Abstract
The thesis is concerned with the regio-, stereo- and facial diastereoselective Diels-Alder reactions of dissymmetric 2,4-cyclohexadienones 24b-e and deals with the kinetic studies of Diels-Alder reactions of 2,4-cyclohexadienones 24a-c. We have observed that 5-methoxy masked o-benzoquinone (24a) and simple dissymmetric 2,4-cyclohexadienones (24b-e) undergo Diels-Alder reactions with methyl vinyl ketone, styrene and benzyl vinyl ether in highly chemo-, regio-, and stereoselective manner and the dissymmetric 2,4-cyclohexadienones (24b-e) reacted with dienophiles form syn adducts (dienophile approach is syn to allylic methoxy group) exclusively. HOMO-LUMO theoretical calculations further helped us to understand the regioselectivity of these reactions. The kinetic studies of 6,6-dimethoxy, 6-methoxy-6-methyl and 6,6-dimethyl-2,4-cyclohexadienones (45) were performed in toluene. In the reactions where diene is 45b the dienophile approaches syn to 6-methoxy. From the experimental data, the Diels-Alder reaction is first order in diene and first order in dienophile, and thus the overall cycloaddition is second order. The rate of Diels-Alder reaction of 6,6-dimethoxyl-2,4-cyclohexadienone (45a) is faster than 6-methoxy-6-methyl- and 6,6-dimethyl-2,4-cyclo- hexadienones (45b-c) with methyl vinyl ketone and ethyl acrylate (about 7-9 times at 63 ℃) as dienophiles. When the less reactive dienophile 2-methoxyfuran was employed, the relative rates of 2,4-cyclohexadienones differed by a large magnitude. From the solvent effect studies, the Diels-Alder reaction is found to be independent of solvent so it may proceed through concerted mechanism. From the kinetic parameters viz. activation energy (Ea) and pre-exponential factor (ln A), enthalpy (ΔH‡), entropy (ΔS‡) of activation and Gibbs free energy (□G‡), we know that these Diels-Alder reactions are exothermic and non-spontaneous reactions. Theoretical calculations have supported the experimentally observed facial selectivity i.e., syn-approach of the dienophile to methoxy group in the diene (whose activation energy is less compared to the dienophile approaching from other face), which is apparently due to the electronic effect of the methoxy group. The transition state calculations showed that these reactions go through asynchronous concerted pathway.