Photo-reorganization of 3-alkoxy-6-chloro-2-(thiophen-3-yl)-4H-chromen-4-ones: Regioselective cyclization via γ-hydrogen abstraction

Regioselective photocyclization of 3-alkoxy-6-chloro-4H-chromen-4-ones bearing thiophen-3-yl moiety at 2-position has been described. These chromenones on irradiation by the pyrex filtered UV-light produced a diverse array of novel angular tetracyclic photoproducts. Of these, mostly the cyclic dihydro and dehydrogenated photoproducts had the gem-dihydro functionality and exocyclic double bonds onto the fused pyran ring respectively, which is unprecedent in these chromenones.

The 1,4-biradicals generated through the Norrish type-II process in 3-alkoxy/allyloxy-2-(thiophen-3-yl)chromenones produced a diverse array of the novel dihydro and dehydrogenated angular tetracyclic photoproducts. Most of these products originated from the Dewar thiophene type intermediate generated in situ.Figure optionsDownload as PowerPoint slideHighlights
► Regioselective photocyclization of thienylchromenones.
► Novel photoproducts having gem-dihydro functionality.
► Intermediacy of Dewar thiophene type moiety.
► [1,7]-H shift.