Endo-templated synthesis

In many cases the casts obtained from templates can themselves by used as templates for a subsequent template synthesis consecutive templating). This often involves a sequence of endo/exo-templating. Further, one can make use of hierarchical templating on different length scales. The scheme in Fig. 19 shows common pathways employing different templating steps to prepare nanoparticles, nanofibers, hollow particles, hollow fibers, and nano- and mesoporous materials. 

The interior cavity of coordination cages has also been exploited as endo-template for the synthesis of highly monodispersed nanoparticles. 

Yamauchi Y, Fujita M (2010) Self-assembled cage as an endo-template for cyclophane synthesis. Chem Commun 46 5897-5899... 

The diastereospecific synthesis of novel [3.6.6.4.7]-fused pentacyclic /3-lactams via a novel 6-exo-trig,7-endo- %-tandem radical cyclization has been reported. The use of this method for the construction of a polycyclic system fused to an azetidin-2-one template has been very little exploited. The mixture of /3-lactams 509 and 510 (R = -f 4 I—Cl C. -Cl I—Cl ICH3) in their racemic forms on treatment with tributyltin hydride gave diastereomeric mixtures of tetracyclic 511 and 512 in the same ratio as in the starting materials (Equation 84) <2003TL1827>. 

More modern studies have made use of copper(I) triflate (CuOTf) as the reagent. This compound is well known to form complexes with dienes and it provides a template on which cycloadditions can be effected. Several examples of this type of cyclization have been reported and cycloadditions based on this approach provide a useful route to cyclobutane derivatives. Thus, a new stereochemical synthesis of grandisol has been developed using the copper(I)-catalysed cycloaddition of the dienol 95 to afford the isomeric bicyclo-heptenols 96 exo endo ratio in this cyclization is solvent-dependent. The racemic... 

For the preparation of nano- and microstructures there are numerous examples of endo- and exo-templates that have been used for the synthesis of particle- and pore structures from the nanometer to the micrometer scale. Templates can be inorganic, organic, or biological materials as are the materials or casts produced therefrom. The hardening process can be physical (crystallization from the melt, glass formation) or chemical (cross-linking, polycondensation). Templates usually have to be extracted in order to obtain the desired material. 

Tricyclic templates such as 246 have been used to obtain stereocontrol in the synthesis of important cyclopentanoic natural products [85]. Normally, the endo-face of a cage-like structure is strongly shielded, and all kinds of additions occur from the exo-side. Thus, cuprate addition to enone 246 selectively gives the exoadduct 247. If, however, an additional substituent is introduced onto the exo-face as in the ether 246b or the sulfide 246c, considerable amounts of the endo-adduct 248 are generated. For X= SPH, the endo-selectivity is completely reversed, yielding exclusively compound 248. 

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