Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0329 479

O

+

Si(Pr-i)3

(i-Pr)3Si

∆ Ph2O : α-MeNaph (1 : 1)

R

R

R R R

(163; R = Si(Pr-i)3)

Liao and coworkers124 studied the Diels –Alder reactions of substituted furans, now acting as dienophiles, with masked o-benzoquinones. A representative reaction has been depicted in equation 50. The masked o-benzoquinones such as 181 reacted with unactivated, electron-rich as well as electron-poor furans such as 182. The substitution pattern of the masked o-benzoquinones proved, however, more important. An electron-withdrawing

group at C(3) or an electron-releasing group at C(2) greatly diminished reactivity. The reactions were completely site selective, i.e. the unsubstituted double bond of the substituted furans reacted in every instance to give adducts like 183.

8. Diels–Alder reactions of pyrones and pyridones

Because ˛-pyrones and ˛-pyridones have some aromatic character, like furans, they undergo Diels –Alder reactions less easily than most cyclic dienes do. Nevertheless, suitable reaction conditions have been developed and their Diels –Alder reactions have been used intensively to generate useful synthetic intermediates35. Unsubstituted ˛-pyrones and ˛-pyridones generally react with electron-poor dienophiles in normal electron demand Diels –Alder reactions. The regioselectivity, determined by the weakly directing endocyclic heteroatom, is often poor. To improve regioselectivity, either electron-releasing or electron-withdrawing substituents must be present. The reaction becomes an inverse electron demand Diels –Alder reaction in the latter case.

By using electron-withdrawing N-substituents, the aromatic character of ˛-pyridones can be reduced and the efficiency as well as the stereoselectivity of the Diels –Alder reaction increased. The efficiency of the reactions of ˛-pyridones can be further improved by using bulky electron-withdrawing N-sulfonyl substituents, as was shown by Afarinkia and Mahmood125. N-Sulfonyl 2-pyridones generally rearrange easily to the thermodynamically more stable 2-(sulfonyloxy)pyridines. The authors found that use of the large 2,4,6-triisopropylbenzenesulfonyl group and solvents of low polarity suppressed this rearrangement, thereby improving the yields of the cycloaddition.

Guitian´ and colleagues126 performed some Diels –Alder reactions between in situ generated cyclohexyne and several ˛-pyrones. The reactions were performed at 100 °C which resulted in immediate loss of carbon dioxide from the primary cycloadducts. Reaction yields were generally above 80%. The reaction between 184 and cyclohexyne, derived from 185, to give 186 has been depicted in equation 51.

MeO2C

(186)

Hsung127 used a [4 C 2] cycloaddition reaction of a -pyrone to synthesize the tetracyclic core of arisugacin, a novel inhibitor of acetylcholinesterase. He noticed an unexpected concentration effect on the stereoselectivity in the reactions of 3-cyano- - benzopyrone derivatives with electron-rich dienes128. When 1-methoxybutadiene (187) reacted with -benzopyrone 188, for example, the ratio between endo adduct 189 and exo adduct 190 depended on the concentration of 188, as demonstrated by the data given in Table 4 (equation 52). Raising the concentration of 188, while keeping the diene concentration twice as high, caused the reaction to become less endo selective. Variation of the diene concentration, while keeping the -benzopyrone concentration constant, did not demonstrate a clear trend.

O

CO2Et

i-Pr3Si

CN

(194; 33)

Krause and colleagues131 reported the cycloadditions of several substituted vinylallenes with symmetrically as well as asymmetrically substituted olefins. The reactions proceeded at a reasonable rate at room temperature, affording adducts with high to complete facial, endo and regioselectivities. Vinylallene 195 proved highly reactive, even at 5 °C, affording dimer 196 as a 70 : 30 mixture of two isomers (equation 54). The regiochemistry was contrary to that generally observed (see above).

Bu-t

CO2Et

Murakami and colleagues132 studied the Diels –Alder reactions of vinylallenes with alkynes catalyzed by a rhodium complex. When a vinylallene lacking substituents at the vinylic terminus was reacted with a terminal alkyne, 1,3,5-trisubstituted benzenes were obtained, the reaction between vinylallene 197 and 1-hexyne (198) being a representative example (equation 55). The reaction was proposed to proceed via a rhodacycle which afforded the primary Diels –Alder adduct via reductive elimination. Aromatization via isomerization of the exocyclic double bond led to the isolation of 199.

The palladium catalyzed reactions of substituted vinylallenes with unactivated 1,3- butadienes proceeded with high selectivity133. A multistep mechanism, involving several palladacycles, was proposed to explain the high selectivities observed.

Allenes generally react with conjugated dienes to give [4 C 2] type of adducts in contrast with ketenes which generally react with dienes in a [2 C 2] kind of way. Some enhancement of reactivity is expected in comparison with olefins because of the significant amount of strain that allenes have. Semi-empirical calculations, however, have shown allene to be less reactive than ethylene due to higher deformation energies135.

Gedanken and colleagues136 investigated the Diels –Alder reactions of trichloromethyl allenyl sulfoxides 203 and cyclopentadiene under ultrasound irradiation. Allenes 203 are generally very sluggish in reactivity. However, when ultrasound was applied, the reactions of allenes 203 with cyclopentadiene were completed within 2 hours (equation 57). Mixtures of endo (204) and exo (205) isomers were obtained in all instances. When the - position of the allenyl sulfoxides was substituted, additional mixtures of E and Z isomers were obtained.

Kanematsu and coworkers studied the reactions of optically pure allene-1,3-dicarboxy- lates with furan137 and cyclopentadiene138. The aluminum trichloride catalyzed reaction of 206 with cyclopentadiene proceeded at 78 °C with virtually complete endo selectivity and complete facial selectivity to afford 207 and 208 in a ratio of 98 : 2 (equation 58), cyclopentadiene approaching the double bond from the face opposite to the perpendicular carboxylate group.

SOCCl3

R1

SOCCl3

(205)

10. Diels–Alder reactions of fullerenes

The Diels –Alder chemistry of fullerenes has proven to be an important method for the preparation of novel organofullerenes. Consequently, a wide variety of cycloadducts have been reported36. In spite of the presence of 30 conjugated double bonds in [60]fullerene, it does not behave as a diene in Diels –Alder reactions. Instead, the carbon –carbon double bonds across two six-membered rings in [60]fullerene serve as dienophiles in reactions with predominantly electron-rich dienes. Although the strain of the -orbitals and the electron-withdrawing ability of [60]fullerene make it reactive toward dienes, the loss of aromaticity of the two six-membered rings involved generally leads to retro Diels –Alder reactions at low temperatures. The well-explored reactions of [60]fullerene with o-quinodimethanes provide especially stable adducts as a consequence of the aromatization process which takes place during product formation. The reactions with heterocyclic o-quinodimethanes have been investigated much less139.

R = (−)-menthyl

+

H

RO2C RO2C

(208; 2)

The trapping of in situ generated pyrimidine o-quinodimethanes 210a–d with [60]fullerene (209) was investigated by Herrera and colleagues140. The reactions were conducted in refluxing o-dichlorobenzene and yielded adducts 211a–d in yields ranging from 54% to 96%, based on the amount of consumed [60]fullerene (equation 59). According to

o-dichlorobenzene, ∆

N R

N

R

(211a−d)