Hydrogen transannular

A number of chemiluminescent reactions may proceed through unstable dioxetane intermediates (12,43). For example, the classical chemiluminescent reactions of lophine [484-47-9] (18), lucigenin [2315-97-7] (20), and transannular peroxide decomposition. Classical chemiluminescence from lophine (18), where R = CgH, is derived from its reaction with oxygen in aqueous alkaline dimethyl sulfoxide or by reaction with hydrogen peroxide and a cooxidant such as sodium hypochlorite or potassium ferricyanide (44). The hydroperoxide (19) has been isolated and independentiy emits light in basic ethanol (45). 

Large ring heterocyclic radicals are not particularly well known as a class. Their behavior often resembles that of their alicyclic counterparts, except for transannular reactions, such as the intramolecular cyclization of 1-azacyclononan-l-yl (Scheme 1) (72CJCH67). As is the case with alicyclic ethers, oxepane in the reaction with r-butoxy radical suffers abstraction of a hydrogen atom from the 2-position in the first reaction step (Scheme 2) (76TL439). 

No product derived from the transannular hydrogen abstraction is observed in the addition of bromotrichloromethane because bromine-atom abstraction is sufficiently rapid to prevent effective competition by the intramolecular hydrogen abstraction. 

By using various trapping reagents, it has been deduced that the transannular fragmentation is rapidly reversible. The cyclization of the fragmented radical C is less favorable, and it is trapped at rates which exceed that for recyclization under most circumstances. " Radicals derived from ethers and acetals by hydrogen abstraction are subject to fragmentation, with formation of a ketone or ester, respectively. 

Whereas exo-norbornene oxide rearranges to nortricyclanol on treatment with strong base through transannular C-H insertion (Scheme 5.11), endo-norbornene oxide 64 gives norcamphor 65 as the major product (Scheme 5.14) [15, 22]. This product arises from 1,2-hydrogen migration very little transannular rearrangement is observed. These two reaction pathways are often found to be in competition with one another, and subtle differences in substrate structure, and even in the base employed, can have a profound influence on product distribution. 

Transannular shifts of hydrogen atoms have also been observed. 

Torsional strain and van der Waals repulsions between hydrogen atoms across rings (transannular strain) cause the small instabilities of these higher cycloalkanes. 

C1, C19, C26 pharmacophoric element C3, C11 transannular hydrogen bonding C7, C20 tunable substituents C8 gem-dimethyl crucial for bryostatin-like biological respose... 

Similar low activities were found in the hydrogenation of 1-octene [47]. The use of [Ni(PPh3)2I2] in the hydrogenation of norbomadiene resulted in considerable amounts of nortricyclene, via transannular ring closure, whereas 1,5-cyclo-octadiene yielded bis-cyclo-[3.3.0]oct-2-ene. According to these authors, the re-... 

Structural Analysis. H2[pz(A4)] A = dispiroketal (190) has been structurally characterized, Fig. 40, and is shown to exhibit a noncrystallographic D2 symmetry the central core is planar to within 0.08 A. The presence of hydrogen atoms on the nitrogen centers distorts the potential D4 symmetry of the non-hydrogen atoms The transannular porphyrin N—N distances differ by-0.15 A (11). 

Substituents larger than hydrogen atoms cannot generally occupy intraannular positions. We have seen about transannular reactions which are special distinctive feature of medium-sized rings. The transannular reactions do not occur at the carbon atom of the chain and they also do not involve the neighbouring atoms. They take place between atoms on opposite sides of the ring. 

Another example of a transannular cyclization that occurs in the solid state is provided by the epoxy alcohol 31. This compound is stable when dissolved in organic solvents and in 0.25N sulfuric acid. However, the crystals transform rapidly to 32. Although the process is accompanied by partial melting, it appears to be a true solid-state one. Interestingly, the reaction is slowed down appreciably when the dry crystals are covered with ether. Hydrogen bromide is eliminated in the reaction and it may be that an acid-catalyzed process is also occurring in the presence of solvent this process may be slowed down by the dissolution of the decomposition products in the solvent (77). 

The larger (Z,Z)-l,5-cyclononadiene (169) reacts141 stereoselectively with PhSeCl in AcOH to give the substituted hydrindan 170 (equation 138). In consideration of the anti addition mode of selenenyl reagents to double bonds, the transannular reactions of 169 have been rationalized on the basis of the two reaction intermediates, 171 or 172, which are liable to place the PhSe- and AcO- groups in a cis- 1,4-relationship and trans to the bridgehead hydrogen (equation 139). The preferential formation of 170 has thus been attributed to the fact that the pathway via 172 should involve a boat transition state. 

The isodrine framework was used for synthesis of pagodane in which one of the key steps is Mackenzie s transannular dyotropic hydrogen transfer101. Also, a very intriguing ... 

Ionic hydrogenation of the same bicyclic diene 382 by Et3SiH in the presence of CF3COOH at room temperature or at 80 °C via ions 387 and 388 is accompanied by transannular cyclizations (equation 139)192. The behavior of diene 382 under Ritter reaction conditions (MeCN, H2SO4) reveals new possibilities to control the transannular cyclizations (equation 140)193. Depending on the sulfuric acid concentration, the reaction temperature and the presence of a nucleophilic solvent, these transformations can be directed to the formation of either the bicyclic amides 389 and 390 having the precursor structure or the tricyclic products 391193. 

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