acid), and P4 is prepared with all templates. For above 10 nm of pore size and spherical pore system, the Barrett-Joyner-Halenda (BJH) method underestimates the characteristics for spherical pores, while the Broekhoff-de Boer-Frenkel-Halsey-Hill (BdB-FHH) model is more accurate than the BJH model at the range 10-100 nm [13]. Therefore, the pore size distribution between 1 and 10 nm and between 10 and 100 nm obtained from the BJH model and BdB-FHH model on the desorption branch of nitrogen isotherm, respectively. N2 isotherm of P2 has typical type IV and hysteresis loop, while that of P3 shows reduced hysteresis loop at P/Po ca. 0.5 and sharp lifting-up hysteresis loop at P/Po > 0.8. This sharp inflection implies a change in the texture, namely, textural macro-porosity [4,14]. It should be noted that P3 shows only macropore due to the PS bead-free from alumina framework. 

The chemical methods for the preparation of nanomaterial could be categorized as either template-directed or template-free. The template synthesis methods commonly used for the production of one-dimensional nanostructured PANI are further subdivided into hard template (physical template) synthesis and soft template (chemical template) synthesis approach according to the solubility of the templates in the reaction media. Non-template routes for the synthesis of one-dimensional nanostructured PANI such as rapid-mixing reaction method, radiolytic synthesis, interfacial polymerization, and sonochemical synthesis have also been reported [56], Other approaches like combined soft and hard template synthesis are also known. An overview of hard-template, soft-template, and template-free procedures are presented in the following paragraphs. 

Strategies to convert selective P450 substrates to suicide inactivators by the incorporation of a suitable activatable function at the position oxidized are not always successful. For instance, the introduction of an acetylenic moiety into the chemical template Af-(3,5-dichloro-4-pyridyl)-... 

A chemical template organizes an assembly of atoms with respect to one or more geometric loci, in order to achieve a particular linking of atoms. 

In the 1960s, Busch carried out pioneering work on metal-directed syntheses of macro cycles establishing the concept of chemical template. As defined by him a chemical template organizes an assembly of atoms, with respect to one or more geometric loci, in order to achieve a particular linking of atoms [20-22]. This provides an efficient route to prepare a specific molecular assembly when several others can be potentially formed. Ideally templates should be removed from the final product once the reaction has reached completion however, templates often form an integral part of the final product hence, they cannot always be removed from it. 

Busch, D.H. Structural definition of chemical templates and the prediction of new and unusual materials. J. Inclusion Phenom. 1992. 12 (1-4), 389-395. 

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