Carbon chain cleavage

Carbon chain cleavage through p-scission of an alkoxyl radical... 

Intramolecular radical substitution of a peroxide-linked fatty acid oligonoer resulting in carbon chain cleavage to aldehyde fragments... 

As may be expected from previous discussion, aldonic acid spectra show a number of fragmentations due to carbon chain cleavages alpha to TMS-ether groups. Charge retention on the non-carbonyl fragment gives a series of ions m/z = 103 + n X 102). The first member of the series, m/z 103, may indicate a terminal hydroxyl on the chain, but can be confused with m/z 103 arising from... 

Free radicals are initially generated whenever polymer chains are broken and carbon radicals are formed. These effects occur during manufacture and in service life. Many elastomers are observed to oxidize at relatively low temperature (about 60°C), where carbon-hydrogen and carbon-carbon bond cleavages are highly unlikely. It has been demonstrated [52] that traces of peroxides impurities in the rubber cause low-temperature oxidation of rubber. These initiating peroxides are present in even the most carefully prepared raw rubber polymer [53]. 

Step 4 of Figure 29.3 Chain Cleavage Acetyl CoA is split off from the chain in the final step of /3-oxidation, leaving an acyl CoA that is two carbon atoms shorter than the original. The reaction is catalyzed by /3-ketoacyl-CoA thiolase and is mechanistically the reverse of a Claisen condensation reaction (Section 23.7). In the forward direction, a Claisen condensation joins two esters together to form a /3-keto ester product. In the reverse direction, a retro-Claisen reaction splits a /3-keto ester (or /3-keto thioester) apart to form two esters (or two thioesters). 

Figure 42-3. Cholesterol side-chain cleavage and basic steroid hormone structures. The basic sterol rings are identified by the letters A-D. The carbon atoms are numbered 1-21 starting with the A ring. Note that the estrane group has 18 carbons (Cl 8), etc.

The biosynthesis of polyketides (including chain initiation, elongation, and termination processes) is catalyzed by large multi-enzyme complexes called polyketide synthases (PKSs). The polyketides are synthesized from starter units such as acetyl-CoA, propionyl-CoA, and other acyl-CoA units. Extender units such as malonyl-CoA and methylmalonyl-CoA are repetitively added via a decarboxylative process to a growing carbon chain. Ultimately, the polyketide chain is released from the PKS by cleavage of the thioester, usually accompanied by chain cyclization [49]. 

Homolytic cleavage of b—d bond gives a diradrical intermediate. If the centers bearing the unpaired elections in a diradical are well separated (for example by a long carbon chain) there will be little interaction between them and each will act as an independent monoradical function. Usually the centers are close enough for molecular interaction and it eliminates their reaction as separate chemical entities. 

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