Conformation isomer formation

A peptoid pentamer of five poro-substituted (S)-N-(l-phenylethyl)glycine monomers, which exhibits the characteristic a-helix-like CD spectrum described above, was further analyzed by 2D-NMR [42]. Although this pentamer has a dynamic structure and adopts a family of conformations in methanol solution, 50-60% of the population exists as a right-handed helical conformer, containing all cis-amide bonds (in agreement with modeling studies [3]), with about three residues per turn and a pitch of 6 A. Minor families of conformational isomers arise from cis/trans-amide bond isomerization. Since many peptoid sequences with chiral aromatic side chains share similar CD characteristics with this helical pentamer, the type of CD spectrum described above can be considered to be indicative of the formation of this class of peptoid helix in general. 

Chirality transfer in catalytic asymmetric hydrogenation can be achieved not only by using powerful chiral ligands such as BINAP or DuPhos but also by the formation of a dynamic conformational isomer. The availability of many enantiomerically pure diols allows the production of electron-deficient, bi-dentate phosphate in the form of 27. The backbone O-R -O can define the chirality of the 0-R2-0 in complex 28, hence realizing the chirality transfer.44... 

In a condis crystal cooperative motion between various conformational isomers is permitted. In the CD-glass this motion is frozen, but the conformationally disordered structure remains. For a condis crystal it is not necessarily expected that all possible conformations can be reached, but all conformations of the same type are involved in the condis crystal motion. If conformational isomers of low energy exist which leave the macromolecules largely in a parallel, extended, low energy conformation, conditions for the formation of condis crystals are given. The conformational changes involve more or less hindered rotations about backbone bonds or side chain bonds and are thus the some degree related to the orientational motion in plastic crystals. 

The geometry in the transition state readily explains the preferential formation of the conformational isomer a (46) (Scheme 2). 

Because of the inherent complexity of the cyclooctatetraene system,these results can be explained without invoking adsorbed intermediates 102 but the possibility remains that adsorption plays an important role. By analogy with electrophilic additions to cyclooctatetraene 1 °3, an adsorbed cyclooctatetraene molecule would be attacked preferentially from the fold of the tub conformation with formation of encfo-8-acetoxyhomotropylium ion (11). Attack by acetate ion on 11 would give the c/s-isomer, whereas the trans-isomer might be formed from exo-8-acetoxyhomotropylium ion 12 (Eq. (33) ). 

It should be realised that minor conformational isomers may be important reaction intermediates. For example, treatment of 6-0-tosyl-D-glucopyranose with base results in the formation of a 1,6-anhydro derivative. The starting material exists mainly in the 4Ci conformation. However, for reaction to occur the alternative 4 conformation has to be adopted (Scheme 1.2). The introduction of protecting groups may alter the preferred conformation of saccharides. 

The cyclic sulfanuric chlorides, (NS0C1) , are of interest because they are isoelectronic with the cyclic phosphonitrilic chlorides, (NPCl2)n- Although the formation of a variety of substances with n = 3 or more appears reasonable, only the cyclic trimers have been isolated.1 At least three, apparently conformational, isomers of composition (NSOCl)3 have been... 

A Lewis acid e.g. aluminium tert-butoxide, boron trifluoride, neutral alumina) is considered to coordinate with both the 17-OH and 20-keto functions, bringing them into a cfs-relationship and effectively locking the side chain in one conformation. Product formation is then determined by the relative ease of migration of the Cps)--C(i ) and C(i6>-C(i7) bonds towards the electron-deficient C<20). The structures of the resulting ketones show that the C i3)-C(i ) bond migrates in the i7j -hydroxy compound (i), and the C(i6)-"C(i7) bond in the i7a-hydroxy isomer (2). The reason for this difference has been the subject of much speculation, and is still not clear. The factors which have been considered [202] as affecting the stability of respective transition states include ... 

