Conformation partial flattened cone

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. 

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]. 

The X-ray structure of calix[4]arene tetraquinone " shows it to be in a flattened partial cone conformer. Similarly, the dialkyl ether diquinones 96a, 96b, and 96c all assume the partial cone conformation but differ with respect to the orientation of the four residues in the cyclic array in 96a and 96b the alkoxyaryl... 

In the H-NMR spectrum of 17, a bridged CH2 shows two AB quartets and one OAc appears upheld. Similar prohles of H-NMR spectra reveal inward flattened partial cone conformations in solution for all 176 having OAc or OMe at C-2 of phenylene attached at N-l, iV-1 of uracil. But 176a, 176c and 176j which have H or OH at this site and show broad signals for -CH2, have flexible structures in solution. However, their variable temperature H-NMR studies show the existence of two or more conformers which equilibrate at room temperature. 

The single crystal X-ray structure of thecalix(4 pyiidine 20 was also solved. " It revealed that the molecule adopts a flattened partial cone conformation in the solid state (the dihedral angles between pyridine rings and plane thntugh the calixpyridine nrc-.s-o-carbon atoms are 10.4(2), 82.5 (2), 80.8(2) and -79.0(2). respectively) (Figure 27). In this... 

The crystal structure of compound 25d revealed that the calix[3Jindole adopts a flattened partial cone conformation in the solid state, that is, one indole NH points up, one down, and one is almost flat in the plane of the macrocycle (Figure 28). ... 

The oxacalixarene literature often assigns the term flattened partial cone to what the calixarene literature describes as a chair conformation (see referenee 51). In an attempt to minimize confusion, this chapter observes oxacalixarene convention in using Hhe flattened partial cone nomenclature. 

A few years ago the synthesis of a new type of molecules, the spherands, has been reported that form complexes with Li" and Na that are not only thermodynamically but also kinetically very stable [27]. Combination of the 1,3-dimethyl ether of p-tert-butylcalix[4]arene with half a spherand molecule afforded the calixspherand 28 as its Na" complex in 23 % yield [28]. The complexes of 28 with Na" and K are kinetically very stable in CDCI3 saturated with D2O, and have half-lives of 3.7 and 2.2 years. The conformation of the calix[4]arene moiety in these complexes is a "flattened" partial cone as in the complexes of the calixcrown ether 26a. Only recently we were able to prove that in the free ligand the calix[4]arene moiety adopts a cone conformation in complete analogy with compound 26a [29]. 

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