Strategy XII Reconnection

We had to be careful in chapter 25 when we wanted to add bromoketones 4 to enolates 3 to make the 1,4-dicarbonyl compound 5. We could not use a lithium enolate because it would be too basic. No such difficulties exist in the reaction of enolates with allylic halides such as 2. Any enol(ate) equivalent will do as there are no acidic hydrogens and allylic halides are good electrophiles for the Sn2 reaction. 

But this does not make 5 instead it makes 1 with an extra carbon atom. So what we need are oxidative methods to convert 1 into 5 by cleavage of the alkene. There are many ways to do this. The most obvious is ozone1 that gives aldehydes from a disubstituted alkene 6 with reductive work-up or acids with oxidative work-up. 

Dihydroxylation with catalytic osmium tetroxide and stoichiometric oxidant such as NMO (TV -methylmorpholine-A -oxide) gives diols that can be cleaved to the same aldehydes with sodium periodiate or lead tetra-acetate. It is also possible to combine either KMnC 4 or catalytic OSO4 with an excess of NaIC 4 and complete the operation in one pot. 

Organic Synthesis The Disconnection Approach. Second Edition Stuart Warren and Paul Wyatt 2008 John Wiley Sons, Ltd 

disconnection of 11 by the methods of chapter 25 gives an enolate from the ester 8 but also a bromoaldehyde 12, a type of compound best avoided. But if we replace O by C 10 we can disconnect to allyl bromide 9—a satisfactory electrophile in every way. You might like to call this an FGA, and we shan t quarrel with that, but read on. .. 

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Strategy XII Reconnections Synthesis of 1,2- and 1,4-Difunctionalised Compounds by C=C Cleavage... 

Chapter 22 Strategy X Use of Aliphatic Nitro Compounds in Synthesis. Chapter 24 Strategy XI Radical Reactions in Synthesis. FGA and its Reverse. Chapter 26 Strategy XII Reconnections. 

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