Anions Anti conformation

The anion is assumed to exist preferentially in the anti conformation A, which offers a stabilizing 7rc -> minimizes steric interactions69,71 11. The nonplanar nature of the carhanionic center and coordination of the lithium to the oxygen atom could be demonstrated for a related compound by an X-ray analysis72. Usually, the electrophile attacks the carhanionic center with stereoinversion. 

Accordingly, dianthryl compounds with a close proximity, such as [6] and even [18], are capable of accepting four electrons although an appreciable electrostatic repulsion is built up (Becker et al., 1991 Huber and Mullen, 1980). When considering the question of how a bis-electrophore accommodates the extra charge it is important to note that, for example, the tetra-anion of di(9-anthryl)ethane [6] adopts an anti-conformation with respect to the central C—C bond, thus minimizing the electrostatic repulsion (Huber et al., 1983). 

The crystal structures of the salts of 10 with racemic amines showed that there is a favorable and common conformation for the anions of 10 [(10a-H)-, (10b-H), (10a-2H)2-, and (10b-2H)2- ] in the crystalline state (Figure 4.31) four carbon atoms (C1-C4) are almost on the same plane, and two carboxylate groups are in an anti conformation while two benzoyl or toluoyl groups are gauche to each other.92... 

The erythro isomer cannot eliminate entirely from an anti conformation because only the cis olefin would be produced. Of the two hydrogen atoms which could be abstracted by a base, the more acidic atom is removed and the elimination probably proceeds through an enolate anion (32). These anions however, have the same configuration as their precursors as isomerisation of the erythro to threo substrate before elimination is not observed, presumably due to steric interactions which cause rotation about the 2-3 carbon-carbon bond to be a slower process than proton capture from the solvent. To minimise steric interactions in the erythro dichloride, the bulky aryl residues will separate to a maximum by occupying the "anti positions (32). 

Fig. 5.5 The structure of S/ anti and N/anti conformers of anionic 2 -deoxyribo-nucleotides. TMP molecule is shown as representative example...

The results of calculations demonstrate that the set of stable conformers of methyl esters of DNTs corresponds well to the stmctures considered in the previous studies [19, 20, 52]. The revealed species include south/anti and north/anti conformers of all molecules (Fig. 5.10), south/syn and north/syn conformers of mGMP (Fig. 5.11), and north/syn conformers of mCMP and mAMP with orthogonal orientation of base with respect to ribose (Fig. 5.12). But in addition to our previorrs investigations, south/syn corrformer of mAMP was also found. Existence of this rare conformer is possible becairse presence of intramolecrrlar C-H...N hydrogen bond. This conformer corresponds to minimum on energy surface which is nearest to syn-conformer of mAMP in ZI-ZH-DNA form. Such a distribirtion of minima on the PES of the molecule of methyl nucleotide repeats and corrfirms the results of the conformational analysis performed for DNTs anions with P-OH fragment. 

Davis and co-workers designed isophthalamide receptors with appended hydroxyl (4c) or methoxyl (4d) groups [23]. In the solid state, the hydroxyl groups of 4c formed intramolecular OI 1—0=C H-bonds with the amide carbonyls to stabilize the syn—syn conformation, which complexed Cl (K = 5,230 M ) in its cleft. On the other hand, 4d did not bind halide anions, because the methoxy groups promoted the formation of the anti-anti conformation. 

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