W-conformation

The trana compound (36) fragments because cyclisation is impossible and because it, unlike (35) can adopt the perfect W conformation (all anti-peri-planar bonds) ideal for fragmentatlon (23),... 

The rate constants, and thus the matrix elements of matrix W, conform to the requirement of detailed balance ... 

J. S., Milne, G. W. Conformational changes of small molecules binding to proteins. Eioorg. Med. Chem. 1995, 3, 411 28. 

Particularly intriguing molecules of the R3R2 A—A—Rj type are those containing adjacent nitrogen and oxygen atoms. Thus, NH2OH is predicted to exist in a crowded W conformation which places the N lone Pair anti to the OH bond. 

Once more, replacement of the hydroxyl hydrogen by a group or an atom more electronegative than hydrogen, replacement of the hydroxyl oxygen by its third period counterpart, or, a combination thereof, will enhance n—a interaction and should lead to a preference for the crowded W conformation over the Y con-. formation. This expectation is confirmed by ab initio calculations in which the preferred conformation for NH2OF is found to be the more crowded W con-former314. ... 

The dissection of the CH2XH system into component fragments, A and B, is depicted below for the W conformation. 

The appropriate interaction diagram is shown in Fig. 50. Arguing as before, we predict that the W conformation is preferred since there is an anti relationship between the carbon lone pair and the X-H bond in this conformation. 

Ab initio calculations of the "CH2XH carbanion show that the Y conformation is the most stable conformation for X=0363. However, in the case of X=S, the Y and W conformations are of comparable energy. Clearly, steric effects are important in determining the preferred conformations of these systems. 

Stereochemical Aspects.—The i.r. and Raman spectra of ethylphosphine and its deuteriated analogue (99) indicate that the fluid phases contain gauche- and trans-conformers, but the solid phase contains the /ra/w-conformer only. The methyl... 

Lecchi, S. Allen, K. E. Pardo, J. P Mason, A. B. Slayman, C. W. Conformational Changes of Yeast Plasma Membrane H+-ATPase during Activation by Glucose Role of Threonine-912 in the Carboxy-Terminal Tail. Biochemistry 2003, 44, 16624-16632. 

Hatano, T. and Hemingway, R.W., Conformational isomerism of phenolic procyanidins preferred conformations in organic solvents and water, J. Chem. Soc. Perkin Transactions, 2, 1035, 1997. 

Kumar, V., Brecht, V., and Frahm, A.W., Conformational analysis of the biflavanoid GB2 and a polyhydroxylated flavanone-chromone of Cratoxylum neriifolium, Planta Med., 70, 646, 2004. 

Mattice, W.L. Suter, U.W. Conformational Theory of Large Molecules. The Rotational Isomeric State Model in Macromolecular Systems Wiley New York, 1994. 

Further studies by Shiner and coworkers on the diastereomeric 4-(trimethylsilyl)-3-methyl-2-butyl brosylates 56 showed that solvolysis of one diastereomer gave mainly substitution products with a retained configuration68. A minor product is a cyclopropane resulting from nucleophilic attack at silicon in the ionic intermediate. From the stereochemistry of the cyclopropanes obtained it was deduced that the W conformation is favoured over the credo-sickle , as shown in Figures 3a and 3b. 

Shiner and coworkers also studied optically active acyclic secondary sulphonate esters and found rate accelerations of about 100 in solvolysis in TFA at 25 °C compared with the alkyl reference (Table 8)93-95. From the product distribution and the stereochemical outcome of the solvolysis Shiner and coworkers proposed the reaction course outlined in equation 37 for the brosylate 23994. The transition state for the ionization adopts the W conformation and, to a smaller extent, the ewrfo-Sickle conformation and results in the formation of /-silicon carbocation which is stabilized by a percaudal interaction. The incoming nucleophile attacks the bridged ion from the front side94. 

Similar fragmentation reactions are observed when the stereoelectronically favorable W conformation is present in 2,4-disubstituted adamantanes. Thus,... 

Perret, F., Bonnard, V., Danylyuk, 0., Suwinska, K., Coleman, A. W., Conformational extremes in the supramolecular assemblies of para-sulfonato-calix[8]arene. NewJ. Chem. 2006, 30, 987-990. 

Eliel, E. L. Nader, F. W. Conformational analysis. XX. The stereochemistry of reaction of Grignard reagents with ortho esters. Synthesis of 1,3-dioxanes with axial substituents at C-2.]. Am. Chem. Soc. 1970, 92, 584-590. 

This unusually high long-range coupling constant is attributed to the W conformation of the four a bonds between HA and HB. 

W coupling (saturated chain in rigid planar W conformation) ... 

Extended allylic systems can be formed by deprotonation of dienes such as 142,144 and 147 with s-BuLi. The dienyllithiums 143, 146 and 148 adopt an extended W conformation, and react to give 1,3-butadienes 146 and 149 with retention of double bond geometry.19 The equivalent species 150 formed by deprotonation with LiCKOR superbases (see section 2.6) adopt a U -shaped configuration. 

Various thermally less stable Zn(C5H7)2L2 complexes (L = (CH3)3NO, THF) were also quite fluxional to well below room temperature, but again incorporation of a chelating ligand such as TMEDA led to freezing out of the terminally a-bonded spectra. Even so, however, at ca. 90° some of the spectra demonstrated fluxionality. While Zn(C5H7)(Cl)(TMEDA) exists entirely in the W conformation, Zn(C5H7)2(TMEDA) and Zn(3-QH9)2(TMEDA) each contained substantial (20%) quantities of the S con-... 

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