Bridged carbocyclic systems

In contrast, the closely related palladium acetate-promoted intramolecular alkylation of alkenes by tri-methylsilyl enol ethers (Scheme 4)6,7 has been used to synthesize a large number of bridged carbocyclic systems (Table 1). In principle, this process should be capable of being made catalytic in palladium(II), since silyl enol ethers are stable to a range of oxidants used to carry the Pd° -> Pd11 redox chemistry required for catalysis. In practice, catalytically efficient conditions have not yet been developed, and the reaction is usually carried out using a full equivalent of palladium(II) acetate. This chemistry has been used in the synthesis of quadrone (equation 2).8 With the more electrophilic palladium(II) trifluoroace-tate, methyl enol ethers underwent this cyclization process (equation 3).9... 

Voliime 6 reviews the literature published during 1976. The format of the earlier volumes is retained, with individual chapters on three-membered rings four-membered rings five- and six-membered rings and related fused systems medium-and large-ring compounds and bridged carbocycles. 

For unactivated carbocyclic systems [14], 6/5 cychzation proceeds more readily than 6/6 cychzation. For activated systems in which the electron-withdrawing group is at the end of the dienophile away from the bridge [15], there seems to be httle preference for 6/5 over 6/6. Finally, for activated systems in which the electron withdrawing group is at the end of the dienophile toward the bridge [16], or is included in the bridge [17], 6/6 cychzation is much more facile than 6/5 cychzation. 

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. 

Polycyclic parent hydrides. These are classified as bridged polyalkanes (also known as von Baeyer bridged systems, from the nomenclature system developed to name them), spiro compounds, fused polycyclic systems and assemblies of identical rings. The four systems may be either carbocyclic or heterocyclic. In developing their names, the following principles are used. 

The methylene-bridged bisdehydroaza[19]annulene (215), a hetero[4w— l]annulene with Ann electrons, has been synthesized . Comparison of the H-n.m.r. spectrum of 215 with that of the open-chain analogue 214 indicates that 215 is a paratropic 20tc electron system just as carbocyclic [4 i]annulenes . 

In order to facilitate comparisons between different rings systems, the base components indene, indacene, pentalene, and naphthalene have been chosen for the design of the carbocyclic pattern of the tricyclic assembly (Table 2). Of these broad classes of structures, [/lannelated- (1), [e]annelated-(5), [a]annelated- (2), and [c,d]annelated-indenes (6) have been the most completely explored, less being known of (l,4)-methano-naphthalenes (7), while [c]annelated- (3), [bridged systems (8-10) and the (3a,6a)-butano-pentalenes (11) have been scarcely mentioned, if at all, in CHEC-I. 

Bredt s Rule is further assessed in a report by Shea, which covers both carbocyclic and heterocyclic bridged systems. 

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