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Density functional theory has been utilized for exploring the mechanism of Diels-Alder reaction between 7H-benzo[a]phenalene and maleic anhydride. 7H-Benzo[a]phenalene is an antiviral compound and information available about its cycloaddition reactions with possible reaction path and mechanism is scarce. In order to work on the synthesis of its further potential derivatives, the mechanism of its reaction with all aspects should be well understood. Two novel intermediates involved in this reaction have been reported. Diels-Alder reaction of maleic anhydride has found many applications in the synthesis of wide range of useful products. The major concern of this work is to evaluate the consequences of introducing electron donating and electron withdrawing substituents on the reactivity of maleic anhydride towards 7H-benzo[a]phenalene. Thermodynamic parameters, activation parameters, energies of frontier orbitals, global reactivity indices and global electron density transfer (GEDT) have been determined for all the reactions. Fukui functions are computed for each reactant in order to identify the most reactive sites. All the reactions have been found to proceed via normal electron demand having polar nature. The substituents with opposite electronic properties were expected to affect the reactivity of dienophile in an inverse manner, however, the results are not according to this assumption. Rather, both kinds of substituents increased the activation barrier of the reaction. This behavior has been explained in the light of various parameters such as the stability of reacting species, gap of frontier molecular orbitals etc. Experimental studies reported previously are in agreement with these results.

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