Tether groups

Faure, S., Piva-Le-Blanc, S., Bertrand, C., Pete, J.-P., Faure, R., and Piva, O. (2002) Asymmetric intramolecular [2 + 2] photocycloadditions a- and (3-Hydroxy acids as chiral tether groups. Journal of Organic Chemistry, 67, 1061-1070. 

Consistent with most tethered photocycloadditions, the photoproducts obtained from silicon tethered irradiations are formed with complete regiocontrol [19]. Several examples of silyl tethered groups have been reported. Scheme 16 lists the variants that have been successful. As mentioned previously, the mechanistic pathway that is most consistent with the results involves interaction between the alkenes prior to cycloaddition. The experimental observation is that cycloadducts are formed only when the alkenes have extended n systems attached. 

Scheme 16 Silyl tethered groups that undergo cycloaddition.

Related materials can be prepared in which the polysaccharides are linked to a silica support by covalently bound tether groups. For example, silica derivatized by 3-aminopropyl groups can be linked to polysaccharides using diisocyanates. These materials seem to adopt organized structural patterns on the surface, and this factor is believed to contribute to their resolving power. The precise structural basis of the chiral recognition and discrimination of derivatized polysaccharides has not been elucidated, but it appears that in addition to polar interactions, tt-tt stacking is important for aromatic compounds. ... 

The use of silyl acetals as tethering groups in the IMDA reaction is often hampered by the relatively poor yields observed in the formation of unsymmetrical systems. However, the preparation of the triene precursors can be facilitated if only one of the r-systems is linked through a silyl ether connection, while its reacting partner is attached directly to the silicon tether [9]. 

Inter molecular Additions to Cyclohexenones and Related Systems. Calculations have been reported that deal with a transition state analysis of the regioselectivity encountered in triplet-state (2 + 2)-cycloaddition reactions of cyclohex-2-enone. Diastereoselective (2 + 2)-cycloaddition of ethene to cyclohexenone carboxylates in the presence of chiral auxiliaries has been described. Yields of bicyclo[4.2.0]octanone derivatives can be obtained with de as high as 81%. The enone (16) can be tethered to a polymer substrate via the tether group R. Irradiation of this material in toluene with ethene at —78°C gives a 68% yield of the adduct (17). The de of the product is reasonable at 72%. Addition of 1,1-diethoxyethene to the cyclohexenone (18, R = Ph) results in the formation of the two stereoisomeric head-to-tail regioisomers (19) and (20). The outcome of the reaction is dependent on the rate of formation of 1,4-biradical intermediates. This can be seen in the dependence of the cisjtrans ratio on the solvent and on the temperature at which the reaction is carried out. Thus with enone (18, R = Ph) in acetone at 3°C, a cjt ratio of 4.4 is obtained, and this changes to 2.1 at —40°C. In acetonitrile the cjt ratio is only 1.9 at the same temperature. With the other derivative of (18), the cjt ratio is 1.3 in acetone and 0.8 in acetonitrile. Photoaddition of ethene to the enone carboxylate [21, R = (-)-8-(2-naphthyl)menthyl] results in the formation of the diastereoisomers (22) and (23) with a de of 56% at —78°C. The diastereo-selectivity can be enhanced by the addition of naphthalene derivatives to the solution. Thus with naphthalene, a de of 71% is obtained, and this can be increased to 83% with 1-phenylnaphthalene. ... 

The nature of the transition state determines whether the newly formed ring junction is ctj-fiised or trani-fused. The endo rule applicable to intermolecular Diels—Alder reaction cannot be relied upon to predict the stereochemical outcome of the IMDA reaction. The IMDA reaction of Z- and -dienes can give either cis- or frani-fused products depending on orientation of the diene relative to the dienophile in the transition state (i.e., whether the addition is via an exo or endo mode). In this case, the terms exo and endo refer to the spatial position of the tether group. When the bulk of the tether group is oriented... 

Scheme 3. Final assembly of the immobiltation reagent incorporating dithiol tether groups and antibody immobilization functionality.

The reagent is fixed at the pretreated optical fibre surface by adsorption. At the surface, at first reactive tether groups form. They should bind as strongly as possible with the receptor molecules in order to avoid bleeding into the sample solution. Some examples are fisted in Table 8.2. 

Ito and Ogawa have also reported the synthesis of P-mannopyranoside linkages by use of the intramolecular aglycone delivery strategy with PMB as tethering group linked to a solid phase (Scheme 5) [14],... 

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