Intramolecular bridging, conducting

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. 

Now, as we have defined wire-like transport by a motion that is assisted by molecular bridges, we may proceed to the mechanistic point of view. In particular, we will contrast theoretical results with experimental data for molecular wire-like behavior in regard to the transfer of electronic charge and/or energy. Intramolecular electron-transfer (ET) rate constants characterize the charge transport in DBA conjugates and in electronic transport junction we can apply the word conductance . 

Mechanism. The electron-transport pathway of nitrite reductase begins with the type 1 copper center of pseudoazurin, continues to the type 1 copper center of nitrite reductase, and from there to its type 2 copper center. This last electron transfer is not conducted directly from copper to copper, but via an intramolecular Cysl36-Hisl35 bridge, similar to the one proposed for ascorbate oxidase [26,208] (Fig. 35). 

Bis-stilbenes connected with an ethane bridge (23) behave similarly to 22 (Scheme 9). The ZZ isomer undergoes almost exclusive one-way isomerization, while the ZE and EE isomers conduct two-way isomerization in the singlet excited state on direct irradiation [99]. The one-way ZZ->ZE isomerization is thought to occur by way of intramolecular excitation transfer from the ZE, where the E chromophore is excited, to Z E, where the Z chromophore is excited. 

Consider the case in which a DBA assembly acts as the molecular wire. There are two mechanisms for the current to pass from one electrode to the other. In the chemical mechanism D transfers an electron to A through the bridge. The newly formed D" " and A are then rapidly reduced and oxidized, respectively, at the electrodes, giving rise to a steady-state situation in which the current is determined by the rate of intramolecular electron transfer from D to A. The conductance via this pathway may be estimated from... 

Williams and co-workers have reported a remarkable reversal of regio-selectivity in the intramolecular Diels-Alder reaction of bis-diene substrate 6.25 in connection with efforts toward the total synthesis of ilicicolin (6.23) [77]. They found that heating a toluene solution of 6.25 (R = Et) at 165°C in a sealed tube resulted in the production of regioisomeric cycloadducts 6.26 and 6.27 in the ratio of 3/1 (Scheme 1.12). However, when this Diels-Alder reaction was conducted in boiling water, the product ratio was reversed in favor of 6.27, the highest selectivity (6.27/6.26 = 8/1) being observed when the free carboxylic acid (R = H) or its sodium salt (R - Na) were used. The formation of both cycloadducts is consistent with exo-bridged Diels-Alder transi-... 

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