Methyl group, oxidation carbonyl

Numerous diamines and aromatic dianhydrides have been investigated. WhoUy aromatic Pis have been stmctiirally modified by incorporating various functional groups, such as ether, carbonyl, sulfide, sulfone, methylene, isopropjlidene, perfluoroisopropyUdene, bipyridyls, sdoxane, methyl phosphine oxide, or various combinations of these, into the polymer backbone to achieve improved properties. The chemistry and apphcations of Pis have been described in several review articles (4). 

In the alcohol oxidations, the sulfonium intermediate (2, nucleophile = R2C(OH)) loses a proton and dimethyl sulfide to give the carbonyl compound (42). The most common mechanism for the decomposition of (2) is attack by a mild base to remove a proton from one of the methyl groups. Subsequent cycHc coUapse leads to the carbonyl compound and dimethyl sulfide (eq. 9) ... 

The solvated phosphorane adds to the polarized carbonyl with the incipient C-21 methyl group pointing away from the bulk of the steroid nucleus. The newly formed carbon-carbon bond must then rotate in order for the tri-phenylphosphine group and oxygen atom to have the proper orientation for the elimination of triphenylphosphine oxide. This places the C-21 methyl in the CIS configuration. 

The removal of angular methyl groups is important in the transformation of steroids and related compounds. In these reactions, the methyl group is oxidized to the aldehyde before fission in which the carbonyl group oxygen is retained in formate (or acetate), and one oxygen atom from dioxygen... 

In contrast, dissimilation of acetate may take place by reversal of the pathway used by organisms snch as Clostridium thermoaceticum for the synthesis of acetate from COj. In the degradation of acetate, the pathway involves a dismutation in which the methyl group is successively oxidized via methyl THF to COj while the carbonyl group is oxidized via bound carbon monoxide. Snch THF-mediated reactions are of great importance in the anaerobic degradation of pnrines, which is discussed in Chapter 10, Part 1. 

Acetate may also be converted into methane by a few methanogens belonging to the genus Meth-anosarcina. The methyl group is initially converted into methyltetrahydromethanopterin (corresponding to methyltetrahydrofolate in the acetate oxidations discussed above) before reduction to methane via methyl-coenzyme M the carbonyl group of acetate is oxidized via bound CO to CO2. 

As for the reaction path from pyruvic acid to citraconic anhydride, it is considered that a condensation reaction first takes place by a reaction between an oxygen atom of carbonyl group and two hydrogn atoms of methyl group in another molecule, followed by oxidative decarboxylation to form citraconic acid. The produced citraconic acid is dehydrated under the reaction conditions used. The proposed reaction path is shown in Figure 7. 

Scheme 12.22 provides some examples of the oxidation of aromatic alkyl substituents to carboxylic acid groups. Entries 1 to 3 are typical oxidations of aromatic methyl groups to carboxylic acids. Entries 4 and 5 bring the carbon adjacent to the aromatic ring to the carbonyl oxidation level. 

Scheme 13.17 depicts a synthesis based on enantioselective reduction of bicyclo[2.2.2]octane-2,6-dione by Baker s yeast.21 This is an example of desym-metrization (see Part A, Topic 2.2). The unreduced carbonyl group was converted to an alkene by the Shapiro reaction. The alcohol was then reoxidized to a ketone. The enantiomerically pure intermediate was converted to the lactone by Baeyer-Villiger oxidation and an allylic rearrangement. The methyl group was introduced stereoselec-tively from the exo face of the bicyclic lactone by an enolate alkylation in Step C-l. 

The stereochemistry of the C(3) hydroxy was established in Step D. The Baeyer-Villiger oxidation proceeds with retention of configuration of the migrating group (see Section 12.5.2), so the correct stereochemistry is established for the C—O bond. The final stereocenter for which configuration must be established is the methyl group at C(6) that was introduced by an enolate alkylation in Step E, but this reaction was not very stereoselective. However, since this center is adjacent to the lactone carbonyl, it can be epimerized through the enolate. The enolate was formed and quenched with acid. The kinetically preferred protonation from the axial direction provides the correct stereochemistry at C(6). 

The teleocidin B4 core 15 is synthesized from the Shiff base of 2-/< //-butyl-5-methoxyaniline, as shown in Scheme 16.161 The key sequence of this synthesis consists of two G-H bond functionalizations, alkenylation and oxidative carbonylation of two methyl groups, via palladacycle formations. 

Cryptopine-type indole alkaloid bumamicine (572) has been synthesized from geissoschizine methyl ether (31) (287). In the first step, geissoschizol (34) was prepared, and then cleavage of the C/D ring fusion was carried out by means of ethyl chloroformate. Finally, C-3 carbonyl and 7V-methyl groups were developed by simple oxidation, reduction, and repeated oxidation steps. 

The first product of the oxidation of alcohol is acetaldehyde and an important end-product is fulminic add, which latter can, however, only be isolated if silver or mercury ions are present. With these ions it forms salts—fulminates—which are stable towards nitric add in them, it must be presumed, the linkage with the metal is homopolar and non-ionogenic, as in mercuric cyanide. The formation of fulminic acid takes place because the carbonyl group of the aldehyde confers reactivity on the adjacent methyl group which then forms a point of attack for the nitrous acid. The various stages in the process are indicated by the following formulae ... 

In the case of the synthesis of cortisone, Oppenhauer oxidation of hydroxyl group at C(14) -after the necessary acetalisation of the unsaturated carbonyl group-leads to intermediate 16a. which, under the basic conditions of Oppenhauer oxidation, spontaneously isomerises to 17a. with the more stable trans-BIC junction. An additional advantage of this sequence is that the carbonyl group activates the C(13) vicinal position, and allows not only the introduction of the methyl group, but facilitates the construction of ring D of the cortisone molecule. 

The reactions unique to the pathway for Methanosarcina thermophila are shown in Figure 11.2 and Table 11.3. In the pathway, the carbon-carbon bond of acetate is cleaved, followed by reduction of the methyl group to methane with electrons originating from oxidation of the carbonyl group to carbon dioxide thus the pathway is a true fermentation. 

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