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Conformation antiperiplanar

These results may be explained either by Cram s cyclic model in the case of lithium alkyls or by Cornforth s dipolar model if copper-boron trifluoride reagents are used. Boron trifluoride causes double complexation of both nitrogen and oxygen atoms which results in the formation of an adduct with rigid antiperiplanar conformation due to electrostatic repulsion (see 4 and 5)9. [Pg.705]

As mentioned in the introductory part of Section 2.2.3, fully extended y9-peptide strands can be generated by populating antiperiplanar conformations around the C a)-C fi) bond (0j values close to 180° Fig. 2.27A). [Pg.76]

Fig. 2.27 The two types of extended /1-peptide strands with conformation requirements around the C(a)-C(/1) bonds. (A) Parallel and antiparallel polar sheets with antiperiplanar conformations around the C(a)-C fl) bond are promoted by unlike-fi -ami-no acids with alkyl side-chains. Antiperiplanar side-chains at C(a) and C(/3) occupy positions approximately perpendicular to the amide planes. (B) Extended strands formed by alternating +)-sc and (-)-sc conformations... Fig. 2.27 The two types of extended /1-peptide strands with conformation requirements around the C(a)-C(/1) bonds. (A) Parallel and antiparallel polar sheets with antiperiplanar conformations around the C(a)-C fl) bond are promoted by unlike-fi -ami-no acids with alkyl side-chains. Antiperiplanar side-chains at C(a) and C(/3) occupy positions approximately perpendicular to the amide planes. (B) Extended strands formed by alternating +)-sc and (-)-sc conformations...
Synclinal and antiperiplanar conformations of the TS are possible. The two TSs are believed to be close in energy and either may be involved in individual systems. An electronic tt interaction between the stannane HOMO and the carbonyl LUMO, as well as polar effects appear to favor the synclinal TS and can overcome the unfavorable steric effects.161bi 162 Generally the synclinal TS seems to be preferred for intramolecular reactions. The steric effects that favor the antiperiplanar TS are not present in intramolecular reactions, since the aldehyde and the stannane substituents are then part of the intramolecular linkage. [Pg.837]

In the case of the antiperiplanar conformer of 1,2-difluoropropane (7) we studied again the influence of the basis for the carbon atom in the methyl group. For all hydrogen atoms we used the minimal basis set apart from the reference calculation where the cc-pVDZ basis set was employed on hydrogens 5H , 6H and 7H . The result, shown in Fig. 7, is the same as for trans-1,2-difluoropropene (3). The electron density around the methyl carbon, although it is out of the F-F bonding path, is not properly described with the minimal basis set. The OP, SD and FC terms are all not well reproduced by this calculation... [Pg.174]

It can be seen from Fig. 8 that the minimal basis set on the hydrogen atoms produces larger errors in the OP term and FC term of 1,2-difiuoroethane in the synperiplanar conformation (8) than in the antiperiplanar conformation (6). The same difference was also observed between the trans and cis couplings in the... [Pg.175]

By a similar analysis of the results for the traus-1,2-difluoropropene, (3), and the antiperiplanar conformation of 1,2-difluoropropane, (7), (see also Table 5) one can see that the previous findings are still valid when the LDBS scheme is apTJ/pT/[pD/m]/m. However, it can he seen in Figs 3 and 7 that for quantitative reproduction of the vicinal F-F coupling it is necessary to describe the carbon atom of the CH3 substituent with a larger than a minimal basis set. In both cases, the cc-pVDZ basis set was used. [Pg.179]

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... [Pg.32]

The gauche effect — which has the same electronic origin as that of the anomeric effect—is able to stabilize a conformation that should, a priori, be disfavored by steric factors. Thus, the gauche effect prevents the trans antiperiplanar conformation of two electronegative substituents borne by two vicinal carbon atoms (Figure 3.2). ° ... [Pg.74]

Antiperiplanar conformation, 118, 253 Arenium ions, 131 Arenonium ions, 131 Arndt-Eistert reaction, 119 Aromaticity, 14... [Pg.206]

