Diels-Alder Reaction of Substituted Cyclopentadienones with Benzothiophene S, S-dioxides: Synthesis of Dibenzothiophenes, Dibenzothiophene S, S-dioxides, Acenaphthenes, Phenanthrenes, Pyrenes, and Fluorenones by Exploiting Thermolysis of Diels-Alder Adducts

Diels-Alder (DA) reaction of a series of tetra substituted cyclopentadienones (CPDs) with benzo[b]thiophene S, S-dioxide analogues in various solvents at different temperatures led to the formation carbocyclic as well as heterocyclic compounds. The intermediate Diels-Alder adducts underwent CO extrusion followed by different aromatization patterns depending upon the nature substituents on dienes as well as dienophiles, and temperature to afford these compounds. While at temperature <150 oC, the adducts mostly furnished annulated benzo[b]thiophenes involving 1,3-H shifts followed by deoxygenation or afforded substituted arenes through SO2 extrusion. However, at an elevated temperature (>200 oC), the intermediate dihydro dibenzothiophene S, S-dioxides underwent an exclusive SO2 elimination pathway followed by an intramolecular cyclization to furnish aryl-/hetero-aryl fused phenanthrene and pyrene derivatives. Relatively, the Diels-Alder reaction of phenanthrene as well as pyrene fused CPDs required high temperature for CO and SO2 extrusions accompanied by annulation to give the corresponding π-conjugated arenes as well as hetero-arenes. The DA reaction of CPDs could also be performed with 2-indenone as a dienophile to assemble unsymmetrical fluorenones via CO extrusion followed by 1,3-H shift and subsequent oxidation of the resulting fluorenols. Alternatively, the DA reaction of acenaphthene based CPD containing electron withdrawing ester units at 7, 9-positions could be carried out with benzo[b]thiophene S, S-dioxides in nitrobenzene at reflux to afford the respective fluorenones involving SO2 extrusion followed by intramolecular anionic cyclization with an ester unit.