Outer-Ring Dienes

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

Ethylene disulfonyl-1,3-butadiene (43) is an example of an outer-ring diene with a non-aromatic six-membered heterocyclic ring containing sulfur. It is prepared by thermolysis of sulfolenes in the presence of a basic catalyst. It is very reactive [43] and even though it is electron-deficient, it readily reacted with both electron-rich and electron-poor dienophiles (Equation 2.15). 

Indole-2,3-quinodimethanes [44] 44 are bicyclic outer-ring dienes that are widely used to prepare a variety of heterocyclic polycyclic compounds. These dienes, generated by extrusion of CO2 from lactones, are then trapped by dienophiles. Some examples of Diels Alder reactions of the dienes 44 are reported in Scheme 2.19. 

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. 

Heterocyclic derivatives as inner-outer ring dienes... 

Starting from some of the above 1-hydroxycarbacephams 54, the synthesis of inner-outer-ring 2-[/< r/-butyldimethylsilyloxy dienes 55 with a carbacepham structure and their totally 7t-facial endo selective Diels-Alder reactions to structurally novel polycyclic p-lactams 56 have been reported (Scheme 19) [64]. 

A more versatile approach uses an inner-outer ring diene system such as compound 496, which reacted with iV-methylmaleimide in toluene at 145 °C in a sealed tube to give cycloadduct 497 in 91% yield <2004TL7255>. Similar procedures with different electrophiles have been reported by other workers who were able to combine consecutively the preparation of the inner-outer diene by RCM reaction (see Section 2.04.9.3) and the Diels-Alder reaction in a one-pot process, which gave increased yields in most cases for the polycyclic compounds when compared with the two-pot approach <2004S2665>. 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

Inner-outer-ring dienes 104 are particularly useful for the synthesis of polycyclic structures, because they enable the incorporation of additional rings into the sixmembered ring formed by the DA reaction (Scheme 10.106) [289]. Highly electron-deficient alkenes and alkynes react smoothly with 104 at room temperature. Benzaldehyde and N-benzyhdeneaniline can also be used as dienophiles in the presence of a catalytic amount of ZnCl2. Overall, the Diels-Alder reactivity of 104 is comparable with that of 102. 

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