Chiral ..softballs

Differently from the previous chiral softball, in this case the dissymmetric inner surface of the cavity easily sensed chiral guests, providing different diastereoisomeric complexes in different amounts. Each diastereoisomer is characterized by four different NH resonances because of the complete loss of symmetry of the complex. The system described was extensively investigated pursuing the synthesis of a total of six different glycoluril monomers (21-26) with various spacers in terms of size and electronic properties on the two sides of the bicyclic six-membered unit, and the relative homodimeric capsules were studied with a total of 20 chiral enantiopure guests (Fig. 23 and Table 2) [53]. 

Fig. 26 A Chiral memory effect applied to the construction of a chiral softball homo-dimeric capsule made of achiral units. B Racemic softball binds a chiral guest leading preferentially to one diastereoisomer. Fast exchange, compared to capsule s racemization rate, with a better achiral guest allows preparation of enantioenriched capsule...

Chiral softballs have also been reported from symmetric (39a and 39 b) and asymmetric (39d) monomers, when optically active camphor derivatives were encapsulated... 

Rivera JM, Martin T, Rebek J Jr (2001) Chiral softballs synthesis and molecular recognition properties. J Am Chem Soc 123(22) 5213-5220... 

Fig. 21 Chiral homodimeric softball capsule from an achiral component bearing two different glycoluril residues. The R1 and R2 groups have been removed for clarity...

Table 2 Diastereoisomeric excesses for chiral racemic softballs as a function of different chiral enantiopure guests ...

Figure 8 Chiral memory effect displayed by softball capsule templated with a chiral guest that is subsequently replaced by an achiral one, leading to an enantiomerically enriched softball that slowly racanizes.

Guest-induced chirality within the cavity of hydrogen-bonded softballs 307, 308, 359, and 360 (a racemic mixtnre of the latter is formed as a resnlt of dimerization of nonsymmetric ligand syntone 148) after encapsnlation of chiral camphor derivatives (Scheme 3.37) has been examined [39] by H NMR method. 

Tokunaga Y, Rebek J Jr (1998) Chiral capsules. 1. Softballs with asymmetric surfaces bind camphor derivatives. J Am Chem Soc 120(l) 66-69... 

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