Carbon-oxygen bond formation reductions

The carbon-oxygen bond formation follows the same pathway. For both nitrogen-carbon and oxygen-carbon bond formation, a competing reaction is 13-hydride elimination (if a hydride is present at the heteroatom fragment), which lowers the yield and the reduced arene is obtained after reductive elimination. Reductive elimination of the C-N or C-0 fragments should be faster than 13-hydride elimination in order to avoid reduction of the aryl moiety. The side-reaction is shown at the bottom of Figure 13.25. 

Carbon-Oxygen Bond Formation The cathodic reduction of some nitrocarhonyl compounds in aqueous acidic medium gives the hydroxylamino derivatives that can undergo a ring-closure reaction affording anthrandic compounds or isoxazolones [102-104] (Schemes 70 and 71). 

Creation of the synthon from the retron. The user carries out on the retron structure the corresponding changes to elaborate the synthon. For example, if we want to convert, in retrosynthetic direction, an alcohol into a carbonyl (the equivalent to a reduction reaction), an alcohol group has to exist in the retron, and the user must indicate "the formation (retrosynthetic) of a carbon-oxygen bond". 

A rather more complex tertracyclic indole based compound lowers blood pressure by selective blockade of a 1-adrenergic receptors. Reaction of the anion from indole (72-1) with butyrolactone (72-2) leads to the scission of the carbon-oxygen bond in the reagent and the formation of the alkylated product (72-3). The acid is then cyclized onto the adjacent 2 position to give the ketone (72-4) by treatment with a Lewis acid such as polyphosphoric acid. Reaction with bromine then leads to the brominated ketone (72-5). This is subjected to reductive alkylation with ethylene... 

Aromatic ketones represent a rather special case in dissolving metal reductions. Under many conditions pinacol formation is the predominent reaction path (see Volume 3, Chapter 2.6). Also, the reduction potentials of aromatic carbonyl compounds are approximately 1 V less negative than their aliphatic counterparts. The reductions of aromatic ketones by metals in ammonia are further complicated by the fact that hydrogenolysis of the carbon-oxygen bond can take place (Chapter 1.13, this volume) and Birch reduction may intervene (Chapter 3.4, this volume). 

The synthetic uses of hydrazones and oximes for carbon-bond formation differ little from those of imines as their anions represent enolate equivalents except for certain methods that have been developed for asymmetric induction. Conversely, the formal replacement of the carbon substituent of imines by a heteroatom (nitrogen for hydrazones and oxygen for imines) opens reaction pathways such as the Beckmann rearrangement of oximes and the Wolff-Kishner and Shapiro reductions of hydrazones that have no analogy in the chemistry of imines. 

On the other hand, ethylene-TPD on p UO3 indicated the desorption of acetaldehyde (490 K). In addition, an unexpected product was also observed. This product was identified as furan (C4H4O, m/e 68, 39) which desorbed at ca. 550 K with a carbon selectivity of ca. 40 %. Furan formation from ethylene on UO3 requiring the formation of one carbon-carbon bond and of one carbon-oxygen bond, is most likely accompanied by oxygen depletion from the UO3 surfaces and subsequent reduction of U cations into lower oxidation states. The observation of furan from ethylene shows that one may obtain oxygenated products with a high carbon number from ethylene (a relatively abundant feed stock) via one single step. 

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