Dienynes, Diels-Alder reactions

Lewis-acid catalysis is effective in intermolecular as well as intramolecular /zomo-Diels-Alder reactions. Thus, complex polycyclic compounds 93 have been obtained in good yield by the cycloaddition of norbornadiene-derived dienynes 92 by using cobalt catalyst, whereas no reaction occurred under thermal conditions [91] (Scheme 3.18). 

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). 

Alkynes are poor dienophiles in the Diels- Alder reaction decomposition occurs by an attempted thermal intramolecular Diels-Alder reaction of dienynes at 160 °C. In contrast, the Ni-catalysed [4+2] cycloaddition of the dienyne 50 proceeded smoothly at room temperature using tri(hexafluoro)isopropyl phosphite to give 51, which was converted to the yohimbine skeleton 52 [15]. The same reaction is catalysed by RhCl(Ph3P)3 in trifluoroethanol [16]. Intramolecular Diels-Alder reactions of the 6,8-dieneyne 53 and the 1,3,8-triene 55, efficiently catalysed by [Rh(dppe)(CH2CH2)2]SbF6 at room temperature, gave 54 and 56 [17],... 

While the syntheses of the acyclic precursors in the examples above each require a couple of steps, symmetrical dienynes with a central triple bond and heteroatoms in the tethers are more easily accessible. They can yield heterotricyclic compounds by the same reaction mode, for example, the diaza- and dioxatricycles 121 are obtained starting from dienynes 119 (Scheme 18) [73]. Yields were best (90%) with N-tosyl linkers, with N-Boc groups the reaction was slower (41% yield), and with N-benzyl linkers only decomposition occurred. This may be due to coordination and blocking of the catalyst by the more Lewis-basic amines. The cis- and frans-diastereomers of 121 were formed in a ratio of 1.8 1, and this ratio did not change in other solvents, at different temperatures, with other catalyst precursors or under high pressure (10 kbar). In view of the apparent influence of the tether, the unsymmetrical oxazaprecursor 122 gave a 7 3 mixture of tricycles 123 and 124. Obviously, the hydridopalladation of the triple bond occurred with some regioselectivity such that intramolecular carbopalladation of the allyl-amine predominated. It is noteworthy that in these cases the intramolecular Diels-Alder reactions of the intermediate trienes 120 already occur under the employed conditions, i.e. at 80 °C. 

An interesting sequence of two consecutive Heck-type cyclizations and a subsequent Diels-Alder addition was observed when the methoxycarbonyl-substituted 2-bromo-trideca-1,1 l-dien-6-yne ( /Z)-63 was treated with the typical palladium catalyst (Scheme 3-20) [172]. The cyclization of dienyne 63 at 80 °C gave two diastereomeric trienes ( /Z)-64. At higher temperature (130 °C), an intramolecular Diels-Alder reaction of only the ( J)-isomer ( )-64 occurred to give the tetracyclic 65, whereas (Z)-64 remained as such, probably due to steric interference of the methoxy group. 

Intramolecular homo Diels-Alder reaction of dienynes. Norbornadicnes bearing... 

Polycyclization of dienynes.9 Treatment of the cnyne 1 with Pd(OAc)2/ triphenylphosphine and a base (Heck conditions) results in cyclization to a mixture of cis-and rrans-tricnes 2. At 130° and with K COj, only trans-2 undergoes an intramolecular Diels-Alder reaction to 3. 

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 ... 

Intramolecular homo Diels-Alder reaction of dienynes. Norbomadienes bearing a tethered alkyne group at C2 undergo an intramolecular homo Diels-Alder reaction when treated with 8 mol% of Co(acac)2, (C2H5)2A1CI (4 equiv.), and dppe at 25°. This reaction is particularly facile when a 5- or 6-membered ring is formed. No reaction is observed on thermal treatment of 1 at 140-170°. 

Scheme 2-44. Attempted intramolecular Diels-Alder reaction of 1,3-dienyne 432.

The thermal Diels—Alder reaction of dienynes (71) produced naphthalene derivatives (72) in the absence of Lewis acids, transition metals, or oxidants. The presence of the silyl group attached to the alkyne is essential for the dehydrogenation reaction to proceed (Scheme 22). The thermal intramolecular Diels—Alder reactions of (l , 7 )-l-iiitro-deca-l,7,9-trienes (73) produced trans-iused decalin cycloadducts (74) (Scheme 23). °... 

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. ... 

The nickel(0)-catalyzed stereoselective intramolecular [4+ 2] cycloaddition between dienes and unactivated allenes or alkynes has been shown to be an efficient complement to the uncatalyzed concerted Diels-Alder reaction that often requires stereoelectronic restrictions. In a typical reaction, treatment of dienyne with 10 mol% Ni(COD)2 and 30mol% of tri-o-biphenyl phosphite at room temperature... 

In 2008, a gold-catalyzed cycloaddition of captodative dienynes with nitriles to provide pyridines was developed [114], The reaction proceeded via intermolecular hetero-dehydro-diels-alder cycloaddition and gave the desired pyridines in good yields regioselectively (Scheme 3.56). 

---