Oxido bridge

The H NMR spectrum of 7.80 was also recorded by Elix. While it was of insufficient quality to allow an extract assignment of structure vide supra), it did allow Elix to conclude that, as expected given its 4n n-electron formulation, compound 7.80 is not aromatic as judged by the absence of any substantial macrocyclic ring-current effects. This conclusion reached, it should be noted, that the formally 4n + 2 pentaoxa[30]pentaphyrin-(2.2.2.2.2) isomers also failed to display any kind of substantial ring-current effects. This led Elix to conclude that there is no fundamental difference in electronic character between large 4n and 4n + 2 7r-electron oxido-bridged annulenes. ... 

Dauben used a related cyclization-cycloaddition approach for the synthesis of tigliane 22. Carbonyl ylide 21, derived from the diazo ketoester 20, underwent intramolecular cycloaddition to form 22, a molecule which contains the C(6), C(9)-oxido bridge of the tigliane ring system (Scheme 5) (93JOC7635). 

Sz/w-1,6 8,13-bridged [14]annulenes having two oxido bridges 1180, 188], one oxido and one methane bridge [189] and one amido and one raethano bridge [190] have also been prepared but not their anti--isomers. These compounds are thermally stable, and their n.m.r. spectra show that they are diatropic. The 1,6-methano-8,13-oxido compound is acetylated by acetic anhydride in the presence of boron trifluoride to give a mixture of 2-, 9- and 10-acetyl derivatives [189] the dioxido compound has been acetylated, brominated and nitrated, in each case providing the 2-substituted product [191]. 

Another type of oxido-bridged annulene which has been prepared... 

With the A-ring effectively disassembled through this strategy, the trans disposition of the vicinal C-10 and C-4 carbon chains on the seven-membered B-ring in 9 now seemed ripe for removal through sequential, stereocontrolled addition reactions to an a,P-unsaturated intermediate such as 10. In this regard, the previously installed oxido bridge would need to play a crucial role for the overall suc-... 

In the parlance of 1,3-dipolar cycloadditions of this type, the endo isomer is readily identified as that in which the substituents on the dipolarophile end up anti to the oxido bridge. Accordingly, the exo isomer is the one in which these substituents are syn to the resultant oxygen bridge. 

In addition to these fundamental contributions, the developed synthesis is also noteworthy for its careful command of stereochemistry throughout its prosecution, as achieved by the combination of asymmetric reactions and exquisite understanding of the inherent topology of the molecule to introduce new steieogenic centers through substrate-controlled events. In particular, the use of an oxido bridge during the majority of the route to ensure conforma-... 

FIGURE 9.13 Nucleophilic addition to key intermediates and elimination of the oxido bridge. Copyright 2006 Taylor Francis Group, LLC... 

Recently, Hulsken et al. have employed STM to probe the mechanism by which manganese porphyrin complexes achieve epoxidation of alkenes with dioxygen [69]. Importantly, this study demonstrates the positive role of solid-support immobilization in precluding the formation of inactive oxido-bridged catalyst dimers (a common deactivation pathway). 

In Fig. 21.23, the oxido-bridge atom lies on an inversion centre. Explain what this means. [Ans. See Section 3.2]... 

D. Epoxides, Ether- or Oxido-Linkages, etc. Epoxides and other oxido-bridges are indicated by a code (e.g. 3/3(—O—)4/3) specifying the two centers of attachment and, if known, the orientivity of attachment. 

The presence of a 22,25-oxido bridge (tetrahydrofuran on the side chain), contrary to the original assumption, is not an artefact produced during the hydrolysis of the saponin (Burnell and ApSimon, 1983). 

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