Potential rings

If R and Z, A or X arc connected to form a ring structure the result is a potential ring opening monomer. For many of the transfer agents in this section there are analogous ring-opening monomers described in Section 4.4.2. 

Figure 3 Potential ring closure routes in the synthesis of 1,2,4-thiadiazoles.

Scheme 4 Potential ring conformations in square planar and (rons-octahedral trien complexes...

Evidently, under nucleophilic conditions the potential ring cleavage reaction 25 54 is extremely reversible because the incipient carbanionic function of 54 can never totally leave the influential sphere of the phosphorus center46,59. Again, no hexacoordinate ate-complexes 55 could be identified in these reactions. 

The potential ring-opening of cyclopropane radical cations, breaking the weakened bond of type A radical cations (21 +), has been a subject of both interest and controversy. The ESR spectra of cyclopropane radical cation and its methyl-substituted derivatives decayed at temperatures near 100 K. They were replaced by secondary spectra, in which the protons at one cyclopropane center do not interact with the electron spin. This coupling pattern was interpreted as evidence for a ring-opened trimethylene species (22 +) in which one terminal carbon has rotated into an orthogonal orientation [105, 106, 140]. 

The open analog is again cyclized after oxidation or dehydration by a metabolic enzyme. We deal here with potential rings, which represent nothing more than metabolic precursors of the active species. 

Potential rings in vivo return to the cyclic derivative... 

The conformational flexibility and thus the number of valid 3D molecular models steeply increase from ring size 9 upward. An explicit use of potential ring conformations becomes more and more infeasible. Some of the programs discussed below therefore refrain from generating 3D structures for macrocyclical and poly-macrocyclical structures such as the trimacrocyclical system in Fig. 2. 

In the second synthesis of lycopodine, that of Stork (61), the closure of ring B between C-4 and C-13 was the key step. The preparation of 197 was therefore undertaken with a view of cyclizing it to 198, modifying the potential ring A and the aromatic ring to yield 199 and converting this by established routes into lycopodine. All of these objectives were realized but not without difficulty. 

While we had access to the natural product core 52b, a number of synthetic challenges remained to complete the synthesis, foremost among them, the reduction of the lactone. Conditions to chemoselectively reduce a lactone in the presence of the ester were required. An additional concern was potential ring-opening of the generated lactol 62, which may exist in equilibrium with hfs-aldehyde 63b, a species that would readily undergo reduction to diol 64b, which can potentially lactonize to afford 65 (Scheme 14). 

An alternative to the threading, shpping and clipping routes dealt with so far, is to join two half-threads in the presence of a potential ring. This synthetic method was successfully apphed to the preparation of singly- and doubly-ringed poly-acetylenic threads (LlllO from Lilli Eq. 4.70). Diedrich s cyclophane (L1112) conferred water solubility on these rotaxanes [223]. 

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