Strategy XIV Rearrangements in Synthesis

Background Needed for this Chapter Reference to Clayden, Organic Chemistry Chapter 37 Rearrangements. 

If the carbon framework of a TM is difficult to construct, one strategy is to make a slightly different framework by conventional reactions and rearrange it into the framework we want. Methods involving rearrangement range from simple chain extensions to deep-seated skeletal rearrangements very difficult to analyse. 

In the last chapter we met diazoalkanes as sources of carbenes in the synthesis of three-membered rings. These same diazoalkanes are useful reagents for rearrangements via carbenes or carbocations. 

Chain Extension with Diazoalkanes The Arndt-Eistert Procedure 

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. 

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