Periplanar orientation

The first step of the structure refinement is the appHcation of distance geometry (DG) calculations which do not use an energy function but only experimentally derived distances and restraints which follow directly from the constitution, the so-caUed holonomic constraints. Those constraints are, for example, distances between geminal protons, which normally are in the range between 1.7 and 1.8 A, or the distance between vicinal protons, which can not exceed 3.1 A when protons are in anti-periplanar orientation. 

When electron pairs or polar bonds are placed or generated on adjacent pyramidal atoms, syn or anti periplanar orientations are disfavored energetically with respect to that structure which contains the maximum number of gauche interactions. 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

Aligning the acidic proton with the bromide in a /ran.v-periplanar orientation allows formation of the illustrated product as shown using arrow pushing. 

Scheme VIII/11. Double fragmentation in a tricyclic system [33]. Boldface bonds are anti-periplanar orientated.

The observed change in stereoselectivity can be rationalized by consideration of the conformation of the 2-(arylsulfinyl)-2-cyclopentenone (24) (Fig. 3). The sul-finyl and carbonyl moieties are normally arranged in an anti periplanar orientation (27). The bulky aromatic substituent on the chiral sulfinyl group shields one face of the alkene and thereby controls the facial selectivity of the reaction. In the presence of the Lewis acid the sulfinyl and carbonyl moieties are locked in a syn orientation (28) as a result of chelation between the two moieties and the metal. Thus, the opposite face of the alkene is shielded and (3-addition results in the other diastereoisomer being formed. 

Diastereomer 8 reacts much more slowly than other isomers in an E2 reaction because no pair of hydrogen and chlorine atoms can adopt the anti periplanar orientation preferred for E2 elimination. 

The subsequent step involves cleavage of the C-S bond in the spiro-intermediates and rotation around the central C - C bond to place the - SO2 and benzothiazolyloxy group in an anff-periplanar orientation. The bulk of the data collected by Julia indicated that final elimination was entirely anti-periplanar when R and R were saturated aliphatic chains (or for 258a where r1 = r2 = Ph). However, when carbocation stabilizing substituent R is involved, the zwitterionic intermediate may collapse to the olefin directly, as shown in Scheme 85. Rotation around the carbon-carbon bond in a stabilized carbocation is also feasible. 

A conformational analysis of various substituted A-homo-steroids is reported. The preferred conformation of the amide group in some acetamido-substituted steroids is of the type (5), with anti-periplanar orientation of C—H and N—H bonds the eclipsing of the C—H and C=0 bonds is similar to that accepted for acetates of secondary alcohols. 

X-ray study of the 4-(3-chlorophenyl) derivative showed that the N containing ring adopted a boat conformation and the cyclohexene has a half-chair conformation (05AX(E)01634). The corresponding 4-(2-chloro-5-nitrophenyl) derivative has the chloro substituent in a syn-periplanar orientation with respect to the pyridine ring plane with the nitro group over it (04AX(E)0711). 

A completely different rationale for the stereochemical outcome of aldol additions relies on open-transition-state models. These involve anti-periplanar orientation of enolate and carbonyl group, in contrast with their syn-clinal conformation assumed in the six-membered cyclic transition states. Open-transition-state structures have been proposed to offer a rationale for those aldol additions that give predominantly syn products, irrespective of enolate geometry [90]. This outcome has been observed in aldol reactions of tin and zirconium enoiates and of naked enoiates generated from enolsilanes by treatment with tris(diethylamino)sulfonium difluoro-methylsiliconate [70]. As shown in Scheme 1.12, the driving force for the... 

The basic assumption of open-chain transition-state models is the anti-periplanar orientation of the enolate and the carbonyl double bond. This... 

The E2 elimination reactions will occur in the anti periplanar orientation as drawn. To draw the product of elimination, maintain the orientation of the remaining groups around the C=C. 

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