Rigid bicyclic systems

Whenever we have a structure where the wedges and dashes are implied but not drawn, it is much easier to use this method. There are other examples of carbon skeletons that, by convention, do not show the wedges and dashes. Most of these examples are rigid bicyclic systems. For example. 

Answer This is a rigid bicyclic system, and the dashes and wedges are not shown. Therefore, we will use the second method for drawing enantiomers. We will place the mirror on the side of the compound, and draw what would appear in the mirror ... 

While structural evidence indicates that radical 44 is delocalized through the five carbon centers, it has been suggested that annelation of the cyclopentadienyl framework with rigid bicyclic systems leads to localized structures, as in compound 45 (Fig. 7.20). The X-ray structure of 45 indicates that the rather long distances between C1-C2 (1.445 A), C1-C5 (1.442 A), and C3-C4 (1.482 A) correspond to... 

P-Cleavage of homoallylic alcohols.1 Homoallylic tertiary potassium alkox-ides undergo cleavage in HMPT (or DMF) of the allylic C—C bond to give the enolate of a ketone (equation I). A rigid bicyclic system is not essential for the cleavage. 

Marchand, A. P. Stereochemical Applications of NMR Studies in Rigid Bicyclic Systems Verlag Chemie International Deerfield Beach, Florida, 1982. 

Proof that a site incapable of undergoing inversion is also incapable of undergoing a second-order substitution reaction has been obtained from bicyclic compounds. The bridgehead carbon of rigid bicyclic systems cannot invert without fragmenting the molecule, and indeed, compounds such as 1-bromotripty-cene (2) and 7,7-dimethyl-[2.2.1. ]-bicycloheptyl-T/i-toluencsulfonate (3) are completely inert when treated with a nucleophile under conditions.14... 

A. P. Marchand, N. W. Marchand, and A. L. Segre, NMR studies of rigid bicyclic systems. II. Evidence for the nonequivalence of exo-exo and endo-endo coupling constants in 7-substituted-l,4-dichloro-2,2,3,3-tetradeuterionorbomanes, Tetrahedron Lett., 59 (1969) 5207-5210. 

Marchand, A. P. In Stereochemical Applications ofNMR in Rigid Bicyclic Systems, Methods in Stereochemical Analysis Verlag Chemie International FL, 1982 Vol. 1. 

Longer-range couplings, those that involve more than three bonds, are common in systems with al-lylic hydrogens (Section 5.7) or aromatic ring hydrogens (Section 5.11) and in compounds that are rigid bicyclic systems (Section 5.13). 

The advantages of the norbornane system are accompanied by disadvantages the skeleton asymmetry explains the difference in the behaviour of e.g. 2-exo-and 2-endo-sulphonates by the different influence of the steric factors in the ground and transition states on the ionization of these epimers, especially as the steric effects are more essential for rigid bicyclic systems than for flexible aliphatic or alicydic substrates... 

As noted above, the skeletal rearrangements in the 8, 1 reactions and the stereospecificity in the formation of reaction products are essential criteria of the intermediate formation of nonclassical ions. Nevertheless the results obtained can be explained by assuming the C—J bond ionization to proved with a simultaneous shift of the a-bond C —C in the isomer 350 and of the a-bond C —C in the compound 351. The structures 347 and 348 must be transition states of ionization processes. The authors of , however, consider this assumption less probable because the synchronous process in question is a reaction of the 8, 2 type at the bridged carbon atom and for the bridged atoms of rigid bicyclic systems such processes are known to be strongly hindered. The formation of ion 318 with bicyclo[3,2,I]octadiene skeleton consecutively from ion 347 rather than directly from compound 350 with the geometry of bicyclo[2,2,2]octadiene is also in fuller accordance with the principle of so-called least motion " 29)... 

In rigid bicyclic systems cis-trans isomerization is not a factor, and so the (tt,tt ) state displays efficient di-ir-methane rearrangement photochemistry (Eq. 16.45). What about the (tt,tt ) state The reason the di-rr-methane rearrangement is often not seen is that other, more conventional reactions such as [2-1-2] photocycloadditions predominate. It is not that the di-TT-methane rearrangement is not feasible, but rather that other reactions are more facile. 

The biosynthesis of the aminoglycoside antibiotics has been reviewed. Apramycin, a broad-spectrum aminoglycoside antibiotic produced by a strain of Streptomyces tenebrarius, has been assigned the structure (361), which contains residues of 4-amino-4-deoxy-D-glucose and an octadiose that exists as a rigid bicyclic system. The structure (361), which was first derived from chemical and spectroscopic evidence, was confirmed by A"-ray crystallographic analysis. 

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