The strain energy values that SpartanBuild calculates have another use besides structure refinement. They also can be used to compare the energies of models that share the same molecular formula—that is, stereoisomers or conformational isomers. Allowed comparisons are shown below. Strain energy differences between these pairs of molecules correspond closely to differences in heat of formation and to differences in free energy. SpartanBuild reports strain energies in kcal/mol (1 kcal/mol = 4.184 kj/mol) in the lower left-hand corner of the SpartanBuild window. 

Several theoretical studies have considered LiH addition as a model for the more computationally difficult L1A1H4 and NaBH4 [45]. However, reaction of aldehydes and ketones with LiH seldom if ever leads to reduction [46]. AIH3, while less commonly used than the complex boron and aluminium hydrides, is useful for reducing carbonyls [47] and therefore is a suitable model for computational study. Calculations [2, 5] show that gas-phase reduction of formaldehyde by AIH3 occurs by formation of complex 1, which rearranges via a four-centre transition state to form an aluminium methoxide product. Two conformational isomers of... 

Curtin-Hammett principle In a chemical reaction that yields one product (X) from one conformational isomer (A ) and a different product (Y) from another conformational isomer (A") (and provided that these two isomers are rapidly interconvertible relative to the rate of product formation, whereas the products do not undergo interconversion), the product composition is not in direct proportion to the relative concentrations of the conformational isomers in the SUBSTRATE it is controlled only by the difference in standard free energies (dA G) of the respective TRANSITION STATES. 

Azumaya et al. reported an interesting example of retention of the molecular chirality when the chiral crystal of l,2-bis(A-benzoyl-A-methylamino)benzene 65 was dissolved in a cold solution (Fig. 6) [37]. Furthermore, Tissot et al. reported a fine example of the formation of optically active complex 67 ( 100% ee in 93% yield) using axially chiral ligand 66 (Scheme 32) prepared by chiral crystallization [38]. Both achiral 65 and 66 exist as mixtures of many conformational isomers or diastereomers in solution at ambient temperature. If the molecular chirality is retained in homogeneous conditions like these examples, the frozen... 

In 1988, it became obvious that the composition of sulfur vapor is even more complex than had been thought before Lenain et al. published a Raman spectroscopic study of the sulfur vapor composition at temperatures of up to 700 °C [20]. 8ignals for the cyclic species 8s, 87, and 8e as well as for the acyclic molecules 84, 83, and 82 were observed and spectroscopic evidence for the presence of two conformational isomers of 84 and for either chain-like (8 ") or branched-bonded species (8 =8 with n>4) was presented. These authors came to the conclusion that the thermodynamic data of Rau et al. [24] are reliable except for the enthalpy of 87 formation from 8s which was said to be too high. 

Let us now consider the dihydoxy ketone 164 which is enantiomeric to 161 at the secondary carbinol center. Cyclization is expected to lead to the formation of two isomeric acetals 165 and 166. The isomer 165 may be expected to exist as an equilibrium mixture of 165a and 165b. Likewise, the isomer 166 may be expected to exist as an equilibrium mixture of 166a and 166b. Whether both 165 and 166 or just one of them, and also whether each of these could indeed exist as an equilibrium mixture of the two possible conformational isomers will be governed by their relative energy differences, calculated as follows ... 

Development in the area of asymmetric aza-annulation reactions paralleled achievements in the analogous area of asymmetric Michael addition reactions with chiral imines.111 Induction of asymmetry has been primarily controlled through substitution at the nitrogen of the imine or enamine that becomes incorporated into the heterocycle. Restricted rotation of this asymmetric substituent led to preferred conformational isomers, which provided stereofacial bias for carbon-carbon bond formation. 

The product ratio is therefore not determined by AG, but by the relative energy of the two transition states A and B. The conclusion that the ratio of products formed from conformational isomers is not determined by the conformational equilibrium ratio is known as the Curtin-Hammett principled Although the rate of the formation of the products is dependent upon the relative concentration of the two conformers, because AGb is decreased relative to AG to the extent of the difference in the two conformational energies, the conformational preequilibrium is established rapidly, relative to the two competing product-forming steps. " The position of the conformational equilibrium cannot control the product ratio. The reaction can proceed through a minor conformation if it is the one that provides access to the lowest-energy transition state. 

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