Transannular migration

The decisive step in this reaction is the transannular migration of the N-oxide oxygen atom (86+87). Such an intramolecular attack seems quite plausible, since model studies demonstrate that the oxygen atom of the N—0 group in 86 can approach within 2.6 A of the pseudo-geminal position in the other benzene ring. Analogous intermolecular reactions, such as oxidation of phenols to quinones by N-oxides, are known 87>. 

The polymerisation of monocyclic non-conjugated diolefins, which takes place by the double bond opening with simultaneous transannular migration, leads to cyclopolymers having bicyclic repeating units present in the main chain. A typical example of such cyclopolymerisation is the polymerisation of 1,5-cyclooctadiene [31,32] ... 

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. 

In the reaction of disubstituted alkynes, 1,3-migration of the acetate takes place to give allenyl esters that can be versatile substrates, especially for [3,3]-Cope rearrangements.333 1,5-Enynes have proved to be versatile substrates for the preparation of perfumery agents such as sabinol334 and sabina ketone.335 Transannular systems undergo similar reactions.336... 

Transannular hydrogen migrations occur readily in medium rings. An example is Equation 9.113.1"... 

Photolysis of the 1,2,5,6-tetrathiocine 31 promoted a reversible, transannular sulfur migration to give 1,2,3,6-tetrathiocine 32 (Equation 7) <1998CJC1093>. The photoisomerized product 32 was readily distinguished by the 19F NMR spectrum, which exhibits an AMXY pattern. 

Several instructive iUnstrations taken from the field of natural products will serve to conclude the Bsent discussion of acid-catalyzed epoxide isomerization. Mwdi has been said about the ooeurrenoe of simple i,2-hydride transfers in boron trifluoiide-CBta yzed epoxy steroid rearrangements. Although initiated by a mineral acid rather than a Lewis acid, an instance of transannular hydride transfer has been reported for a 9/3,11 -epoxy steroid, as shown in Eq. <484. Such migrations can in principle occur elsewhere. 

Transannular 1,5-hydride shifts are often seen across medium-sized (8-, 9-, and 10-membered) rings. The atoms directly across from each other in these rings are quite close together, so migration is facile. In general, though, most cationic rearrangements can be accounted for by a series of 1,2-shifts. 

Double bonds can also migrate transannularly if the conformative requirements are fulfilled. Characteristic examples are the [1,2,(3)7]-, and [l,2,(3)8]-eliminations of 17291) and 7 73 98) to give the bicyclic products 156 (12%) and 174 (14%). The [l,2,(3)5]-elimination type can be realized with cycloalkenes that have a suitable side chain. These complex eliminations succeed thermally, under the influence of bases,... 

A remarkable transannular phenyl migration occurs in the tricyclic system (380) which hydrolyzes to products derived from the allylic cation (381)3l5 Aryl-assisted ionization gives rise to a cyclopropyl cation which undergoes ring opening to (381). Alkyl shift predominates in the epimeric tosylate (382) although some leakage toward (381) is also found. 

Preparation by ring expansion can be contrasted with that of ring contraction recently reported, whereby 1,6-benzothiazocine derivatives (79) undergo ring contraction in the presence of lead tetraacetate to give the 2H-1,4-benzothiazine derivatives 80 and 81. The proposed mechanism for the reaction involves a transannular sulfur migration. The structure of 81 was confirmed by X-ray crystallography.123... 

The 5 -perhydroazulene structure (524) is accessible by solvolysis of a la-mesylate (523)." The comparable reaction in the 5a-series requires solvolysis of a 1 -substituted steroid. In each case the migrating C jj-C o) bond has an onti-periplanar relationship to the leaving group. Related perhydroazulenes (526) are obtained by transannular cyclisation of the 5,10-seco-steroid (525). ... 

Intramolecular insertion reactions lead to rearranged structures and are equivalent to a 1,2-migration of hydrogen. Alkylcarbenes almost invariably react by intramolecular C-H insertion to give cycloalkanes, which is transannular in the case of cycloalkylidenes, or by rearrangement to give alkenes (Scheme 5.43). 

Through-space pinacol rearrangements have also been observed. When paracyclophane 18, a diol with hydroxyl groups in pseudo-vicinal proximity, was exposed to acid, a surprising transannular pinacol-like hydride migration occurred to yield ketone 19. ... 

The seventh item in Table 8.7 is a transannular (across the ring) pinacol-pinacolone rearrangement in which it can be argued that a carbocation forms and that the hydride ( H, deuteride) shift occurs with participation of the oxygen after the tertiary hydroxyl is lost. As shown in Scheme 8.82, the transannular hydride ( H deuteride can be quite close to the carbocation (or incipient carbocation if loss of water is accompanied by hydride ( H, deuteride) migration. 

Scheme 8.82. A representation of a pinacol-type rearrangement involving a transannular 1,6-hydride ( H, deuteride) migration in a 10-membered ring.

A hydrogen from across the ring has migrated to yield the products. This kind of a migration is referred to as a transannular shift. In Chapter 2 transannular strain (Figure 2.13) was discussed. It arises from steric repul-... 

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