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

Abstract

The Diels–Alder (DA) reaction of a series of tetrasubstituted 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 DA adducts underwent CO extrusion followed by different aromatization patterns depending upon the nature of substituents on dienes as well as dienophiles, and temperature to afford these compounds. At temperatures <150 °C, 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 °C), the intermediate dihydrodibenzothiophene S,S-dioxides underwent an exclusive SO₂ elimination pathway followed by an intramolecular cyclization to furnish aryl-/hetero-aryl fused phenanthrene and pyrene derivatives. In contrast, the DA 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 CPDs 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.