Conformation CONFOR

FIGURE 10 6 Confor mations and electron delo calization in 1 3 butadiene The s CIS and the s trans con formations permit the 2p or bitalsto be aligned parallel to one another for maxi mum TT electron delocaliza tion The s trans conformation is more stable than the s CIS Stabilization resulting from tt electron de localization is least in the perpendicular conformation which IS a transition state for rotation about the C 2—C 3 single bond The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases... 

The various conformations of cyclohexane are in rapid equilibrium with one another, but at any moment almost all of the molecules exist in the chair confor mation. Not more than one or two molecules per thousand are present in the skew boat conformation. Thus, the discussion of cyclohexane confor-mational analysis that follows focuses exclusively on the chair confor mation. 

Beginning with cycloheptane, which has four confomnations of similar energy, conformational analysis of cycloalkanes becomes more complicated. The sane fundamental principles apply to medium and large rings as apply to smaller ones—but there ar e more atoms and more bonds to consider and more confor-mational possibilities. 

These remarkable observations stimulated an investigation to understand the origin of the directing effect. To clarify the contribution of the axially- and equato-rially-oriented oxygen atoms in the ketal, a survey of the reaction of three confor-mationally biased t-butyl cyclohexenone ketals 78, 81 and 84 was undertaken (Scheme 3.26) [56]. In each case, careful conformational analysis provides critical clues to rationalizing selectivity. 

The influence of the solvent on the oxidation of film under conformational relaxation control is illustrated in Fig. 47, which shows chronoamperograms obtained by steps from -2000 to 300 mV vs. SCE at room temperature (25°C) over 50 s in 0.1 M LiC104 solutions of different solvents acetonitrile, acetone, propylene carbonate, (PC), dimethyl sulfoxide (DMSO), and sulfolane. Films were reduced over 120 s in the corresponding background solution. Despite the large differences observed in the relative shape of the curves obtained in different solvents, shifts in the times for the current maxima (/max) are not important. This fact points to a low influence of the solvent on the rate at which confor-... 

Finally, it is worth while to consider the applicability of these relaxation methods to molecules having flexible conformations. Examples given in the previous Section demonstrated that relaxation rates are able to define either the most probable conformation of a flexible molecular segment, or to specify a range of allowed conformations from the total number of confor-... 

Egodage etal. [103] have described a novel application of monoclonal antibodies for the probing of conformation and orientation of the adsorbed protein using cytochrome c(3) films on mercury electrode. Antibodies were utilized to confirm the presence of three confor-mationally distinct electrochemical forms of cytochrome dependent on the applied potential. 

Neglecting high energy conformations, cis-1,4,5,8-tetramethylperhy-dro[2,3-b]pyrazinopyrazine (416) exists as an enantiomeric pair of confor-mers 417 and 418 interconverting by ring inversion. For the trans isomer, only conformations 419 and 420 need to be considered. A 13C- and H-NMR... 

In conformity with the mechanical losses we can calculate the orientation polarisation using Eq. (28). It will btA= - Ho or exp/cof. If there are three possible confor-... 

A new strategy for the synthesis of erythromycin A and closely related mac-rolide antibiotics was elaborated in our laboratory (88). This new approach to synthesis is based on the knowledge that stereoelectronic effects control the conformation of acetals. The strategy is based on the 1,7-dioxaspiro-[5.5]undecane system which was found to be conformationally rigid, existing in conformation J50 only (see Chapter 2). This is so because in this confor-mation, steric effects are at their minimum and the acetal function has... 

With the tricyclic system 113 (Fig. 5), two isomers 20 and 21 are possible from the cyclization of the dihydroxyketone 2j)> and each can exist in two conformations. It was, however, predicted that each isomer would be confor-mationally rigid, existing in the conformation 20A (0 kcal/mol) and 21A (2.4 kcal/mol) respectively. Furthermore, since isomers 20 and 21 can be... 

An interesting comparison can be made looking at the a- and (5-tetralin derivatives entries—15/16 and —17 in Table 1 which can be regarded as cyclic confor-mationally constrained analogues of phenylglycine and phenylalanine. In an interesting study, 6-hydroxy-2-aminotetralin-2-carboxylic acid 12 (Hat) has been incorporated as a conformationally constrained tyrosine analogue into S-opioid receptor selective tetrapeptides.114 15 Whereas entry 15, the (S)-a-tetralin deriva-... 

Small-angle X-ray diffraction was used to identify the time-averaged location of amiodarone in a synthetic lipid bilayer. The drug was located about 6 A from the center of the lipid bilayer (Figure 4.13) [125, 126]. A dielectric constant of k = 2, which is similar to that of the bilayer hydrocarbon region, was used to calculate the minimum energy conformation of amiodarone bound to the membrane. The studies were performed below the thermal phase transition and at relatively low hydration of lipid. The calculated conformation differed from that of the crystal structure of amiodarone. Even though the specific steric effects of the lipid acyl chains on the confor-... 

In the debate about existence of pre-ordered states in the polymer melt, as advocated recently, polyolefins with chiral side chains may well become a major investigation tool. Indeed, the macromolecular amplification induces a pre-organization, or at least a preferred helical conformation in the polymer melt or solution. As such, these polymers display very precisely the behavior that is assumed by some of the recent crystallization schemes or scenarios. Furthermore, for the P4MH1 systems considered so far at least, the confor-mationally racemic character of the stable crystal structure implies that half of the stems must change their helical hands at some stage in the crystallization process - which may greatly delay the formation of this stable crystal structure, as illustrated by P(S)4MH1. 

Purposely, as models for copolymers, the cis/trans-isomers of 2,5-disubstituted 1,3-dithianes (cf. Scheme 22) were synthesized and confor-mationally studied (02CEJ1546). The conformational equilibrium in the trans isomers always arranges the benzyl and the thienyl groups in the... 

A plausible explanation can be given if we consider the allosteric nature of the typical receptor protein and its interaction with the ligand. An allosteric protein has two conformational states that are in equilibrium (Figure 3.12). With most receptors, the inactive state will be more prevalent in the absence of agonists. However, an agonist will exclusively bind to and therefore favour the active confor-... 

Th chair confor-maiion of cyclohexane has many chemical consequences. Por XHmp1 . w 11 MU tn Section 11.13 that the ehetnicat behavior of many substitute cycluhexanee is directly controlled by their conformation. Another consequence of the chair conformation is that tbero are two kinds of positions far substituents on the ring oxiof positions and equatorial... 

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