Synthetic studies directed towards understanding facial selection in diels-alder reactions (1991)
AuthorsFong, Siew Taishow all
Chapter 1 of this thesis briefly introduces the Diels-Alder reaction and highlights the stereochemical aspects of the reaction including endo versus exo addition, regioselectivity and π-facial selectivity. Some examples of systematic studies of π-facial selectivities are given. In Chapter 2 the syntheses, incidental chemistry and Diels-Alder reactions of two monosubstituted cage-fused dienes, the monoethylene acetal 52 and monothioethylene acetal 53 are described. With alkene dienophiles, the cycloaddition reactions gave almost exclusively "bottom face" adducts, while acetylenic and azo dienophiles gave mixed selectivities. An X-ray determined crystal structure of the monoethylene acetalnitrosobenzene adduct 99 is reported. Reactions of the cage diketone 35 with various alcohols and a crystallographic study of the diisopropoxyacetal 65 are also reported. Syntheses and Diels-Alder reactions of two symmetrically modified cage dienes, the dialkane 103 and dioxime 104, are described in Chapter 3. X-ray crystal structures of the dialkane-dimethyl acetylenedicarboxylate 117 and dioxime-N-phenyltriazoline dione 119 adducts are reported. The latter study revealed the oxime moieties are oriented anti to the cyclobutane ring. Diels-Alder reactions of dialkane 103 gave exclusively "bottom face" adducts showing that, in the absence of unfavourable electronic effects, the "bottom face" is sterically favoured. The dioxime 104 gave mixed selectivities. In Chapter 4 syntheses of cage ether 121, cyclic acetal 122 and amide 140, incidental chemistry of the cage compounds and their Diels-Alder reactions are reported. With the alkene dienophiles preference for "bottom face" attack was observed. The reactions of ether 121 and cyclic acetal 122 with acetylenic and azo dienophiles showed a high preference for the "top face". This is attributed to "closed shell repulsion". Diels-Alder reactions of hydroxyketone 143 and the acetate 144 are described in Chapter 5. These reactions gave only "bottom face" adducts. Thus, hydrogen bonding interactions as a determining factor in π-facial selectivities is ruled out. Molecular mechanics modelling of the adducts and diastereomeric "transition state" (TS) structures were undertaken to rationalise the observed π-facial selectivities. Product stabilities were shown to be unimportant. A model based on steric and torsional effects at the TS can qualitatively predict the π facial selectivities for reactions of all the dienes with alkene dienophiles. With acetylenic dienophiles the TS model predicted a high preference for the "bottom face". This prediction was in agreement with experiment only for the dialkane-DMAD adduct. An alternative new explanation is given for these reactions involving through-space filled orbital repulsive interactions between the diene substituents and the orbitals of the dienophile which are orthogonal to the bond forming HOMO π orbitals.