Bridge bicyclo octane ring

Two possible intramolecular disconnections are available for the [2.2.2] bicyclo-octane ring system (path A and path B, Scheme 1.4). The choice between the initial [4+2] disconnections A and B at first appears inconsequential leading to idealized intermediates of comparable complexity (54 and 57). However, when the [4+2] and [3+2] disconnections are considered in sequence, the difference becomes clear. For path A, retrosynthetic [3+2] disconnection of intermediate 54 leads to the conceptual precursor 56, which embodies a considerable simplification. In contrast, path B reveals a retrosynthetic [3+2] disconnection of intermediate 57 to provide the precursor 59, a considerably less simplified medium-ring bridged macrocycle. Thus, unification of the [3+2]/[4+2] dual cycloaddition strategy, using the staging... 

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. 

The silyl-bridged four-membered rings are not A, B observed in these reactions with aminofluorosilyl-substituted rings. These Li salts react by one route to triply substituted products they also form bicyclo[4.2.0]octanes by an intramolecular reaction with LiF elimination and silyl group and methanide ion migration . Figure 4 shows the structure of compound... 

Bicyclo[3,2,0]heptane and Bicyclo[4,2,0]octane Derivatives.—The bridged bicyclo-[l,l,0]butanes (281), (282), and (283) rearrange thermally to the bicyclo[3,2,0]hept-enes (284) and (285). The rates of rearrangement are most rapid where radical stabilization by bridging functionality is possible, i.e. (283) reacts more rapidly than (282) which is in turn more reactive than (281). Use of substrates with deuterium specifically incorporated at the 6- and 7-positions showed that these positions become the olefinic positions in the newly formed cyclobutene ring. 

Bridge bicyclo[3.2.1]octane ring from quinones... 

Figures 16.8.2(e) and (f) show two other isomeric forms of Teg+. In TcgiWCIe )2> the Teg+ cation is composed of two flve-membered rings, each taking an envelope conformation, with an average Te-Te bond length of 275 pm and a relatively short transannular Te- Te bond of 295 pm. In (Te6)(Te8)(WCl6)4, Teg+ exhibits a bicyclo[2.2.2]octane geometry with two bridge-head Te atoms.

Many bridged ring systems are named by the von Baeyer system. Von Baeyer names are used mostly for bridged ring systems and occasionally for nonbridged ring systems. Examples of von Baeyer names are bicyclo[3.2.1]octane and tricyclo[7.4.1.0 ]tetradecane. 

Bicyclo denotes two rings and octane denotes a total of eight skeletal atoms in the ring system. [3.2.1] gives the sizes of the three bridges connecting two bridgehead atoms. 

In contrast to the results for monocyclic compounds, in bicyclic compounds the rigidity of bridged structure frequently permits more substantial effects than in acyclic or monocyclic compounds. The bridging locks the cyclohexane ring into the boat and/or twist-bond conformation. For example, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have one and two boat cyclohexane rings respectively. The enthalpies of the boat and twist-boat conformers of cyclohexane are 27 and 23 kJ/mol higher than that of the chair conformer. Therefore, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have ring strains of 64 and 54 kJ/mol, respectively, as shown in Table 4. 

The bridged bicyclic systems bicyclo[2.2.1]heptane, also called norbornane, and bicyclo[2.2.2]octane contain locked boat cyclohexane rings. Because of their rigidity, these systems have played an important role in the development of theories of structure-reactivity relationships. 

Similar procedures are also successful in the construction of cyclobutane rings in other stereochemical environments, including the formation of bicyclo[3.1.1]heptane (67)and bridged tricyclic systems such as (68), (69) ° and (70). ° A variety of annelated cyclobutanes have been prepared by the photochemical procedure as above, yields are sometimes poor or only moderate. Examples include bicy-clo[2.1.0]pentanes (71), °° bicyclo[2.2.0]hexanes (72)"° and cyclobuteno aromatics (e.g, 73), " o-nor-steroids e.g. 74)," A-bisnorsteroids" and triterpenes" as well as the highly strained tricyclo[4.2.0.0 ]octane system (75)," and the [4.4.4.5]fenestrane (76)." ... 

Molecules containing a bicyclo[2.2.2]octen-2-one subunit fused or bridged to an additional ring have been investigated in detail, as their oxa-7c-methane rearrangements afford tetracyclic ketones, usually in very good yields. The commonest examples are 7,8-fused substrates 33 which afford 6,7-exo-fused tricyclo[3.3.0.0 ]octan-3-ones 34. 

Gold(I) catalysis has been reported to convert cyclopentenes to cyclohexadienes and involves ring expansion and a [l,2]-alkynyl shift. It is thought to be initiated by the cleavage of the bridging C-C bond and the proposed allyl-gold cation intermediate is trapped with alcohol to give bicyclo[3.2.1]octadiene and tricyclo[3.2.1.0 ]octane derivatives (Scheme 110). ° ... 

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