Browsing by Author "Fosu, Evans"

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    Diels-Alder cycloaddition versus ring-opening esterification: Acomputational study of the mechanism of formation of oxa-norbonenelactones from the reaction of furfuryl alcohol and itaconic anhydride
    (Elsevier B.V., 2018) Fosu, Evans; Botchway, Cecil Humphrey; Tia, Richard; Adei, Evans
    The reactions of furfuryl alcohol with itaconic anhydride can proceedviafour plausible pathways–two path-ways involving initial Diels-Alder cycloaddition followed by lactonization and two pathways involving initialesterification followed by intramolecular Diels-Alder cycloaddition–to afford two distinct norbornene productsbearing either afive- or six-membered butyrolactone ring respectively. DFT calculations reported herein showthat thefirst-step Diels-Alder cycloaddition reactions have barriers of 8.5 and 11.8 kcal/mol which are far lowerthan the barriers of 29.6 and 36.9 kcal/mol for the esterification pathways. The calculated energies for theexperimentally observedfive- and six membered oxa-norbornene lactone isomers are exergonic with reactionenergies of−17.1 and−12.8 kcal/mol respectively. Thermodynamically thefive-membered isomer is morestable adduct than the six-membered adduct. Thus the most preferred pathway is the one involving initial[4 + 2] cycloaddition followed by lactonization to form thefive-membered butylrolactone ring. Substitution ofmethyl groups on the carbon bearing the hydroxyl group shows that the substituents do not affects the reactionpathways markedly.
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    Mechanistic studies on Diels-Alder [4þ2] cycloaddition reactions ofa,b-substituted cyclobutenones: Role of substituents in regio- andstereoselectivity
    (Elsevier Ltd., 216-11-03) Fosu, Evans; Tia, Richard; Adei, Evans
    Diels-Alder reactions of substituted cyclobutenones with 6-methoxy-1-vinyl-3,4-dihydronaphthaleneand methoxy-substituted-1,3-butadiene have been studied with DFT. In the reactions of 6-methoxy-1-vinyl-3,4-dihydronaphthalene with cyclobutenone anda-bromocyclobutenone, the formation of themetaandorthoisomers have the same barriers, indicating that the two isomers might be formed in equalproportions, contrary to earlier reports. The regiochemistry of the reaction is mainly controlled by theketone functionality at C1 on the dienophiles. In the reactions of methoxy-substituted-1,3-butadienewith cyclobutenone anda,b-substituted cyclobutenones theortho/endoandpara/endostereo-isomericpathways are the most favorable pathways, changing toexoselectivity when OH, Br, CH3are placedon theb-carbon of the cyclobutenone, but still withorthoandpararegioselectivity. The stereoselectivityis independent of the bulkiness of substituents. The stability of substituted cycloadducts are lowercompared to unsubstituted adducts and this explains why thea-cyanoketones anda-bromoketoneproducts readily undergotrans-methylation and angular-alkylation as electrophiles.