Drastic Increase in the Flexibility of Open Host Frameworks of a Steroidal Host Compound upon Shortening Its Spacer

Abstract

Nordeoxycholic acid (NDCA), which has a shorter side chain than the steroidal host compound, deoxycholic acid (DCA), forms inclusion crystals with various organic substances at 1:1 or 2:1 host−guest ratios. X‐ray crystallographic studies reveal that four types of host frameworks (bilayer, monolayer, tape, and hexagonal) are generated by NDCA, in stark contrast to the robust and dominant bilayer frameworks formed from DCA, although they have the same functional groups capable of hydrogen bonding. The first three host frameworks are further divided into three, two, and four sub‐types by arrangements of the hydrogen‐bonded host columns, tapes, or layers, respectively. A total of nine types of open host frameworks are observed depending on the nature of the guest compounds. The sizes of the guest compounds induce isomerization of these host frameworks having molecular cavities, which is rationalized by the range of values of PCcavity, which are ratios of the volumes between the guest compound and the host cavities. Such a drastic increase of flexibility of the open host framework by the slight chemical modification from DCA to NDCA deserves attention with respect to crystal engineering. Unique 21 helical assemblies of DCA are linked by the carboxyl group at the side chain to form a robust sheet‐like structural motif. In NDCA, on the other hand, the linkage between the helices becomes impossible because of the shortening of the side chain’s length, which results in destruction of the sheet‐like motif to yield renewed molecular voids among the unique 21 helical assemblies. These remarkable differences in the flexibility of the open host frameworks provide us with a possible strategy for designing of new host compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Supramolecular Chirality in Crystalline Assemblies of Bile Acids and Their Derivatives; Three-Axial, Tilt, Helical, and Bundle Chirality

“…Shortening by one methylene unit brings about a change of the orientation of the carboxyl group, leading to a great change of the hydrogen bond networks. Norbile acids, which have shorter side-chain length by one methylene unit than original bile acids, form inclusion crystals with various organic substances [52][53][54]. Among them, IIe exhibits a notable inclusion ability [54].…”

“…Norbile acids, which have shorter side-chain length by one methylene unit than original bile acids, form inclusion crystals with various organic substances [52][53][54]. Among them, IIe exhibits a notable inclusion ability [54]. On the other hand, the elongation of the side-chain may give rise to expansion of the host cavity.…”

Guest-Induced Supramolecular Isomerism and Chirality of Brucine Inclusion Crystals with Aliphatic Alcohols: A Hierarchical Interpretation

“…30,31 However, the inclusion crystals with 3,7,8,12,13,14,15,24,29, and 35 have a 32 and pantolactone. 33 All the resultant crystals could be categorized into three structural types of supramolecular isomers composed of common 2 1 helical tape-like assemblies stabilized by weak multiple hydrogen bonds, such as CH/O and CH/ interactions: 29 C10-HÁÁÁO3, C11-HÁÁÁO4 (O3 and O4: methoxy), C23-HÁÁÁO2 (O2: ether), and C20-HÁÁÁ (arene) hydrogen bonds as shown in Fig.…”

“…[10][11][12][13][14][15][16] On the basis of a series of the studies, we have proposed that the supramolecular isomerism in their inclusion crystals can be interpreted by the hierarchical structural analysis through supramolecular chirality. [12][13][14][15][16][17][18] The structural analysis is performed in the following four phases (Scheme 1): (i) fixing a conformation of a host molecule with three-directional chirality, 16 i.e., head and tail (or leg), right and left, and belly and back, (ii) forming a bimolecular aggregate and stacking of the bimolecular aggregate to form a motif, such as columnar assembly with the three-directional chirality, (iii) stacking the columns in a parallel or anti-parallel fashion to produce a bundle of the columns, (iv) accommodating guest molecules in the cavities of the bundles to yield host-guest complexes characterized by molecular recognition. It is noteworthy that the chirality of a single molecule dominates the supramolecular chirality of the columns, bundles, and complexes in the corresponding steps.…”

“…[12][13][14][15][16] However, it is not clear whether the analysis is a useful approach for the structural elucidation of the supramolecular isomers constructed with intermolecular interactions weaker than hydrogen bonds. Our aim in this study has been to extend the applicability of the proposed structural analysis to a chiral compound without hydrogen-bonding donor groups.…”