π-Facial selectivity in the Diels-Alder reactions of the 1,3-cyclohexadienes bearing allylic oxygen substitution

Gillard, James R. (1992) π-Facial selectivity in the Diels-Alder reactions of the 1,3-cyclohexadienes bearing allylic oxygen substitution. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Diels-Alder reaction of cis-3,5-cyclohexadiene-1,2-diol (121) and a number of its acyclic derivatives, cis-1,2-diacetoxy-3,5-cyclohexadiene (109), cis-1,2-bis- (trimethylsiloxy)-3I5-cyclohexadiene(122)andcis-1,2-dimethoxy-3l5-cyclohexadiene(124) were found to add N-phenylmaleimide in a kinetically controlled manner to yield adducts that arose mainly by attachment of the dienophile to the face of the diene syn to the oxygen atoms. Some cyclic derivatives of 121, cis-3a,7a-dihydro-2,2-dimethyl -1,3- benzodioxole (114) ,cis-3a,7a-dihydro-2,2-dimethyl-1,3,2-ben2odioxasilole (123) and cis-2- ethyl-3a,7a-dihydro-2,1,3-benzoboradioxoie (125), afforded nearly equal quantities of both syn and anti adducts. Benzylidene-protected derivatives (2α,3aβ,7aβ)-3a,7a-dihydro-2- phenyl-1,3-benzodioxole (132) and (2α,3aα,7aβ)-3a,7a-dihydro-2-phenyl-1,3-benzodioxole (133) also reacted with N-phenylmaleimide, but they gave predominantly anti-addition products. -- 1,3,5-Cyclohexatriene-1,2-oxide (142) and its more substituted derivatives, 1,2- dimethyl-1,3,5-cyclohexatriene-1,2-oxide (148a) and 10-oxatricyclo[4.3.1.0]deca-2,4- diene (148b) reacted also with N-phenylmaleimide to give only products that resulted from addition of the addends to the face of the diene anti to the epoxide oxygen. -- The results obtained have been discussed in the context of the many theories that attempt to define the controlling factors involved in determining (syn/anti) π-facial selectivity of plane-nonsymmetric dienes bearing an allylic heteroatom. Steric effects and stereoelectronic effects are invoked in this study to rationalize the results we report here. In particular, our contrasting results with the diol-derived dienes and the benzene oxides are consistent with syn-addition being determined by the presence of a favorable secondary orbital interaction between the LUMO of the dienophile and the components of the highest occupied molecular orbitals that reside on the allylic oxygens of the dienes.

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