Conformation flattened cone

An unusual scoop conformation of 24 has been characterised in the hydrate (C-methylcalix[4]resordnarene)(H20) [65]. The scoop conformation is a hybrid of the bowl and flattened cone conformations and there are two intramolecular O-H... 0 hydrogen bonds from adjacent resorcinol units. Adjacent 24 molecules form O-H... 0 hydrogen bonds and there are additional links via water molecules resulting a 2D sheet structure. The sheets pack in a bi-layer arrangement. 

The flexibility of the calix[4]arene basis is reflected in the molecular structure of 54a as determined by X-ray diffraction analysis (Fig. 22). The calix[4]arene adopts a flattened cone conformation with intramolecular 7i-stacking of two terpyridyl... 

Figure 11. X-ray crystal structure of 104 (a) shows a flattened cone conformation and (b) shows the dimeric form. (Reprinted with permission from 2002CEJ1134, Copyright 2002, John Wiley Sons Inc.)...

Different solid state structures of p-fert-butylcalix[7]arene C[7] have been determined by X-ray crystallography. The most common conformation found was the so-called double-cone-pinched [90] one (Fig. 7.25a) in which a tetrameric conelike (blu in Fig. 7.25a) and a trimeric %-cone-Uke (yellow in Fig. 7.25a) substructures were stabilized by the strong intramolecular H-bonds between OH groups. The structure reported by Perrin et al. [91] for p-ethylcalix[7]arene lb (Fig. 7.25b) is characterized by a more flattened %-cone-like trimeric subunit and by a tetrameric subunit with the two central phenolic rings inverted with respect to... 

Depending on the reaction conditions and the nature of the ester side chain in the cinnamates, the resorc[4]arenes can adopt different conformational states, namely, 1,2-altemate, 1,3-altemate, or flattened-cone). In particular, when ( )-2,4-dimethoxycinnamic acid methyl ester 1 was reacted with BF3 Et20 at room temperature, only the 1,2-altemate 2a and flattened-cone 2b stereoisomers were obtained in a 2 3 ratio and 75 % overall yield. In the 1,2-altemate conformation, the assignment of the 12- and 16-OMe signals, by INEPT experiments in conjunction with DIF NOE measurements, allowed to establish the correct stereochemistry at C(14). On the other hand, the presence in the H- and C-NMR spectra of only one signal for both external (H-5) and internal (H-28) aromatic protons and the related carbons, as well as a similar pattern for the methoxy group and the aliphatic... 

A different pattern of reaction products was obtained with the ethyl 3 and isopropyl 5 cinnamic esters. In both cases, comparable chemical yields were obtained, but in addition to the 1,2-altemate (4a, 6a, respectively) and the flattened-cone (4b, 6b, respectively), a third type of stereoisomers (namely, 4c and 6c) emerged from the reaction mixture. An accurate analysis of H and NMR spectra pointed out the existence of two symmetry planes. DIF NOE experiments revealed the proximity of the methine and the methylene groups with the aromatic protons. Therefore, on the basis of only NMR evidences, 4c and 6c were assigned the 1,3-altemate conformation with the pseudoequatorial side chains in an all -cis position. Later, thanks to the obtainment of suitable crystals for X-ray analysis, the stereoisomer 4c was definitively reassigned the chair conformation [6]. 

Notably, not only the 2,4-dimethoxycinnamates underwent the BF3-Et20 mediated tetramerization reaction, and this has proved the versatility of the reaction. In fact, also 2,4-dimethoxycinnamic acid amides such as 12 (Scheme 8.5), derived from the mixed anhydride of 2,4-dimethoxycinnamic acid with 1- or d-vaUne ethyl ester, upon treatment with BF3-Et20 yielded the amido-resorc[4]arenes 13a-C which proved to be chiral for the presence of four axial pendants containing the chiral valine residue [9]. A novel flattened-partial cone conformation was isolated for the first time from the reaction mixture, together with three other stereoisomers (namely, 1,2-altemate, flattened-cone, and chair). The flattened-cone stereoisomer, which was identified by molecular modeling as the most stable, proved to form as minor product in kinetic conditions, and to become the main component of the reaction mixture under more drastic conditions (namely, substrate/ BF3 Et20 = 1 400 M ratio). The chiral amido-resorc[4]arenes 13a-c were obtained in enantiomerically pure forms, as checked by enantioselective HPLC. 

Treatment of tetraalcohol 25b with glutaroyl, adipoyl and pimeloyl dichlorides in the presence of Et3N gave the corresponding double-spanned resorcarenes 26 (n=l), 27 (n = 2), and 28 ( = 3) [15], in which the insertion of the methylene bridges led to the formation of a cavity shaped architecture resembling a basket (Scheme 8.8) H- and C-NMR spectra. X-ray analysis, as well as molecular dynamic studies agreed that the basket derivative had a frozen flattened-cone conformation. 

The crown ether bridge in calix[4]arene 19 freezes the calixarene stmcture in a rigid flattened-cone conformation also pushing the two metal ions apart. The bimetallic complex 19—Zu2 cleaves HPNP eight times less effectively than the flexible 2—Zu2 [17], thus confirming that a certain degree of flexibility is beneficial and is an important requisite in a supramolecular catalyst. 

There is only eight crystal structures of calix[7]arenes deposited in CSD. The reason for this is much more difficult synthesis of seven-membered calixarenes and/or difficulties in obtaining monocrystals of this compounds and their complexes/co-crystals. Like for smaller calixarenes discussed above also here the self-inclusion of one of the substituent may occur like in para-t-hvXy -carboxymethoxycalix[7]arene/deuterochloroform clathrate (Fig. 38.25a) [52] or inclusion complexes are obtained. For the later, due to a great degree of flexibility, many conformations of calixarene host molecule is possible but the most observed is a double partial cone, e.g. 1 2 para-t-butyl-calix[7]arene/pyridine inclusion complex where one guest molecule is located in each partial cone cavity (Fig. 38.25b) [53]. In one case the flattened cone conformation was found where the (Fig. 38.25c). In this crystal structure one toluene and one disordered benzene/ toluene molecule are involved in inclusion complex formation with one para-t-butyl-calix[7]arene molecule [54]. 

For compoimds 16, this conformation is in slow exchange (AG = 70-88 kJ/mol at 328K in CDCI3) with the 1,2,3-alternate conformation, the latter being less stable (AG° = 2-7 kJ/mol). ° Therefore it is possible to conclude that three important factors stabilize the flattened cone structure, viz. (i) the self-inclusion of the methoxy groups in the apolar cavity driven by CH-ti interactions, (ii) the presence of three bulky groups in 2,4,6 positions at the lower rim, and (iii) the presence of the ter/-butyl groups at the upper... 

However, most of the tetra-O-alkylated calix[4]arene cone isomers adopt a flattened cone (pinched cone) conformation in the solid state, showing a > symmetry. This... 

The crystal structure of 11 was described in our preliminary communication [13]. The molecule adopts a conformation in which each of the two OP(OAr)3 subunits exists in a flattened cone conformation with one ring nearly coplanar to the reference plane and the two remaining rings twisted. The two P=0 bonds are oriented in a parallel fashion and exo to the macrocyclic plane. 

---