Disconnection-reconnection carbon

Figure 10.35 Application of the disconnection-reconnection strategy to the carbon-14 labeling of complex molecules [ C2]protoporphyrin IX dimethyl ester...

The cis stereochemistry of (7) is irrelevant to (10 or (11), as cyclisation can give only ais fusion of two five-membered rings, but it might be a problem for (9). In any case, no disconnection, other than removing single carbon atoms, can easily be made on (9). Much the same is true of (12), but reconnection of the 1,6-dicarbonyl in (13) provides a good starting material (14). 

FGA and carbonyl strategy the synthesis of long chain fatty acids such as (16) can be achieved in a few steps by suitable FGA. Adding a carbonyl group in a 1,6 relationship allows reconnection to (17) and hence the disconnection of the six carbon atoms in the ring. Analysis... 

But what are we to call the retrosynthetic transformation of 11 into 10 It isn t a disconnection rather an extra carbon atom has been added. So we call this operation a reconnection joining the target molecule back up to something to reveal the precursor. So, consider the synthesis of the m-enone 21, a structure found in insect pheromones, perfumes and flavourings. A Wittig... 

This extension, known as the Amdt-Eistert procedure,1 is useful if the relationship between functional groups is unhelpful in the TM but becomes helpful if the chain is retrosynthetically shortened. Other methods, such as cyanide displacement, also increase the chain length by one carbon and we saw a chain extension by two carbon atoms in the last chapter. The disconnections are strange both C-C bonds between R and CO are made in the reaction so we must disconnect both 5a. You might like to think of this as a reconnection strategy (chapter 26) or as an extrusion of a CH2 group. 

Disconnection of the bicyclic diketone 21 starts reasonably to reveal 22 but the two carbonyl groups are now 1,6-related suggesting a reconnection strategy (chapter 27). But this is impossible as the bridgehead alkene 23 is too strained to exist. However, if we extrude the carbon atom in the seven-membered ring between the ketone and the branchpoint 22a, we get a new ketoester 24 with a 1,5 relationship that can be made by conjugate addition (chapter 21). 

Disconnections may involve either the carbon skeleton or functional groups. Normally the first step is to disconnect the sections of a molecule that are held together by functional groups such as esters, amides and acetals, as it is usually easier to find reconnection reactions for these functional groups. Heterolytic disconnections usually provide the most useful approach to synthesis design. 

Blank Analysis Place a square piece of preheated asbestos pad in the combustion boat, and cover it with sodium carbonate. Stop the nitrogen flow, disconnect the ball-joint, quickly insert the boat into the combustion tube with large forceps, and reconnect the joint. Note the time, allow the boat to sit in the tube with no nitrogen flow for exactly 1 min, and then restart the flow of nitrogen. Mercury elutes almost immediately with the reinstated nitrogen flow note the recorder response. Allow about 30 s between runs. 

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