Disconnections Umpolung Polarity Reversal

In the same way, disconnection of a 1,4-diketone requires either an acyl anion equivalent reacting with a normal /3-carbonyl electrophile or a normal o -carbonyl nucleophile reacting with an abnormal cr-carbonyl electrophile. These abnormal or reversed-polarity reagents are said to have umpolung reactivity. 

It is therefore necessary to use an umpolung (reverse polarity) reagent in place of one of the usual components. A nitro group on a carbon facilitates the removal of a proton from that carbon, giving a nucleophilic nitronate anion suitable for the 1,2 disconnection. Later the nitro group and carbon may be converted to a carbonyl group (which itself would have been electrophilic). 2,5-Heptanedione was prepared in this way (Eq. 8.9) [14]. 

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