1.3- altemate conformer

Calix[4]arene tetraethers can be converted into mono Cr(CO)3 complexes 72, which makes one of the phenolic units different from the others (AAAB).135 Thus, a single chiral compound is obtained from the 1,2-altemate conformer, while for the partial cone conformer only one of the three possible products is asymmetric (compare Figure 13). Mono derivatives fixed in the cone and 1,3-altemate conformation contain a symmetry plane. 

It should be mentioned that a tetraether in the 1,3-altemate conformation (54) or 1,2-altemate conformation (Cj) or an anti-1,3-diether (C,) are not chiral. 

The related monodentate ligands 5.33 and 5.34 form ML4 type complexes with the relatively inert platinum(II). The complex [Pt(5.33)4] adopts a 1,2-alternate conformation (by analogy with the calixarenes. Section 3.14) giving two anion-binding pockets that can bind planar anions such as nitrate and acetate in a 1 2 ratio. Acetate displays a positive allosteric effect with A 12 being more than twice Kii. Tetrahedral anions are bound in a 1 1 ratio, however, and it is likely the complex can adopt a variety of conformations in solution. Related to [Pt(5.33)4] + is [Pt(5.34)4] +. Like the complex with 5.33, X-ray diffraction results show that the chloride complex adopts a 1,2-altemate conformation with a... 

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... 

The corresponding X-ray crystal structures of these triptycene-derived calixarenes and analogues were also obtained. As shown in Fig. 18.2a, b, macrocycle la adopted a typical cone conformation with a highly symmetric structure, while trans-isomer 2a adopted a chair conformation with two opposite phenol rings in the same plane [11a, b]. Similar stmctural features were also found in the crystals of their derivatives 5 and 6 [11c]. In the case of 7a, b and e [1 Id, e] (Fig. 18.2c) and 8a [12] (Fig. 18.2d), the crystal structures showed that they adopted cone conformations with fixed cavities while 7g [lie] was in a 1,2-altemate conformation. 

In 1,2-altemate conformation the in calix[4]arene molecules with unsubstituted OH group very often two sallow cavities are formed symmetrically on both sides of the... 

Fig. 38.14 Molecular complexes of calix[4]arenes in 1,2-altemate conformation (a) t-butyl-bis (mesityl)calix[4]arene/chloroform 1 2 complex [30] (b) para-sulfonatocalix[4]arene/...

Substitution of hydroxyl hydrogen with methyl group (metoxy derivative) in calix[4]arenes in 1,2-altemate conformation results in creation slightly deeper and better defined cavity similar to that characteristic for calix[4]arenes in cone conformation. Several of such molecular complexes was reported by C. Fischer et al. [32]. Two of them is shown in Fig. 38.14c, d. 

The 1,2-altemate conformation cannot yet be obtained from direct alkylation reactions in attractive yields. We were able to show that ethylation of the own-1,3-diethyl ether of 9b with KOtBu in tetrahydrofuran afforded the 1,2-altemate conformation of the tetraethyl ether in 64 % yield [16]. Apparently, the 1,3-diethyl ether is not formed in large quantities in the reaction starting from p-/err-butylcalix[4]arene 9b under the same conditions. 

Calix[4]arene ethers, like the tetrapropylethers, can be converted into mono-Cr(CO)3 complexes [37]. Starting with the cone or the 1,3-altemate isomer, these derivatives still have a symmetry plane, while the corresponding derivative of the isomer in the 1,2-altemate conformation becomes asymmetric. Starting with a partial cone conformer, an asymmetric molecule can be obtained only by introducing the Cr(CO)3 at one of the aromatic rings next to the inverted phenol ring. 

Due to the equivalence of the phenolic units, it is now possible to construct in an unambiguous way all kinds of 0-alkylation products (from mono- to tetraethers), as with achiral calix[4]arenes. Of course, the symmetry is reduced in partially 0-alkylated derivatives. Mono- and triether derivatives are asymmetric (Ci) while 1,3-diether derivatives with the usual jyn-arrangement of the O-alkyl groups have C2 symmetry. An anri-1,3-diether (the formation of which is not observed under usual reaction conditions) would have an inversion center in its 1,2-altemate conformation, hence being achiral. 

Although theoretically four different disubstituted regioisomers 3a-d can exist (Scheme 1), direct substitution on the parent calix[4]arene 1 in the presence of the above bases regioselectively affords 3a or 3b, with no trace of the other two possible anti regioisomers. However, the isolation of tetra-O-alkylated 1,2-altemate conformer 5c in the Cs2C03-catalyzed exhaustive alkylation of 1 suggests that the obvious anti-(l,2)- or anft -(l,3)-di-0-a]kylated precursors are transient species under the experimental conditions employed [35]. 

Loeb and co-workers reported the preparation of complex [Pt°(34)4] from 34. Due to the free rotation around the Pt -N bond, the complex could assume four basic up and down conformations of the butyl urea substituents. The crystal structure of [Pt (34)4] S04 showed that the receptor was arranged in a cone conformation, with all eight N-H groups oriented toward the cavity including the sulfate anion (Fig. 5.24a), with > 10 M . In chloride complex [Pt (34)4] (Cl )2, the receptor adopted a 1,2-altemate conformation (Fig. 5.24b), while the two chloride ions were located above and below the Pt° center, with = 1.2 x 10 M and A a2 = 2.2 X 10 M respectively [72]. 

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