Inner-Ring Dienes

The investigation on the use of K-10 montmorillonite under free solvent conditions was then extended to inner ring dienes such as furan and its 2,5-dimethyl derivative [9] (Table 4.3). The cycloadditions generally proceed slowly, and Zn(II)-doped clay and microwave irradiation were used to accelerate the reactions. The reaction with maleic anhydride preferentially affords the thermodynamically favored exo adduct. 

Diels-Alder reaction of dienophiles, N-allylic enamides and a,/l-unsaturated lactam derivatives with open chain and inner ring dienes is promoted by iodine [98]. Thus the cycloaddition of N-benzyl-N-methallyl acrylamide 147 with cyclo-pentadiene (1) proceeds smoothly in DMF at —78 °C in the presence of I2 (2 eq.) to give a prevalence of endo adduct l Vd) in 88% yield (Equation 4.17). 

Dienes can be open-chain, inner-ring (e.g., 80), outer-ring856 (e.g., 81), across rings (e.g., 82). or inner-outer (e.g., 83), except that they may not be frozen into a transoid conformation... 

Reactions with Cyclic Transition States 3.3.1.8.1 Heterocycles as inner ring dienes... 

Heterocycles as inner-ring dienes (96CHEC-il(2)8l, 96chec-ii(2)322, 96CHEC-ii(3)277>... 

Splitting of inner ring around the meri-dien. Equatorial outer ring splitting. Optically un ia-xial. 

Splitting of inner ring aifound meri-dien. Outer ring splitting into equatorial doublet. 

Inner-outer-ring dienes are very useful in the synthesis of polycyclic molecules. Their reactivity in the Diels-Alder reaction depends on the type of ring (carbo-cyclic, heterocyclic, aromatic) that bears the ethenyl group or on the electronic effects of substituents at the diene moiety [30]. 

Arylethenes are inner-outer-ring dienes in which the vinyl group is linked to an aromatic system. These dienes are poorly or moderately reactive the presence of electron-donating substituents in the diene moiety markedly increases their reactivity. Their cycloadditions are usually accelerated in order to be carried out under mild conditions. 1-Vinylnaphthalene is more reactive than 2-vinyl-naphthalene and styrenes. 

---