Diastereomeric crystals hydrogen bond networks

Thus, in the cases of these diastereomeric salts which could not be efficiently separated by crystallization, the columnar hydrogen-bond network is not reinforced by additional hydrogen bonds formed by the hydroxy group as that in the less-soluble salts in success. This makes the difference in stability between the pair of diastereomeric salts small, resulting in low efficiency of resolution for this combination. 

Thus the crystallographic analyses of these less soluble diastereomeric salts revealed that the dominant interactions in the crystals are (1) the hydrogenbonding interaction, which constructs a consistent columnar supramolecular hydrogen-bond network, and (2) the CH tt interaction, which packs the columns tightly with each other. Thus the less soluble diastereomeric salts are efficiently stabilized by these two kinds of interactions. 

Of these crystal structures, 1-phenylethylammonium 2-phenylpropionate (NMACEP, PMACEP, Figure 4.38)115 and 2-phenylbutyrate (PEAPEA, PBUPEA)115,116 are the earliest examples reported by Brianso. The less soluble salt is the combination of the 5-acid with the R-amine (PMACEP and PEAPEA) in each case. The crystal structures of both pairs of the diastereomeric salts (less and more soluble salts) have been determined to show that these two systems have quite similar crystalline characteristics. In both less and more soluble salts a columnar hydrogen-bond network, formed by the carboxylate and ammonium groups, is commonly built around a two fold screw axis two molecules of the carboxylate part and two molecules of the ammonium part form a unit, which is piled up along a 2i-axis to form a supramolecular hydrogen-bond column. 

The crystal structures of the diastereomeric salts of 1-phenylethylamine with chiral carboxylic acids are subsequently reported by other groups (Table 4.7). The comparison of the crystal structure of JAGHEE with that of JAGHII also indicates that columnar hydrogen-bond networks are formed in these crystals, respectively, and that the difference between them can be found in the relative orientation of the substituent on the chiral carbon of the carboxylate moiety in the supramolecular column. 

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