Molecular Diels-Alder reaction

Both inter- and intra- molecular Diels-Alder reactions of 2-benzopyran-3-ones occur with high endo-selectivity and have been used to synthesise (-)-podophyllotoxin (14) and 4a-substituted cis-BC fused hexahydrophenenthrenes (15), respectively <96JCS(P1)151, 96JCS(P1)705>. 

Zndo Selectivity Example The inlra-molecular Diels-Alder reaction was used to make (19) needed for a synthesis of the natural product torreyol. The intermediate (IS) could not be isolated as oxidation of alcohol (17) gave (19) directly. The intramolecular Diels-Alder must be very efficient. 

Condensed benzo[i>]furan molecules can be prepared by inter- or intra-molecular Diels-Alder reactions from furo[3,4-b]benzofurans, and some interesting intermolecular examples are listed below. As can be seen, the furo[3,4-i>]benzofuran 60 underegoes Diels-Alder reactions with naphtho-l,4-quinone in the presence of Znl2 as a Lewis acid to form the aromatized cycloadduct. When the diene precursor reacts with benzo-l,4-quinone in the absence of Znl2, the product is obtained as an endo-exo mixture <00JCS(P1)1387>. 

Diene Reactivity Dienophile Reactivity Intra Molecular Diels-Alder Reactions... 

On the other hand, a C-5 cyclopentadiene moiety induced almost no face discrimination on addition of dimethyl acetylenedicarboxylate to 9,10-dihydrofulvalene (266) (Scheme 63). Products (268) and (270) result from a tandem inter- ntra-molecular Diels-Alder reaction (domino Diels-Alder reactions). 

The cross-coupling/Diels-Alder cycloaddition has been used in other syntheses. Thus, in work towards the synthesis of the marine alkaloids zoanthamine and norzoanthamine, reported by Tanner, the Diels-Alder precursors were constructed by a Pd(0)- and Cu(I)-catalyzed coupling. A synthesis of the decahydrofluorene nucleus of GKK1032s was achieved using a Stille-coupling/intra-molecular Diels-Alder reaction. ... 

Until recently, attempts to trap the unstable o-quinone methide imines generated by this fluoride-induced 1,4-elimination in inter-molecular Diels-Alder reactions with typical dienophiles were unsuccessful.388... 

Isolated carbon-carbon double or triple bonds do not usually take part in inter-molecular Diels-Alder reactions, but a number of cyclic alkenes and alkynes with pronounced angular strain are reactive dienophiles. The driving force for these reactions is thought to be the reduction in angular strain associated with the transition state for the addition. Thus, cyclopropene reacts rapidly and stereoselectively with cyclopentadiene at 0°C to form the endo adduct 7 in 97% yield, and butadiene gives norcarene 8 in 37% yield (3.13). ... 

The Diels-Alder reaction of cyclopropenyl ketones with cyclic dienes and 2,3-dimethylbutadiene yielded substituted tricyclo[3.2.2.0 " ]non-6-ones in high yields (90%). The inter- and intra-molecular Diels-Alder reactions of 2-halocycloalkenones with dienes formed [4,2,0]octenones. The 2-bromocycloalkenones are highly reactive and endo-sdective. ... 

SCHEME 3.29 Microwave-assisted inter-molecular Diels-Alder reaction of benzocyclob-utenes. 

S. Akai, T. Naka, S. Omura, K. Tanimoto, M. Imanishi, Y. Takebe, M. Matsugi, Y. Kita, Lipase-catalyzed domino kinetic resolution/intra-molecular Diels-Alder reaction one-pot synthesis of optically active 7-oxabicyclo[2.2.1]heptenes from furfuryl alcohols and p-substituted... 

SCHEME 43.8. Williard and de Laszlo s synthesis of Plocamium natural product via an inter-molecular Diels-Alder reaction. 

Stereoselective construction of cis-transoid-ds-tricydo(7.3.0.0) dodecanes by an intra-molecular Diels-Alder reaction a formal total synthesis of ( )-A( )-capnellene. /. Chem. Soc. Perkin Trans. 1. 865-873. 

Chiral enamine, dienamine, and trienamine intermediates have been shown to catalyse asymmetric Diels-Alder reactions via HOMO activation. These Diels-Alder reactions generally occur with high chemo-, regio-, and stereo-selectivities. ° The effect of the viscosity of pyridinium-based ionic liquids, with tetrafluoroborate and bis(trifluorosulfonimide) anions, on the kinetics of intra- and inter-molecular Diels-Alder reactions has been investigated. Results show that the intramolecular cycloaddition reaction is less susceptible to viscosity variations than the intermolecular reaction. 

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