Ketones fragmentation

Low -molecular-weight ozonides are explosive and are theretore not isolated. Instead, the ozonide is immediately treated with a reducing agent such as zinc metal in acetic acid to convert it to carbonyl compounds. The net result of the ozonolysis/reduction sequence is that the C=C bond is cleaved and oxygen becomes doubly bonded to each of the original alkene carbons. If an alkene with a letrasubstituted double bond is ozonized, two ketone fragments result if an alkene with a trisubstituted double bond is ozonized, one ketone and one aldehyde result and so on. 

Carbonyl compounds exhibit the normal fragmentation that happens with organic compounds, but they also display some other important features. Smaller aldehydes usually have a prominent CHO peak (m/e 29). For ketones, fragmentation may occur on either side of the carbonyl group, which can influence the position of the group. The base peak is often CH3CO (m/e 43) for methyl ketones, and aliphatic and aromatic acids may have a COOH" peak (m/e 45). 

Sm(II) mediated formation of very sophisticated chelating ligands has been observed. The powerful reducing agent CpJSm can induce facile double bond insertion of CO into C=C and C=N double bonds (Eq. 13) [176]. Carbon monoxide also inserts into the Y-pyridyl bond of CpfY(2-pyridyl) to form a bridging /i-f/2 >/2-dipyridyl ketone fragment (Eq. 14) [177]. 

Protonated carbonyl compounds can be electron sinks too (remember the dienone-phenol rearrangement from Chapter 37 ), and this bicyclic methoxy ketone fragments to a seven-membered ring in acid. Note the same 1,2,3,4 arrangement, with the bond between carbon atoms 2-3 fragmenting. 

If the MeO group is replaced by a leaving group such as MsO, it can exercise the pull and the carbonyl can provide the push after it has been attacked by a nucleophile. This next five-membered cyclic ketone fragments on treatment with base—can you detect hints of the benzylic acid rearrangement ... 

Explain why both of these tricyclic ketones fragment to the same diastereoisomer of the same cyclo-octadione. 

Figure 3. Map of a theoretical biological activity. The biological activity has been contoured against the structures arising from a particular combination of amine and ketone fragments. If at first the amine fragment is held constant (amine A) the ketone associated with maximal activity is 1. If the ketone is held constant as 1, the most potent amine is B, resulting in the false optimum B-l (in square). The true optimum C-3 (in hexagon) is never found.

On electron impact, the 4-methyl-5-nitroimidazole molecular ion loses H2O. This loss seems to require an NH adjacent to the nitro group i.e., loss of H2O comes from what is expected to be the least dominant tau-tomer (Scheme 19). In l-alkyl-5-nitroimidazoles (86) a similar hydrogen transfer is followed by an unusual rearrangement which results in loss of an aldehyde or ketone fragment from the molecular ion (Scheme 20). 

Thermal degradation of poly(ethylene-co-carbon monoxide) takes place by chain scission, yielding alkenes and ketone fragments. Around 500° C the decomposition products consist of CO, H2O, ethene, and series of ketones with the general structure R-[-CO-CH2-CH2-]n-CO-R where R, R = -CH3, -CH2-CH3 or -CH=CH2 [2, 3]. 

The cyclopropyl ketone system in 3a,7,7-trimethyloctahydrocyclopropa[c]inden-2-one (26, thujopsene) was converted to the 3-methyl ketone fragment 27 upon reduction with lithium. ... 

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