Structures pseudocyclic

Fig. 3. Pseudocyclic structure of 12-Li+ salt on the basis of the CPK model. (Cited from Ref.I7>)...

The non-cyclic ethers E-2 (Figure 10.26), with two pyrenes linked at both ends of the chain, show strong intramolecular excimer formation. Addition of alkaline earth metal ions leads to an increase in monomer emission at the expense of the excimer band. The helical structure of the 1 1 complexes is supported by NMR spectra. Thanks to the pseudocyclic structure, the stability constants of the complexes with Ca2+, Sr2+ and Ba2+ in acetonitrile are quite high (106-107 for n — 5), but the selectivity is poor as a consequence of the flexibility of the oxyethylene chain. 

Similarly to A-(2-iodylphenyl)acylamides, the tosyl derivatives of 2-iodylaniline 503 and 2-iodylphenol 505 were prepared by the dimethyldioxirane oxidation of the corresponding 2-iodophenyltosylamides 502 or 2-iodophenyl tosylate (504) (Scheme 2.143) and isolated as stable, microcrystalline products [665], A singlecrystal X-ray diffraction analysis of tosylamide 503 (R = Me) revealed a pseudocyclic structure formed by intramolecular I—O interactions between the hypervalent iodine center and the sulfonyl oxygens in the... 

Iminoiodane 209 has an excellent solubility in organic solvents (up to 0.14 M in chloroform at room temperature, at least a 50-fold increase over PhINTs). Compound 209 was analyzed by NMR in solution and by X-ray diffraction in the solid state. A single-crystal X-ray analysis of compound 209 demonstrated a pseudocyclic structure with an intramolecular distance of2.677(3)A between one of the sulfone oxygen atoms and the hypervalent iodine center (1999JA7164). Molecules of 209 are loosely associated into centrosymmetric dimers by long-range intermolecular I N and I 0 bonds (longer than 3.0 A), quite unlike the infinite polymeric chains adopted in the soHd state for PhINTs. 

Detailed NMR conformational analysis of y -peptides 139-141 (Fig. 2.35) in pyri-dine-d5 revealed that y-peptides as short as four residues adopt a 2.6-hehcal fold stabilized by H-bonds between C=0 and NH +3 which close 14-membered pseudocycles [200, 201]. The 2.614-helical structure of a low energy conformer of y-hex-apeptide 141 as determined from NMR measurements in pyridine-d5 [200], is shown in Fig. 2.36A and B). Determination of the structure of y" -peptides in CD3OH was hampered by the much lower dispersion of the diasterotopic H-C(a) protons compared to their dispersion in pyridine-d5. However, the characteristic and properly resolved i/ir-2 NOE crosspeacks between H-C(y) and NH +2 in the NH/H-C(y) region of the ROESY spectrum were an indication that the 2.6-helical structure is at least partially populated in CD3OH. 

Adapted from [200]). (B) Top view of 141 (derived from NMR restraints) [200]. (C) X-ray crystal structure of y -peptide 146 built with (/ ,/ ,R)-amino acids 138a and 138c [206 207]. It is characterized by two H-bonded 14-membered pseudocycles. H-bond N--0 dis-... 

Optimal pre-organization of the y-peptide backbone towards the formation of open-chain turn-like motifs is promoted by unlike-y " -amino acid residues. This design principle can be rationalized by examination of the two conformers free of syn-pentane interaction (f and II", Fig. 2.34). Tetrapeptide 150 built from homo-chiral unlike-y -amino acid building blocks 128e has been shown by NMR experiments in pyridine to adopt a reverse turn-like structure stabilized by a 14-mem-bered H-bond pseudocycle [202] (Fig. 2.37 A). 

It can be seen that the pseudocyclic intermediate (84a) strongly resembles the stable alkylperoxy-mercury compound (84b) prepared from the reaction of TBHP with an alkene in the presence of mercury(II) carboxylate.238 The X-ray structure of the similar BrHg CH(Ph)CH(Ph)(OOBu1) compound has clearly shown the pseudocyclic nature of this adduct by the interaction existing between mercury and the OBu1 group.259 The transmetalation of mercury by palladium in (84b) produces acetophenone in 95% yield, presumably via the formation of the pseudocyclic intermediate (85 equation 85).42... 

Strukul and co workers recently prepared a series of cis and trans isomers of the platinum- (-butyl peroxide complexes Pt(R)(OOBu )L2 (R = CF3, Ph, etc. L = tertiary phosphine). The X-ray crystal structure of the (ranx-[Pt(Ph)(OOBu )(PPh3)2] (88) revealed a square-planar arrangement with end-bonded OOBu group. Interestingly, only the trans isomers were found to be capable of oxidizing terminal alkenes to methyl ketones, whereas the cis isomers were inactive. The suggested mechanism involves the coordination of the alkene leading to a five-coordinate intermediate, followed by a pseudocyclic peroxymetalation, as in Scheme 6 (equation 92). ... 

An alternative mechanism for oxygen transfer was proposed by Mimoun [21-25]. In this mechanism (eq. (11)) initial coordination of the olefin to the metal is followed by its rate-limiting insertion into the metal-oxygen bond giving a pseudocyclic dioxometallocyclopentane (Structure 3). The latter decomposes to the epoxide and the metal alkoxide. 

Owing to the nature of hypervalent bonding and the T-shaped geometry of the iodine(ni) center, the formation of six-membered iodine heterocycles is highly unfavorable. Several such compounds have been reported in the literature [328,342-346] however. X-ray structural data on six-membered iodine(III) heterocycles is not available. Moreover, based on the available X-ray single-crystal data for several pseudocyclic six-membered iodine(V) derivatives (Section 2.2.2), it can be expected that these compounds may exist as their noncyclic tautomers [345]. 

The six-membered cyclic IBX analogues 528 have been synthesized by oxidation of the corresponding 2-iodophenylacetic acids 527. Interestingly, an X-ray structural study demonstrated that products 528 exist in the solid state as pseudocyclic acids 529 (Scheme 2.148) [345]. 

The chelation leads to a pseudocyclization whereby a coordination network is formed. The structure shown in Equation (28-49) is therefore only one of many possibilities. According to the nature of the metal ion and the molar ratio of metal/PTO, the color varies between yellow (Zr /PTO = 0.35) through orange (Zn /PTO=2) and olive green (Cu /PTO = 0.66) to brown (Ca /PTO = 1) and black (Fe /PTO = 1). White and blue shades are not obtained. The chelated polymers are very flame resistant, especially when chelated with zinc, tin, or iron ions. Mercury ions produce radiation-proof but not fire-proof polymers. 

CONCLUSIONS The sequence homology betweeen pig adenodoxin and spinach ferredoxin is very low. The cystein residues (involved in the iron sulfur binding) are not even conserved in the primary structure. Despite these marked differences, the proteins can be changed for one another in several oxidoreckiction reactions including pseudocyclic... 

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