Asymmetric induction with chiral templates

In summary, chiral Br0nsted acids have been used to protonate prochiral intermediates that were photochemically produced. Optimization of the chiral amino alcohols and the conditions led to excellent enantioselectivities of up to 91% ee employing only 0.1 equivalent of chiral inductor with respect to the substrate. These results demonstrate that the desirable goal of chiral amplification can be reached with chiral templates. The asymmetric induction varies strongly for different substrates, and therefore the general applicability as a synthetic tool seems to be limited. 

In summary, chiral solvents have only induced limited enantioselectivity into different types of photochemical reactions as pinacolization, cyclization, and isomerization reactions. These studies are nevertheless very important, because they are among the early examples of chiral induction by an asymmetric environ ment. Based on our classification of chiral solvents as chiral inductors that only act as passive reaction matrices, effective asymmetric induction by this means seems to be intrinsically difficult. From the observed enantioselectivities it can be postulated that defined interactions with the prochiral substrate during the conversion to the product are a prerequisite for effective template induced enantioselectivity. 

A suggestion has been made that asymmetric induction is possible in cycloaddition reactions leading to azetidinones. The reaction of dimethylketene methyl-trimethylsilylacetal with the Schifl s bases of chiral a-aminoesters in the presence of titanium tetrachloride gave rise to the corresponding /3-lactams with extremely high stereoselectivity. The authors propose the formation of a template (183) between titanium and the Schiff s base leading to stereocontrol (Scheme 24). 

A variety of bimolecular photoreactions that are suppressed in the solution phase proceed smoothly and selectively upon complexation with supramolecular hosts. Intermolecular photochemical reactions promoted by an achiral host or template, such as crown ether [74], cucurbituril [75,76] as well as self-assembled coordination cages [77], can occur with remarkable efficiency and chemoselectivity, for which the close proximity and the highly regulated orientation and special arrangement between substrates in the supramolecular aggregate are jointly responsible. The main advantage of CDs over the above-mentioned hosts is the inherent chirality, which makes them good cradles for asymmetric induction in bimolecular photoreactions. 

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