Bridged bicyclic ring system

In bridged bicyclic ring systems, two rings share more than two atoms. In these cases, there may be fewer than 2" isomers because of the structure of the system. For example, there are only two isomers of camphor (a pair of enantiomers), although it has two chiral carbons. In both isomers, the methyl and hydrogen are cis. The trans pair of enantiomers is impossible in this case, since the bridge must be cis. The... 

Intramolecular cycloadditions are among the most efficient methods for the synthesis of fused bicyclic ring systems [30]. From this perspective, the hetisine skeleton encompasses two key retro-cycloaddition key elements. (1) a bridging pyrrolidine ring accessible via a [3+2] azomethine dipolar cycloaddition and (2) a [2.2.2] bicyclo-octane accessible via a [4+2] Diels-Alder carbocyclic cycloaddition (Chart 1.4). While intramolecular [4+2] Diels—Alder cycloadditions to form [2.2.2] bicycle-octane systems have extensive precedence [3+2], azomethine dipolar cycloadditions to form highly fused aza systems are rare [31-33]. The staging of these two operations in sequence is critical to a unified synthetic plan. As the proposed [3+2] dipolar cycloaddition is expected to be the more challenging of the two transformations, it should be conducted in an early phase in the forward synthetic direction. As a result, a retrosynthetic analysis would entail initial consideration of the [4+2] cycloaddition to arrive at the optimal retrosynthetic C-C bond disconnections for this transformation. 

Bond formation takes place at the end of the diene system to give a bridged bicyclic ring system. 

The final option available to a u-alkylpalladium intermediate from Heck alkylation occurs if another alkene or alkyne function is situated properly to participate in a further Heck-type carbopalladation (equation 161)318,319. In properly constructed systems, more than one further carbopalladation is feasible, and many examples of these cascade car-bopalladations have been reported. Several have been quite spectacular (equation 162)320. Fused, spirocyclic and bridged bicyclic ring systems have been prepared in this manner. The process may also create as many as five rings in one step, with five-,six- and three-membered rings321 being the most suitable for preparation (equation 163). Alternatively, the proper orientation of double and triple bonds allows cyclotrimerization to highly functionalized arenes or fulvenes (equation 164)322,279. 

By replacement of just one oxygen atom in 5 by a methylene group, affinity of the corresponding cryptand 15 to NH4+ decreases by a factor of ca. 100 (NH4+ c 15 log /Ca = 4.3). The macrobicycle 16 has almost completely lost the complexation ability and the selectivity of 5 (NH4+ c 16 log /Ca = 1.7). This dramatic effect results from the removal of one bridge of 5, i.e., from a decrease in cyclic order from the tricyclic to the bicyclic ring system, demonstrating the importance of the spherical macrotricyclic structure for the binding properties of 5. 

In general, high levels of diastereoselectivity are observed for 5-evo-cyclisa-tions leading to both fused and bridged bicyclic ring systems. For example,... 

Bicyclic ring systems have rings that are connected by bridges. 

Two possible alcohols might be formed by hydroboration/oxidation of the alkene shown. One product results from addition of BH3 to the top face of the double bond (not formed), and the other product results from addition to the bottom face of the double bond (formed). Addition from the top face does not occur because a methyl group on the bridge of the bicyclic ring system blocks approach of the borane. 

A substituent in a bicyclic ring system is endo if it is syn to the larger of the other two bridges. 

A bicyclic ring system in which the two rings share non-adjacent carbon atoms is called a bridged ring system. 

Fused and bridged bicyclic ring systems are compared in Figure 16.10. 

Bicyclic ring systems—compounds that contain two rings that share one or two carbon atoms-can be bridged, fused, or spiro. 

Bredt s rule (Julius Bredt, 1924) is a qualitative generalization that bridged, bicyclic ring systems cannot have a double bond at the bridgehead. This rule does not apply to fused or large-member ring systems. 

Samarium(II) iodide-promoted radical cydizations have also played a key role in the total synthesis of (-)-grayanotoxin III [43b]. Among the methods utilized for the synthesis of this molecule was an intramolecular ketyl olefin coupling reaction generating a bridged bicyclic ring system (Eq. 68). 

When we treat a molecular model of cw-l,3-dimethylcyclopentane in the same way, we obtain another carbon skeleton, that of norbomane. Norbomane is an example of a bridged bicyclic ring system. In bridged bicyclic systems, two nonadjacent carbon atoms, the bridgehead carbons, belong to both rings. 

Bicyclic ring systems may be fused or bridged. Fusion can be cis or trans. 

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