Torsional control

Keywords it-Facial selectivity, a/ir Interaction, CH/ir Interaction, Ciplak effect, Diels-AIder reaction, Electrostatic interaction, Orbital mixing rule. Orbital phase environment, Secondary orbital interaction, Steric repulsion, Torsional control... 

Scheme 38 Torsional control of 7t-facial selectivity in the reactions of isodicyclopentadiene 79...

If one can find ways to effect SN2 reactions of glycosyl halides with oxygen nucleophiles (e.g., sugar alcohols), selective access to both sets of anomeric halides could prove invaluable. Other studies which employ SN2 chemistry at glycosidic centers include the halide-assisted glycosylation developed by Lemieux et al.,45 the use of a participating solvent such as acetonitrile,46 and exploitation of torsional control of anomeric reactivity.47 This latter approach has been very successfully exploited by Crich and Sun in the synthesis of P-mannosides (Fig. 19).48... 

A direct way to simulate the chromic phenomena starting with the single chain is to include in the model a possibility for torsion, controlled by torsion potentials. The nature of these potentials is then chosen to represent the interaction of side chains with the main chain/side chains, in such a way as to obtain a realistic model of the system. It is also important to account for the solid-state packing by including some potential for the interaction between adjacent chains. Results from such simulations have been reported [57], and indicate that the length of the side chain has no influence on the energetics of interaction, but only the regularity of the polymer. 

Kafinke, P. Gnauert, U. ATC Active Torsion Control zur Optimierung des Schwingungskomforts bei Cabriolets. Proc. Adaptronic Congress 2002, Potsdam (23-24 April 2002)... 

Lucero MJ, Houk KN. Torsional control of epoxidation stereoselectivity in 1,2-dihydronaphthalenes. Transition state modeling with semiempirical quantum mechanics. J. Org. Chem. 1998 63 6973-6977. 

Loads applied on products induce tension, compression, flexure, torsion, and/or shear, as well as distributing the loading modes. The product s particular shape will control the type of materials data required for analyzing it. The location and magnitude of the applied loads in regard to the position and nature of such other constraints as holes, attachment... 

Systematic searches exhaustively sample conformational space by sequentially incrementing the torsional angles of aU of the rotatable bonds in a given molecule. This conceptually simple approach is straightforward to implement, but scales exponentially with respect to the number of rotatable bonds. To control the exponential increase in the number of potential conformers obtained, systematic searches are usually combined with tree-based search techniques taken from computer science. Even the best implementations of systematic searches become impractical beyond several rotatable bonds (typically greater than 10). Stochastic searches are based on probabiHstic theories and are better suited to calculations... 

With less hindered hydride donors, particularly NaBH4 and LiAlH4, confor-mationally biased cyclohexanones give predominantly the equatorial alcohol, which is normally the more stable of the two isomers. However, hydride reductions are exothermic reactions with low activation energies. The TS should resemble starting ketone, so product stability should not control the stereoselectivity. A major factor in the preference for the equatorial isomer is the torsional strain that develops in the formation of the axial alcohol.117... 

As an example, this approach was applied to the calculation of the PMF for alanine dipeptide as a function of the two torsion angles

resulting free energy surface is shown in Fig. 4.5. Bilinear Qi elements were used to approximate the free energy. Control points were chosen such that there are four of them around each data point. This was done in order to increase the smoothness and quality of the reconstructed free energy. The position of the Q i nodes and control points is shown in Fig. 4.6. 

This observation illustrates an interesting possibility of probing torsion angles by a controlled evolution of the intramolecular MQ signals, just as discussed by Levitt and co-workers [61]. 

Chemically Controlled Conductance Torsion-Angle Dependence in Single-Molecule... 

These moieties (particularly (5.95b)) are prominent in protein chemistry, and mechanisms for controlling their torsional stiffness have obvious potential implications for protein-folding processes. 

Figure 5.65 provides theoretical evidence that resonance-assisted H-bonding can serve as an effective mechanism for switching a methyl rotor from one preferred conformation to another, or for controlling the stiffness of torsional motions in alkylated amides. In particular, the torsional potentials of proteins (more specifically, the Ramachandran b angle at Ca) should be sensitive to N—H- O and related H-bonding interactions involving the amide backbone. In principle, this electronic... 

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