U-shaped conformer

Instead, evidence now favors the intermediacy of the highly organized mono-osmium complex 15. Various factors could contribute to its excellent enantioselectivity, the most important one being the formation of a binding site consisting of the two methoxy quinolines and the pyridazine linker. As confirmed by X-ray analysis and NMR studies [35,36], this pocket adopts a U-shaped conformation and is capable of perfectly binding aromatic substrates such as styrene through attractive interactions with the methoxyquinolines. In contrast, bulky... 

The simplest member of the series, the 2,4-pentadienyl metallo species, has been shown by NMR studies,132 133 to adopt either a W-shape or a U-shape conformation with the metal being lithium or potassium, respectively, when both are dissolved in tetrahydrofuran.134 136 The W-form is adopted by the 2,4-pentadienylpotassium when the organometallic species is generated in liquid ammonia137 or in paraffinic suspension.138 This has also been confirmed by the stereochemistry of the products formed after reaction with electrophiles.134,135,138,139... 

The long alkyl side chain of heme a is in an almost extended conformation whereas that of heme % is in a U-shaped conformation (Tsukihara... 

As may be seen from the X-ray structure (Figure 1) and the solution structure the molecule adopts a U-shaped conformation, where the internal (Z)-alkenes are functioning as conformational locks by minimizing 1,3-allylic strain between their respective substituents and 5y/i-pentane interactions along the backbone are avoided, while the y-lactone is held in a boat-like conformation (Figure 1). 

The CM calculations reported by Bamett-Nonis and co-workers (1998) discussed earlier, identified both extended conformers and folded or U-shaped conformers to be major conformational families of AA. The more compact (U-shaped) structure was found to predominate in water, whereas the extended shape was found to predominate in CHCI3 (and in vacuo). These results were taken to be consistent with the idea that in HjO, AA would minimize the exposure of its hydrophobic... 

Diene A is similar m structure to 1,3-cydohexadiene. which reacts well in Diels-Alder cycloadditions O ablc 14- IX In diene B, however, the ends of the diene are locked in a zigzag conformation (called "s-traas") that puts the end carbons too far apart to bond to the two alkene carbons of a dienophile. Figure 14-9 illustrates how the Diels-Alder reaction involves a diene in a U-shaped conformation (called s-cis ), which puts its end carbons close together. Dienes (like B) that cannot achieve this conformation will not participate with dienophiles in Diels-Adler reactions. 

One of the most interesting features of this type of ligands is that they present a binding cleft. In the conformational analysis of the 0s04(DHQD)2PHAL system, Norrby et al. observed that despite the large number of conformations a priori possible for this system, only a few variations of this U-shape conformation were found [39]. This U-shape conformation is formed by one quinoline in one side, the heterocyclic spacer at the bottom, and the OSO4 unit and sometimes the other quinoline at the other side (see Fig. 3). 

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