Enynes/dienynes, Diels-Alder reactions

One productive facet of Pd-catalyzed domino reactions is the cycloisomerization of enynes and allenes, as shown by Trost and coworkers [19]. Thus, transformation of the dienyne 6/1-10 using Pd(OAc)2 led to 6/1-13 in 72% yield, in which the last step is a Diels-Alder reaction of the intermediate 6/1-12 (Scheme 6/1.2). 

The acetylenic bond of enynes and dienynes of types C=C—C=C and C=C—C=C—C=C can also participate. As always in the Diels-Alder reaction, there is 1,4-addition, but in such cases with shift of hydrogen ... 

Potycyclization of dienynes Treatment of the enyne 1 with Pd(0Ac)2/ triphenylphosphine and a base (Heck conditions) results in cyclization to a mixture of cis-and rrans-trienes 2. At 130° and with K2CO3, only trans-2 undergoes an intramolecular Diels-Alder reaction to 3. 

Reactions with conjugated enynes as dienes in Diels-Alder reactions yield cyclohexadiene or benzene products on reaction with alkene or alkyne dienophiles, respectively. These reactions proceed via a stepwise mechanism to avoid formation of a cyclic allene and are referred to as dehydro-Diels-Alder reactions. In 2008, Barluenga and Aguilar demonstrated that gold catalysts promote intermolecular hetero-dehydro-Diels-Alder reactions between dienynes and nitriles. Dienyne 99 combines with phenyinitrile (100) to afford substituted pyridine 101. ... 

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