Guanidinium chiral

Chiral bicyclic guanidinium receptors (e.g. 12-14, S,S-configuration shown) have been developed in our group from aminoacid precursors [16]. Improved synthetic procedures for such compounds were later reported by Schmidtchen [17] (who in 1980 presented a first type of achiral bicycloguanidinium hosts [18] that form complexes with several oxoanions [19]). Most derivatives are highly... 

The general affinity of guanidinium cations for oxoanions and the spatial direction of the two NH-protons makes the bicycloguanidinium core presented in the preceding chapters also a useful building block for the design of nucleotide receptors [96]. Besides carboxylates (see Sect. 2), bis(naphthoyl) host 12a binds several phophoesters in chloroform, for example, l,r-binaphthyl-2,2 -diyl phosphate the complex of the (S)-enantiomer is shown in formula 64. Still, despite the chiral nature of 12a, no enatioselectivity was observed [97]. 

Scheme 4.2 Synthesis of chiral guanidinium hosts starting from chiral amino acids either via cycli-sation of an open chain triamine or unsymmetrically substituted thiourea.29...

Metzger, A., Peschke, W., Schmidtchen, F. P, A Convenient access to chiral monofunctionalized bicyclic guanidinium receptor groups. Synthesis-Stuttgart 1995, 566-570. 

Chiral guanidinium-based ligands have also been used for recognition of diastereomeric salts of saccharides [45]. Some promising ligands with guani-dinium structure have not been studied yet [46], and some of them have been used as catalysts for the nitroaldol reaction [47] and Michael addition to a,P-unsaturated ketones [48]. 

In 1992, de Mendoza and coworkers reported the synthesis of the bifunction-alized chiral bicyclic guanidinium 34 for the purpose of enantioselectively binding zwitterionic aromatic amino acids (i.e., tryptophan and phenylalanine) [55]. By including binding elements for the carboxylate and ammonium of the amino acids which were noncomplementary, they hoped to avoid the potential... 

Calixarene-based host 37 was utilized in the chiral recognition of various zwitterionic amino acids [59]. In competitive extraction experiments, a 9 1 enantioselectivity was observed for L-phenylalanine over the D-isomer with the bis(S,S-guanidinium) 37. 

NMR. No similar catalysis was observed for a,p-unsaturated esters due to the s-cis conformation of the ester. In a later study, analogs of 47 with aromatic substituents on the bicyclic guanidinium were synthesized. These compounds were shown to increase catalysis, but no chiral recognition was observed [80]. 

Fig. 18 Supramolecular chiral aggregates based on bis-guanidinium functional groups A chiral tetra-guanidinium units self-assemble around sulfate anions into a left-handed double helical structure B chiral bis-guanidinium unit associates with a complementary bis-anionic counterpart leading to a left-handed double helix...

Guanidines have been implemented early as recognition elements, guided by the apparent function of arginine in protein structures. The C2-symmetric, chiral anion receptor 52 was introduced by Lehn, Schmidtchen and de Mendoza consecutively and studied in various modifications (Scheme 13) [23c]. For example, an elaborate system based on 52 provided reasonable enantioselective recognition of amino acids [23c, 28]. Furthermore, bis(guanidinium) compounds catalyze RNA hydrolysis in the presence of external base via phosphodiester complexation [29]. The,se functional elements were joined in receptor 53 to yield a functional transesterification catalyst [30]. 

In a follow-up paper, Jacobsen showed that the chiral guanidinium 80 provides catalytic power and enantioselectivity in the Claisen rearrangement systems such as Reaction 6.36. Here the reaction proceeded in 81 percent yield with an ee of 84 percent. The reaction is also diasteroselective for Reaction 6.37, the diastere-oselectivity increases from 11 1 without catalyst to 16 1 with 80. 

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