Periselectivity and Stereoselectivity

Periselectivity and Stereoselectivity.—The thermal stabilities of the Diels-Alder adducts (179) and (180), which are obtained as a mixture from reaction of 2,5-dimethyl-3,4-diphenylcyclopentadienone with ds-3,4-dichlorocyclobutene, are widely different in respect to cheletr opic loss of carbon monoxide. The major isomer (179) remains 

The FMO analysis of the cycloaddition behaviour of 7,7-dimethylfulvene toward cyclopentadiene has been critieally examined. There are three [4 2]addition modes (the one in whieh cyclopentadiene acts as the diene and a ring C=C unit of the fulvene plays the part of the dienophile leads to the experimentally observed product) and one [6 4- 4] mode. The [6 -I- 4] mode has previously been predicted to be the preferred combination on the basis of FMO results. However, superjacent orbital effects i.e. effects arising from contributions of MO s lying above the frontier M O manifold) are now shown to be decisive, with a marked preference for the observed [4 + 2] addition. It is recommended that in the application of FMO theory an inspection of the coefficients of those MO s which lie outside the FMO manifold be carried out. It should nevertheless be noted that the identity if the kinetically controlled product of the above cycloaddition has yet to be verified although, apparently, such studies are now in progress. 

Reaction of 4-methyl-2-pyrone with maleic anhydride or N-phenylmaleimide yields the endo-adducts (189 X = O or NPh) whereas similar reaction of 6-methyl-2-pyrone 

In the Diels-Alder reactions of some 1,4-disubstituted cyclopentadienes with a variety of terminal olefin dienophiles a linear relationship between log(fc,ei) og endo/exo) is found. The substantial deviation from the linear correlation in the cases of maleic anhydride and N-phenylmaleimide, where a higher endo-stereoselectivity is observed than is expected from the k,ei values, is ascribed to the double secondary orbital interactions that can occur with these two cyclic dienophiles. It is difficult, however, to see how one can ignore the appreciable differences in steric effects between dienophiles of different types. 

Hamanaka, and M. Ogawa, Bull Chem. Soc. Japan, 1975, 48, 733. 

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