Synthesis chiral template strategy

In another study,a chiral templating procedure was employed to direct the helicity of an oligo-bipyridine, double-stranded copper(I) helicate. The strategy employed is illustrated in Figure 6.29 in which spiro-bisindanol 71 and dimethyl-biphenic acid 72 were employed as the chiral template starters for the twisting process inherent in helicate formation. The above approach serves as a model for a general route towards the synthesis of enantio-pure helicates and is also of considerable intrinsic interest since it illustrates the manner by which stereochemical information may be transmitted over nanometer distances. 

A chiral template may be temporarily attached to an achiral molecule and a new stereogenic center made under that influence, and then the original template is removed [26]. Such a strategy was employed in the synthesis of (-i-)-lactacystin, as shown in Equation 3.5. Both enantiomers of the starting tricycloiminolactone are readily available and useful in a variety of synthetic reactions [27]. 

The stereochemical outcome of the addition of carbanions to ketones yielding tertiary alcohols (or secondary alcohols in the case of aldehydes) is variable and depends on the substrate, the counterion and the solvent. Numerous applications of this strategy to natural product synthesis from carbohydrates can be found in the literature and this approach was fruitful in pioneering syntheses of polyketide-type products. Here again, the template effect of the sugar plays a tremendous role in the stereochemical outcome of the reaction. Chelation controlled nucleophilic addition can also be used to form chiral centers in a highly predictable way. 

---