Carbonyl groups, by oxidation

Surprisingly, some polymers which have no chromophores in their structure such as PVC, polyethylene, poly (methyl methacrylate), nylon and polystyrene are degraded by exposure to radiation with wavelengths around 200nm, undergoing discolouration and weakening. This may be attributed to the formation of chromophoric carbonyl groups, by oxidation. Impurities or additives in plastics may also contribute chromophores. 

Two routes were originally developed for the synthesis of intermediate diketone 18 ( R= butyl).One involved introduction of the two carbonyl groups by oxidation of the corresponding dibutyl heptacyclic terpyridine. The other route is similar to that shown in Fig. 2, except that 14 was prepared by pyrolysis of the trimethylhydrazonium derivative of 12. An improved method for this 1/3+1/3 coupling reaction involves condensing the HCl salt of Mannich derivative 13 with ketone 12 and ammonium acetate, giving heptacyclic terpyridyl 14 in 70% yield. The synthesis of 9-butyloctahydroacridine (9) and its conversion to N-oxide... 

Thus, oxidation of the rhenium complex, [Re(OD)3Cp ], with four equivalents of dimethyldioxirane (determined by spectroscopic titration) in anhydrous acetone solution yields IReC Cp ]. Both K and GC analyses of the liberated gas indicated it to be a mixture of carbon monoxide and carbon dioxide. The first step of the reaction is likely to be the loss of a single carbonyl group by oxidative elimination, Eq. 8.58 ... 

All of these ehimnddon reacdons contain fi-carbonyl groups in the nltro compounds Of course, masked carbonyl groups are also frequently employed for such fi-elimination of HNO, as shown in Eq 7131, Eq 7 133, and Eq 7 133In these cases, the sulfinylmethyl or hydroxymethyl group is converted into the carbonyl group by the Pummerer rearrangement or by simple oxidation... 

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. 

The allylic alcohols that are the initial oxidation products can be further oxidized to carbonyl groups by Se02 and the conjugated carbonyl compound is usually isolated. If the alcohol is the desired product, the oxidation can be run in acetic acid, in which case acetate esters are formed. 

Nitro group in an oxime is reduced preferentially to the oximino group with ammonium sulfide [240]. In a monoxime of a diketone, the oximino group is reduced to amino group to the exclusion of the carbonyl group by catalytic hydrogenation over platinum oxide in methanolic hydrochloric acid 9-keto-10-oximino-1,2,3,4-tetrahydrophenanthraquinone afforded 10-amino-9-keto-l,2,3,4-tetrahydrophenanthrene in 78% yield [949]. 

Addition of LiBr or LiCl to a solution of Sml2 in THF causes a color change from blue to purple. Oxidation potential of Sml2 in THF changes from —1.33 V to —1.98 0.01 V upon addition of I2 or LiBr (more than 1 equiv.), or to —2.11 0.01 V by addition of 12 or more equiv. of LiCl. In the presence of 4-12 equiv. of the bromide or chloride salt, the pinacol coupling reaction of cyclohexanone is accelerated. These salts should be dried before use otherwise, simple reduction to cyclohexanol occurs. The co-existing lithium cation can also act as a Lewis acid to activate the carbonyl group by coordination. ... 

In die -elimination reactions discussed next, the electron-withdrawing groups are first introduced chemically into the polysaccharide this may be done by esterification of a carboxylic acid group, by oxidation of an alcohol group to a carbonyl group, or by nucleophilic-displacement reactions. 

An alternative and more generally used oxidation method employs chromic acid. This process is an exception to our general theme, because here the alcohol is transformed to a carbonyl group by removal of electron density from oxygen rather than from carbon. The first step has been shown to be a rapid equilibrium between the alcohol and its chromate ester, followed by rate-determining decomposition of the ester in the manner shown in Scheme 7.42 It will be noted that the species eliminated from the carbon that becomes the carbonyl carbon is a Lewis acid, not a Lewis base. 

An initial addition of a ruthenium-oxygen double bond to a a-C—H bond leads to an intermediate containing a carbon-ruthenium bond. This bond suffers a homolytic scission leading to a carbon radical, which is oxidized to a carbocation that provides a carbonyl group by deprotonation. 

Mithramycin shows a completely P-linked chain of D-conflgurated saccharides. This requires a totally different approach for the synthesis which is also done by application of the DBE method. The previously obtained disaccharide 180 is P-glycosylated with the monosaccharide precursor 176 to give the trisaccharide 185. After reductive debromination (Bu3SnH), an acid deformylation deblocked the C-3" position which is oxidized with pyridinium dichromate. Nucleophilic attack at the carbonyl group by methyl lithium affords a 1 1.2 mixture of 186 and 187 none of which is the desired compound [93]. Obviously, the methyl branch is formed exclusively in the axial way. 

One of the earliest attempts to prepare analogues of FA as potential inhibitors involved the synthesis of 2-amino-4,7-dihydroxypteridine-6-carboxylyl-p-ami-nobenzoic acid (612) (in which the change from the structure of FA itself is exchange of the methylene bridge for a carbonyl group, and oxidation of position 7 to a lactam). This compound, which was a surprisingly effective inhibitor, was prepared from isoxanthopterin carboxylic acid (611) by in situ conversion to its acid chloride with a mixture of phosphorus oxychloride and phosphorus pentachloride, followed by addition of p-aminobenzoylglutamic acid (Scheme 3.132) [115]. 

The dominant effect of carbonyl groups on the degradation of oxidized cellulose in an alkaline medium is evident also from the conversion of alkali-unstable oxidized celluloses into stable materials by reduction with sodium tetrahydroborate in a weakly alkaline medium, i.e., under conditions such that aldehyde groups are converted into alcoholic groups. The stability of oxidized celluloses is considerably enhanced also when carbonyl groups are oxidized to carboxy groups. 

Methoxy(phenylsulfanyl)(trimethylsilyl)methyllithium 324 has been used as acyllithium and, depending on the electrophile and the deprotection conditions, it can transfer the acyltrimethylsilyl, the methoxycarbonyl or the (phenylsulfanyl)carbonyl group. By alkylation of intermediate 324 with primary alkyl iodides, bromides and chlorides in the presence of HMPA, followed by oxidation of compound 325 with NaI04, acylsilanes... 

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