S-E conformation

Photolysis of the acetylsilane 218 at 10 K in the absence of oxygen was said to yield initially the s-Z vinyloxysilane 219 which slowly isomerized to the s-E conformer 220, or more rapidly at 45 K as shown in equation 24. 

The most reasonable explanation for this is that the photochemical [l,2]silyl shift in 14e yields carbene 13e in its s-E conformation. This carbene either rearranges back to I4e or produces the conformational isomer s-Z-13e (Scheme 7). Since the migrating trimethylsilyl group is located syn to the methyl group, the silyl shift in i-Z-13e is expected to be less feasible than in the s-E conformer, and thus the [1,2]H shift to s-Z-16 can compete. Since the vinyloxysilanes 16 and 8 are both formed in the s-Z conformation it is tempting to assume that a similar mechanism is operating. 

Plavec J, Thibaudeau C, Viswanadham G, Sund C, Sandstrran A, Chattopadhyaya J (1995) The interaction of the 2 -OH group with the vicinal phosphate in ribonucleoside 3 -(ethylphosphate) drives the sugar phosphate backbone into unique (S, e) conformational state. Tetrahedron 51 11775-11792... 

Volkov, S.N. Conformational transitions and the mechanism of transmission of long-range effects in DNA. Preprint ITP-88-12E, Kiev (1988) 22 Krumhansl, J.A., Alexander, D.M. Nonlinear dynamics and conformational exitations in biomolecular materials. In Structure and dynamics nucleic acids and proteins. (Clementi, E., Sarma, R.H., eds) Adenine Press, New York (1983) 61-80... 

Barrows S E, J W Storer, C J Cramer, A D French and D G Truhlar 1998. Factors Controlli Relative Stability of Anomers and Hydroxymethyl Conformers of Glucopyranose. Journal Computational Chemistry 19 1111-1129. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

We assume that the double bonds in 1,3-butadiene would be the same as in ethylene if they did not interact with one another. Introduction of the known geometry of 1,3-butadiene in the s-trans conformation and the monopole charge of 0.49 e on each carbon yields an interaction energy <5 — 0.48 ev between the two double bonds. Simpson found the empirical value <5 = 1.91 ev from his assumption that only a London interaction was present. Hence it appears that only a small part of the interaction between double bonds in 1,3-butadiene is a London type of second-order electrical effect and the larger part is a conjugation or resonance associated with the structure with a double bond in the central position. 

As well as the disubstituted C2-symmelrie pyrrolidines E and F, the monosubstituted (f> )-2-(mcthoxymethyl)pyrrolidine G can be used as chiral auxiliary for the diastereoselecti ve addition of organomctallic reagents to a-oxo amides16. As with the phenylglyoxylic acid derivatives derived from amines E and F. methyllithium or methylmagnesium bromide in diethyl ether preferentially attack the (,S)-mms-conformer 11 (R = ( 6H5), leading to predominant formation of the (2 S)-diastercomer by Re-side attack. 

On the other hand, in the presence of Lewis acids such as titanium(lV) chloride or eerium(TIT) chloride, the (S)-e s-conformer predominates via chelation of the two carbonyl groups and a reversed stereochemistry of the addition reaction is observed1 °. 

The diazo ketone can exist in two conformations, called s-(E) and s-(Z). Studies have shown that Wolff rearrangement takes place preferentially from the s-(Z) conformation. 

Morris, G. A. Castile, J. Smith, A. Adams, G.G. Harding, S.E. 2009. Macromolecular conformation of chitosan in dilute solution A new global hydrodynamic approach. Carbohydrate Polymers 76, 616-621. 

Enders and coworkers <96AG(E)1725> have developed an interesting general one-pot method for the asymmetric epoxidation of enones with oxygen in the presence of diethylzinc and (l ,/ )-N-methylpseudoephedrine (30), which provides a, P - epoxyketones in very high yield and high enantiomeric excess (e.g., 33 —> 34). The actual reactive species is believed to be the chirally modified alkoxy(ethylperoxy)zinc 31, which attacks the si face of the s-cis conformation of the (E) enones (cf. 32). 

