Tandem cycloisomerization-cycloaddition

RXN73 Tandem Cycloisomerization-Cycloaddition of Dienynes with 1,3-Dienes via Metallodienes... 

As a part of a program directed toward the synthesis of the potent topisomerase I inhibitors, the lamellarins (e.g., 153 and 154), Porco has reported the silver triflate-catalyzed tandem cycloisomerization-azomethine ylide cycloaddition of 155 (Scheme 2.42).75 The postulated mechanism of this intriguing and highly efficient process is shown in Scheme 2.43. Silver-catalyzed addition of the imine nitrogen to the alkyne results, on subsequent deprotonation, in the formation of an azomethine ylide 160. This ylide participates in [3+2] cycloaddition with the alkyne component leading to formation of a dehydropyrrole 161. Finally, oxidation by adventitious oxygen leads to formation of the product 162. 

Intramolecular Reaction of 1,6-Dienes, -Enynes, and -Diynes Cycloisomerization, Tandem Addition-Cyclization, and Cycloaddition... 

Scheme 15. Synthesis of Heterocycles from 1,6-Dienes, Enynes, and Diynes via (a) Cycloisomerization, (b) Tandem Addition—Cyclization, and (c) Cycloaddition...

As 1,/2-alkenes, 1,6-derivatives are used very frequently leading to five-membered heterocycles, while the use of 1,7-derivatives, which produce six-mem-bered heterocycles, is very rare. The reactions of 1,6-dienes, -enynes, and -diynes are classified into three groups (a) cycloisomerization, (b) tandem addition— cyclization, and (c) cycloaddition, such as the Pau-son—Khand reaction, cyclotrimerization, and the Diels—Alder reaction (Scheme 15).97 In these reactions five-membered heterocycles are constructed upon the carbon—carbon bond-forming processes. 

---