Syn-periplanar arrangement

The stereochemistry at C-20 does not affect the reaction. However, the 16j5-mesyloxy analogs give poor yields of fragmentation product, as would be predicted from the syn periplanar arrangement of the bonds involved. 

The dihedral angle for the C-H and C-X bonds equals 0° for the syn periplanar arrangement and 180° for the anti periplanar arrangement. 

In those cases where the developing anion cannot attain antiperiplanarity to a good acceptor, lesser but still significant stabilization can be provided by the syn-periplanar arrangement. 

The appertaining stereodescriptors depend on whether the syn-periplanar arrangement of connecting lines 1-2 and 3-4 (corresponding to a superposition of planes 1,2,3 and 2,3> 4) requires a right (P) or left (M) turn around axis 2-3. 

Effects that arise because one spatial arrangement of electrons (or orbitals or bonds) IS more stable than another are called stereoelectronic effects There is a stereoelec tromc preference for the anti coplanar arrangement of proton and leaving group in E2 reactions Although coplanarity of the p orbitals is the best geometry for the E2 process modest deviations from this ideal can be tolerated In such cases the terms used are syn periplanar and anti periplanar... 

Periplanar (Section 11.8) A conformation in which bonds to neighboring atoms have a parallel arrangement. In an eclipsed conformation, the neighboring bonds are syn periplanar in a staggered conformation, the bonds are anti periplanar. 

An anti-periplanar arrangement of C-Br and C-H is attainable with a vinylic bromide too, provided the Br and H are trans to one another. E2 elimination from the Z isomer of a vinyl bromide gives an alkyne rather faster than elimination from the E isomer, because in the E isomer the C-H and C-Br bonds are syn-periplanar. 

One of the steric requirements of E2 elimination is the need for periplanar geometry, which optimizes orbital overlap in the transition state leading to alkene product. Two types of periplanar arrangements of substituents are possible — syn and anti. 

Hyperconjugation of C H Bonds with C H Bonds. Using hybridised orbitals for C H bonds, and mixing them in the usual way to show conjugation, creates the molecular orbitals of Fig. 2.13, which is set up for the anti-periplanar interaction. There is an equivalent set of orbitals interacting in a syn-coplanar arrangement, the relative merits of which are discussed on pp. 98-100. 

The studies reveal [8] that the delivery of 9-BBN from 3a to an aldehyde shows a greater trans-selectivity regardless of the actual structural arrangement of H-C-C-B array (i.e. syn periplanar or gauche). 

There are two ways in which the C—H and C—X bonds can be parallel they can be either on the same side of the molecule—an arrangement called syn-periplanar—or on opposite sides of the molecule—an arrangement called anti-periplanar. 

As discussed above (see Section 4.10, The Importance of Pyranose to Fura-nose Interconversion, page 75) current thinking requires that we consider saccharidic forms where a 5yn-periplanar arrangement of the anomeric hydroxyl pair can be attained. Generally, this requires formation of the furanose form of the saccharide. However, computational work has shown that in the case of D-galactose the a-D-furanose form of the saccharide is not the only species that can be considered with a syn-periplanar alignment of the anomeric hydroxyl pair (Figure 32). 

Both arrangements allow n overlap of indpient parallel 2p orbitals as the C bonds are broken. The more common anti periplanar geometry corresponds to the staggered anticonformation, which is easily achieved in conformationally flexible molecules. The syn periplanar geometry corresponds to an eclipsed conformation, which is important only in some rigid, cyclic compounds. 

The stereoelectronic effect reflects the geometry of the developing 7i bond in the transition state for the reaction. The anti periplanar arrangement is favored because it aligns the O orbitals of the sp -hybridized C—H and C—X bonds so, they can partially overlap as they become the 2p n orbitals in the product (Figure 9.8). A similar argument holds for the syn periplanar transition state. 

Z and E rotamers result from rotation around the CO—NH bond, with the Z arrangement being the predominant one. The Ca—CO rotation generates the two usual conformers for this type of system sp (OMe and carbonyl in a syn-periplanar disposition) and ap (OMe and carbonyl in an anti-periplanar disposition) (Figure 106). In both the sp and ap forms, the OMe is anti, with respect to the Ca—Ar bond, and the phenyl group is coplanar to the Ca—H bond. 

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. 

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