The diene needs to be able to adopt the S-cis conformation in order to react, e.g. no reaction is observed for 2,4-hexadiene because of steric hindrance. 

Studies of the effect of permeant s size on the translational diffusion in membranes suggest that a free-volume model is appropriate for the description of diffusion processes in the bilayers [93]. The dynamic motion of the chains of the membrane lipids and proteins may result in the formation of transient pockets of free volume or cavities into which a permeant molecule can enter. Diffusion occurs when a permeant jumps from a donor to an acceptor cavity. Results from recent molecular dynamics simulations suggest that the free volume transport mechanism is more likely to be operative in the core of the bilayer [84]. In the more ordered region of the bilayer, a kink shift diffusion mechanism is more likely to occur [84,94]. Kinks may be pictured as dynamic structural defects representing small, mobile free volumes in the hydrocarbon phase of the membrane, i.e., conformational kink g tg ) isomers of the hydrocarbon chains resulting from thermal motion [52] (Fig. 8). Small molecules can enter the small free volumes of the kinks and migrate across the membrane together with the kinks. 

The reaction shows a dependence on the E- or Z-stereochemistry of the enolate. Z-enolates favor anti adducts and E-enolates favor syn adducts. These tendencies can be understood in terms of an eight-membered chelated TS.299 The enone in this TS is in an s-cis conformation. The stereochemistry is influenced by the s-cis/s-trans equilibria. Bulky R4 groups favor the s-cis con former and enhance the stereoselectivity of the reaction. A computational study on the reaction also suggested an eight-membered TS.300... 

Harding, S. E. Analysis of Polysaccharides by Ultracentrifugation. Size, Conformation and Interactions in Solution. Vol. 186, pp. 211-254. 

Interestingly, we were intrigued by the ESI mass spectrum of the compound, as the observed base peak consisted of [M-S02+Na]+. This led us to explore a thermal retro-Diels-Alder reaction that could afford the desired enone 69. It is noteworthy that the chemistry of cyclic enol-sulfites would appear to be an under-explored area with a few references reporting their isolation being found [57]. At last, we were also able to prepare epoxy ketone 70 from 69 in three steps, albeit epoxidation did not take place unless the TES group was removed. Spartan models reaffirmed our initial conformational assessment of enone 69 and epoxy ketone 70, which contain sp3-hybridized C8a and s/r-hybridized C8b (p s e u d o-. v/r - h y b r i d i zed C8b for 70) at the AB-ring junction (Fig. 8.12) and displayed the desired twisted-boat conformation in A-ring. 

Furthermore, whenever the 1,3-diene group is not embedded in a rigid structure, e.g. in 612 and 713, it assumes the most stable s-trans conformation, just as in the case of 1,3-butadiene. Again dissymmetrically disposed substituents perturb the n —> it transition, which acquires some magnetic moment parallel to the electric one. 

Bousset, L., Thomson, N. H., Radford, S. E., and Melki, R. (2002). The yeast prion Ure2p retains its native a-helical conformation upon assembly into protein fibrils in vitro. EMBOJ. 21, 2903-2911. 

Jones, E. M., and Surewicz, W. K. (2005). Fibril conformation as the basis of species- and strain-dependent seeding specificity of mammalian prion amyloids. Cell 121, 63-72. Kad, N. M., Myers, S. L., Smith, D. P., Smith, D. A., Radford, S. E., and Thomson, N. H. (2003). Hierarchical assembly of beta2-microglobulin amyloid in vitro revealed by atomic force microscopy./. Mol. Biol. 330, 785-797. 

Halverson, K., Fraser, P. E., Kirschner, D. A., and Lansbury, P. T., Jr. (1990). Molecular determinants of amyloid deposition in Alzheimer s disease Conformational studies of synthetic beta-protein fragments. Biochemistry 29, 2639-2644. 

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