Strategy XVI Pericyclic Reactions in Synthesis Special Methods for Five-Membered Rings

Background Needed for this Chapter Reference to Clayden, Organic Chemistry  

Chapter 36 Pericyclic Reactions II Sigmatropic and Electrocyclic Reactions. 

The only pericyclic reactions we have used so far have been cycloadditions the Diels-Alder reaction in chapter 17 and 2 + 2 cycloadditions in chapter 33. Electrocyclic and sigmatropic reactions are also used in synthesis and, as each is the basis for a synthesis of five-membered rings, they are grouped together here. 

An electrocyclic reaction is the formation of a new o-bond across the ends of a conjugated 7T-system or the reverse. They thus lead to the creation or destruction of one a-bond. Hexatrienes 1 can cyclise to six-membered rings 2 in a disrotatory fashion but we shall be more interested in versions of the conrotatory cyclisation of pentadienyl cations 3 to give cyclopentenyl cations 4. The different stereochemistry results from the different number of rt-electrons involved.1 

The Nazarov2 is probably the most important of reactions like 3. The cation 6 is formed from a dienone 5 by protonation and cyclises to the allylic cation 7. Though this is presumably a conrotatory process, the stereochemistry is usually lost in the formation of the cyclopentenone 9. 

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Strategy XVI Pericyclic Reactions in Synthesis Special Methods for Five-Membered Rings... 

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