Anti conformation definition

The bases are in syn or anti orientation. The orientation of the base relative to the sugar moiety is defined by torsion angle x which is constrained by steric interactions, and by the anomeric effect. The main conformations are referred to as syn and anti (see Fig. 17.5). In syn, j X is close to 0° (sp for torsion angle definition, see Box 13.3 Fig. 13.12), and the base is I oriented "above the ribose ring causing steric interactions which in the anti conformation with x close to 180° (ap) are avoided. The anti conformation is therefore preferred and is the j only form observed in double-helical DNA and RNA. An exception is the left-handed Z-DNA with alternating purine/pyrimidine nucleotide sequence where the purines are in the syn conformation. 

In contrast to this equal propensity toward formation of a H-bond, there is a definite preference for a proton to locate on the syn side when coming much closer and forming a covalent bond. That is, the syn conformer of RCOOH is more stable than the anti structure by about 5 kcal/mol in the gas phase. If this is the case, why then is there so little apparent pK difference between the syn and anti structures The answer to this question resides in solvation phenomena. The presence of the polarizable medium preferentially stabilizes the anti conformer of the carboxylic acid due to its higher dipole moment. 

For a monosubstituted oxane, the excess free enthalpy of the conformation with an axial substituent over the conformation with an equatorial substituent is, as we know by definition, the conformational free energy (CFE) of the substituent in oxane at this position. These values, possibly measured indirectly by utilizing intennediate compounds, are shown in Table 2.2. Equatorial conformations correspond to anti conformations in butane and methoxyethane, and the axial conformations (not represented) to gauche conformations. We can observe that the environment of derivative 2.20 is closest to that of the cyclohexane and that the CFE is of the same order. On the other hand, the presence of the cyclic oxygen lowers notably the CFE of derivative 2.19. The important point is the noteworthy increase in the CFE of compound 2.18, where the methyl group is close to the cyclic oxygen and possesses, on one side, an environment similar to that of methoxyethane. Let us look at the equilibrium (2.4) of the dimethylated derivative 2.21. 

Figure 1. Preinsertion complex (n complex) of propene with ethyl as model of the growing polymer chain for the ethylenebis(l-indenyl) ligand. ( ) Methyl groups. (A) refacial coordination of propene with anti conformation of the two methyl groups (B) si-faclal coordination of propene with syn conformation of the two methyl groups. For definition of r>i, see Figure 2. ° (Reprinted with permission from ref 9. Copyright 1988.)...

FIGURE 3.2. Cyd in the anti conformation (a) and Guo in the syn conformation (b). Definition of the giycosidic torsion angle 0CN (= cn) purine nucleoside. In (c) (anti) are also shown the pseudorotational angles of the ribose molecule thus is the angle formed between the planes formed between bonds Oii-Cj. and Cg-C3, and so on. The torsion angle for the syn conformation is also shown (d). 

Masamune s descriptors syn and anti, appealing because of their conformity with modern customs of writing formulas, are currently widely in use. But, assuming they are used according to definition, they suffer from exactly the same fundamental limitations as c-erythrojc-threo or any other soft descriptor. Unfortunately, Masamune did not exhaust the area of an unambiguous soft description by restricting the simply defined models to C-C(HX)-C(HY)-C rather than C —C(XY) —C(XZ) —C (see Table 10). 

The mixture of ester and amine plus amide and alcohol products obtained from the imidate ion 54 (R=CHg ) and 55 (R=CgH-]i ) can also be explained. These ions could exist in the anti form only. However, this would be surprising on the basis of the steric argument discussed previously. Also, the results obtained from the carbonyl-oxygen exchange in tertiary amides definitely show that conformational change can easily compete with the breakdown at room temperature only when R=H in tertiary amide (RCONRj1). Thus,... 

Further studies are required to unravel this mystery of how the methoxy substitutions and the a, p-unsaturated p-diketone moiety actually influence conformational changes, lipophillicity, electron density distribution, and redox properties of curcuminoids. Correlating these physicochemical properties with the unique pleiotropic effects of curcuminoids is a rewarding exercise. Such studies would definitely provide proper reasoning in understanding these markedly different antioxidant, antitumor, and anti-inflammatory activities of natural curcuminoids from turmeric. 

This is the common definition of conformers. The IUPAC definition also requires that a conformer correspond to a distinct potential energy minimum, such as the anti and gauche conformations of butane. 

A definite barrier to conformational isomerization is readily imagined in the case of ions (255) and (263). Much smaller conformational barriers would be expected to separate the bicyclo [2.2.2] octenyl cations (270a) and (270b). Nevertheless the distribution of products obtained from the ring expansions of anti- and syn-2-nor-bornenyl-7-methylamines, (269) and (272), demonstrate the existence of a memory... 

Figure 7.11. Terminology definitions for hydroboration transition structures [141] (a) The auxiliary may be either syn or anti to the alkene substituents, but anti to the substituent (R) on the nearest carbon, (b) A stereocenter attached to boron, in a staggered conformation with respect to the forming C-B bond, has substituents in anti, inside, and outside positions, (c) Definition of the Large, Medium, and Small substituents of IpcBH2.

Figure 1.2.2 The description of conformers of an A- X-Y-B system. First row definition of torsion angle ((). Second row definition of left- and right-handed as well as of syn(cis) and anti (trans). Third row definition of defined conformers, always relating to the relative position of A and B.

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