Anti relationship

Eclipsed bonds are characterized by a torsion angle of 0° When the torsion angle is approximately 60° we say that the spatial relationship is gauche, and when it is 180° we say that it is anti Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms... 

Red circles gauche 60° and 300° Red circles anti 180° Gauche and anti relationships occur only in staggered conformations therefore ignore the eclipsed conformations (0° 120° 240° 360°)... 

The relative configurations of vicinal protons follow from the characteristic values of their coupling constants. Thus 16.1 Hz confirms the trans relationship of the protons on C-8 and C-9, 10.8 Hz confirms the cis relationship of the protons on C-6 and C-1. The 2.0 Hz coupling is common to the oxirane protons at = 3.00 and i.27 this value fixes the trans relationship of the protons at C-4 and C-5 following a comparison with the corresponding coupling in the methyloxirane (2.6 Hz). The anti relationship of the protons A-H and h-H can be recognised from their 8.7 Hz coup-... 

Attack ty acetate at C-1 of C-2 would be equally likely and would result in equal amounts of the enantiomeric acetates. The acetate ester would be exo because reaction must occur from the direction opposite the bridging interaction. The nonclassical ion can be formed directly only from the exo-brosylate because it has the proper anti relationship between the C(l)—C(6) bond and the leaving group. The bridged ion can be formed from the endo-brosylate only after an unassisted ionization. This would explain the rate difference between the exo and endo isomers. 

Further examination of the chiral ketals reveals that the lone pairs available for reagent coordination are oriented either in a syw or an anti relationship to the neighboring methyl substituents. The influence of the chiral auxiliary over the reaction is now clear. If zinc coordination must occur proximal to the double bond. 

The most stable conformation of a substituted alkane is generally a staggered one in which large groups have an anti relationship. The least stable conformation is generally an eclipsed one in which large groups are as close as possible. 

Note also the stereochemistry. In some cases, two new stereogenic centers are formed. The hydroxyl group and any C(2) substituent on the enolate can be in a syn or anti relationship. For many aldol addition reactions, the stereochemical outcome of the reaction can be predicted and analyzed on the basis of the detailed mechanism of the reaction. Entry 1 is a mixed ketone-aldehyde aldol addition carried out by kinetic formation of the less-substituted ketone enolate. Entries 2 to 4 are similar reactions but with more highly substituted reactants. Entries 5 and 6 involve boron enolates, which are discussed in Section 2.1.2.2. Entry 7 shows the formation of a boron enolate of an amide reactions of this type are considered in Section 2.1.3. Entries 8 to 10 show titanium, tin, and zirconium enolates and are discussed in Section 2.1.2.3. 

In each instance, the silyl enol ether approaches anti to the methyl substituent on the chelate. This results in a 3,4-syn relationship between the hydroxy and alkoxy groups for a-alkoxy aldehydes and a 3,5-anti relationship for (3-alkoxy aldehydes with the main chain in the extended conformation. 

Entry 15 involves a benzyloxy group at C(2) and is consistent with control by a (3-oxy substituent, which in this instance is part of a ring. The anti relationship between the C(2) and the C(3) groups results from steric control by the branched substituent in the silyl enol ether. The stereogenic center in the ring has only a modest effect. 

Reagent-controlled stereoselectivity can provide stereochemical relationships over several centers when a combination of acyclic and chelation control and cyclic TS resulting from transmetallation is utilized. In reactions mediated by BF3 or MgBr2 the new centers are syn. Indium reagents can be used to create an anti relationship between two new chiral centers. The indium reagents are formed by transmetallation and react... 

Entries 20 to 23 involve additions to C=N double bonds in oxime ethers and hydrazones. These reactions result in installation of a nitrogen substituent on the newly formed rings. Entry 20 involves the addition of the triphenylstannyl radical to the terminal alkyne followed by cyclization of the resulting vinyl radical. The product can be proto-destannylated in good yield. The ring closure generates an anti relationship for the amino substituent, which is consistent with the TS shown below. 

The present homoallylation with siloxy- and methoxy-substituted dienes may be of great synthetic use. The product 42 is easily converted to anti-5-phenyl-5-hydroxy-3-methylpentanal (Scheme 9) hence the diene 41f may be regarded as a synthetic equivalent of a bis-homoenolate of 3-methylbutanal, being capable of introducing 1,3-anti relationship between the methyl and hydroxy groups in the product. 

The appropriate interaction diagram is shown in Fig. 50. Arguing as before, we predict that the W conformation is preferred since there is an anti relationship between the carbon lone pair and the X-H bond in this conformation. 

The second rule is directly relevant to conformational isomerism. In Part IV of this work we have provided the theoretical justification of this rule and also identified the situation where this rule may break down. A typical exception is shown below and involves a molecule where two vicinal polar bonds constitute the best donor-acceptor fragment combination and, thus, lead to a conformational preference placing them in an anti-relationship. 

Scheme 2.6 provides an overall view of our strategy towards solving this problem. As depicted, our late generation synthesis embraces three key discoveries that were crucial to its success. We anticipated that the difficult Cl-Cll polypropionate domain could be assembled through a double stereodifferentiating aldol condensation of the C5-C6 Z-metalloenolate system B and chiral aldehyde C. Two potentially serious problems are apparent upon examination of this strategy. First was the condition that the aldol reaction must afford the requisite syn connectivity between the emerging stereocenters at C6-C7 (by uk addition) concomitant with the necessary anti relationship relative to the resident chirality at C8 (by Ik diastereoface addition). Secondly, it would be necessary to steer the required aldol condensation to C6 in preference to the more readily enolizable center at C2. 

Although our initial attempts in this direction proved unavailing, our efforts were not completely futile in that we were able to confirm that the critical aldol reaction with S-aldehyde 63 did indeed provide the desired C6-C7 syn and C7-C8 anti relationship (by anti-Felkin Ahn addition) as the major diastereomer. 

The H-N and C=0 of the amide (H-N-C=0) groups were placed in the more stable anti relationship (13). 

The general preference of 1,3-anti relationship induced by the substrate structure may be understood by assuming that the transition states me those shown in Scheme 58. The preference for the anti transition feructure originates primarily from the (7 )-BINAP chirality (Scheme 51). addition, the six-membered ring in the syn-generating transition ucture suffers l,3-R2/0 repulsion, which is absent in the diastereo-iric anti transition state. 

The probable structure of the 1,3-phenyl-dimeric catalyst 4 is shown in Figure 4.7. This is based on the results of an X-ray crystallographic study in which the conformation of the two cinchona alkaloid units are placed in a direction of anti-relationship to each other. The figure also shows that each cinchona alkaloid unit... 

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