Stereoselective using template reaction

In the cases of dimethylsilyl IMDA precursors, the exolendo selectivity was poor. However, this ratio could be readily and quite dramatically influenced by varying the alkyl substituents on the silicon template [12]. Thus with dienol 24, tether formation with dimethylvinylsilyl chloride and subsequent IMDA reaction afforded a 4 1 mixture of exolendo products (Scheme 10-7). The ratio could be further improved to 10 1 by using a diphenylsilyl tether, and when bulky Bu groups were used, a single stereoisomer, resulting from exo addition, was observed. This example once more illustrates the potential for tuning the stereoselectivity of the reaction by varying the steric interactions with the tether. 

The results are consistent with a common deprotonated intermediate for the exchange and inversion processes. A knowledge of the relative rates of proton exchange in complexes of this type can be used to advantage in the synthesis of new complexes by intramolecular template reactions, sometimes with stereoselectivity. 

In conclusion, the use of glycosylamincs as chiral templates in the Ugi reaction provides an efficient and highly stereoselective access to both l- and D-amino acids. 

Stereoselective inverse-demand hetero (4 + 2) cycloadditions. A Chiral Template for C-Aryl Glycoside Synthesis. Chiral allenamides2 4 had been used in highly stereoselective inverse-demand hetero (4 + 2) cycloaddition reactions with heterodienes.5 These reactions lead to stereoselective synthesis of highly functionalized pyranyl heterocycles. Further elaboration of these cycloadducts provides a unique entry to C-aryl-glycosides and pyranyl structures that are common in other natural products (Scheme 1). 

The oxazinones 74 and 79, already described as chiral glycine templates in Section 11.11.6.3, have been prepared by the PET cyclisation of 252 by irradiation in the presence of 1,4-dicyanonaphthalene as the electron acceptor and methyl viologen as electron-transfer mediator. When the reaction was carried out under strictly anhydrous conditions, compound 79 was isolated, whereas when the reaction was carried out in wet MeCN, compound 74 was the exclusive product (Scheme 33). In any case, the products were obtained with high stereoselectivity, which is the condition required to use them as chiral auxiliaries <2000EJ0657>. 

The use of chiral azomethine imines in asymmetric 1,3-dipolar cycloadditions with alkenes is limited. In the first example of this reaction, chiral azomethine imines were applied for the stereoselective synthesis of C-nucleosides (100-102). Recent work by Hus son and co-workers (103) showed the application of the chiral template 66 for the formation of a new enantiopure azomethine imine (Scheme 12.23). This template is very similar to the azomethine ylide precursor 52 described in Scheme 12.19. In the presence of benzaldehyde at elevated temperature, the azomethine imine 67 is formed. 1,3-Dipole 67 was subjected to reactions with a series of electron-deficient alkenes and alkynes and the reactions proceeded in several cases with very high selectivities. Most interestingly, it was also demonstrated that the azomethine imine underwent reaction with the electronically neutral 1-octene as shown in Scheme 12.23. Although a long reaction time was required, compound 68 was obtained as the only detectable regio- and diastereomer in 50% yield. This pioneering work demonstrates that there are several opportunities for the development of new highly selective reactions of azomethine imines (103). 

In the very early years of imprinting, carboxylic acid ester moieties were used as binding groups. The aim was the preparation of microreactors for regio- and stereoselective reactions. For this, the cavity was first imprinted with a possible product of the reaction and a precursor was then embedded into the cavity. The idea was to favour the formation of the product used as template by running the reaction within the imprinted cavity. The first experiments were carried out by the research groups of Shea [90,91] and Neckers [92], who performed cycloadditions that led to cyclo-propanedicarboxylic and cyclobutanedicarboxylic acids, respectively. The latter... 

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