ONE-GROUP DISCONNECTIONS

Simply by looking for a food mechanism, you should be able to suggest a good disconnection for this alcohol  

Cyanide is a good anion, and the cation is stabilised by a lone pair of electrons on oxygen. Draw the disconnection again using the lone pair. 

What is the real reaction which is the reverse of this disconnection  

More usually, none of the substituents gives a stable anion and so we use the synthetic equivalent of the anion - the Grignard reagent or alkyl lithium. - We refer to Et as a SYNTHON for which EtMgBr is the synthetic equivalent. 

There are two stages in designing a synthesis—analysis and synthesis. To summarize, the following applies  

Students often experience a moment of panic when the phrase readily available SMs appears—how are they, with only limited experience of organic chemistry, to judge what is readily available Different instructors will want to tackle this in different ways, but as far as problems in the text are concerned, either you will be given a range of molecules with which you can start, or, if there is no specification, you can have anything in the Aldrich catalog.  

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Finally in our treatment of one group disconnections we ought to consider how to synhiesise Mly saturated hydrocarbons - compormds witli no FG at all These are often made by hydrogenation of a double bond, and so the discoimection can be made anywhere we like ... 

Although you have analysed the synthesis of many compounds and considered mechanisms of many reactions, we have collected only a handful of important one group disconnections. Can you fill in the details of these ... 

If the target molecule is monofunctional, the disconnection process is classified as a one-group disconnection. The bond initially considered for cleavage would be, if present, the a-carbon-heteroatom single bond (i.e. the C—O, C—X, C—N, C—S bonds) as would be found in, for example, alcohols (Section 5.4), alkyl halides (Section 5.5), ethers (Section 5.6), nitroalkanes (Section 5.15), amines (Section 5.16), thiols and thioethers (Section 5.17). 

The disconnection 47 gives a different synthon 48 at the carbonyl oxidation level and the best reagents are the a-halo carbonyl compounds 49. At first this looks like a one-group disconnection but there are two reasons why it isn t. The presence of the carbonyl group makes the Sn2 displacement of halide enormously faster and the a-halo carbonyl compounds 49 are made from the ketone 51 by acid-catalysed halogenation of the enol 50. 

Two-group disconnections reduce the complexity of a target molecule more efficiently than one-group disconnections, and you should always be on the look-out for them. You will meet more two-group disconnections in the next section, which deals with how to disconnect C-C bonds,... 

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