Dehydrohalogenation of 1 //-2-chlorodecafluorocyclohexane (38B) gives products 39 and 21.79 Preferential elimination of hydrogen fluoride occurs in compound 38B, where hydrogen and fluorine are in an antiperiplanar conformation. [Pg.108]

In 2-trimethylsilylethyl 4-phenylbenzoate 44, the C—Si and C—O ester bonds are in an antiperiplanar conformation, and the C—O ester bond is significantly longer than expected for an alkyl ester bond at a primary centre. This is consistent with a oq si T c o interaction in the ground state. In /raw.v-2-(di methyl phenyl si ly I )cyclohexyl-3,5-dinitrobcnzoate 45, the dihedral angle between the C—Si and C—O ester bonds is found to be just over 60°, and there appears to be little lengthening of the C—O bond. [Pg.374]

Analysis of the crystal structure of the p-nitrobenzoate and 2,4-dinitrobenzoate derivatives of ewrfo-3-(trimethylsilyl)-ewrfo-2-norborneol 47, in which the Si—C and C—O bonds are in the synperiplanar geometry, showed that there was no significant lengthening of the C—O bond in these cases64. By comparison, the cyclohexyl ester 48, in which the Si—C and C—O bonds are in the antiperiplanar conformation, did show a significant lengthening of the C—O bond compared to model compounds64. [Pg.375]

Fig. 20 Thermodynamically controlled synthesis of capsules 39 and 40132 note that a molecule of ethane was found to reside inside 40. The favorable antiperiplanar conformation about the central C-C bond in a diimine linkage of 40 is shown. (For color version of this figure, the reader is referred to the web version of this book.)... Fig. 20 Thermodynamically controlled synthesis of capsules 39 and 40132 note that a molecule of ethane was found to reside inside 40. The favorable antiperiplanar conformation about the central C-C bond in a diimine linkage of 40 is shown. (For color version of this figure, the reader is referred to the web version of this book.)...
Draw the antiperiplanar conformation of erythro-3-chloro-2-hydroxybutan-oic acid in Newman projection. [Pg.38]

In order to determine the conformation, the substitution pattern at carbon atoms C2 and C3 must first of all be established. Three different groups are attached to each. Hence, the groups of highest priority on each are taken as the reference groups (shown bold in the diagram below), thus this must be the antiperiplanar conformation. In order to transform the formula into a Fischer projection formula, the part of the formula which is closer to the observer is rotated until an eclipsed conformation is obtained in which the groups forming the main chain are oriented synperiplanar to each other. [Pg.100]

Eplivanserin contains three stereogenic units. In addition to the E-config-ured alkene double bond there is a Z-configured substituted oxime. The single bond between both double bonds has partial double bond character. The formula shown below represents the s-trans isomer. This means that it represents an antiperiplanar conformation. [Pg.126]

Relying on this adsorption structure, a model mechanism has been put forward by Knozinger and co-workers (349, 350). The activation is assumed to be initiated by proton fluctuations between adsorbed alcohol molecule and surface which may result in polarization of the molecule. The alcohol molecule itself is suggested to possess some vibrational or rotational freedom relative to the surface so that the 0 proton may approach a basic O2- ion while the alcohol is in the necessary antiperiplanar conformation. [Pg.252]


See other pages where Conformation antiperiplanar is mentioned: [Pg.534]    [Pg.230]    [Pg.188]    [Pg.185]    [Pg.132]    [Pg.68]    [Pg.161]    [Pg.173]    [Pg.167]    [Pg.23]    [Pg.96]    [Pg.105]    [Pg.121]    [Pg.136]    [Pg.450]    [Pg.917]    [Pg.940]    [Pg.96]    [Pg.105]    [Pg.23]    [Pg.280]    [Pg.43]    [Pg.618]    [Pg.620]    [Pg.802]    [Pg.218]    [Pg.29]    [Pg.276]    [Pg.865]    [Pg.1036]   
See also in sourсe #XX -- [ Pg.118 , Pg.253 ]

See also in sourсe #XX -- [ Pg.118 , Pg.253 ]

See also in sourсe #XX -- [ Pg.231 ]

See also in sourсe #XX -- [ Pg.30 ]

See also in sourсe #XX -- [ Pg.198 , Pg.199 ]




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