Photoinduced Double Addition of Acetylene to 3‐Oxocyclopent‐1‐ene‐1‐carbonitrile or 3‐oxocyclopent‐1‐enyl acetate leading to 2,3‐dihydro‐1 H ‐inden‐1‐one and other rearranged products

UV Irradiation of 3‐oxocyclopent‐1‐enyl acetate ( 17 ) and acetylene in MeCN at 0° gives, besides the product of normal enone‐alkyne [2 + 2] cycloaddition ( cis ‐4‐oxobicyclo[3.2.0] hept‐6‐en‐1‐yl acetate, 18 ) and its product of oxa‐di‐π‐methane rearrangement (5‐oxotricyclo[4.1.0.0 2,7 ]hept‐2‐yl acetate, 19 ), unexpected products of further addition of a molar equivalent of acetylene. These are indanone ( = 2,3‐dihydro‐1 H ‐inden‐1‐one, 16 ), in 21% yield, cis ‐1‐ cisoid ‐1,2‐ cis ‐2‐ ( 20 ) and cis ‐1‐ transoid ‐1,2‐ cis ‐2‐7‐oxotricyclo[4.3.0.0 2,5 ]non‐3‐en‐1‐yl acetate ( 21 ), 4‐oxo‐7‐‘ exo ’‐vinyltricyclo[3.2.0.0 2,6 ]hept‐2‐yl acetate ( 22 ), cis ‐4‐oxo‐6‐‘ endo ’‐ ( 23 ) and cis ‐4‐oxo‐6‐‘ exo ’‐vinylbicyclo[3.2.0]hept‐1‐yl acetate ( 24 ), and cis ‐4‐oxo‐7‐‘ exo ’‐vinylbicyclo[3.2.0]hept‐1‐yl acetate ( 25 ). At least in part, indanone must be formed via intermediates 20 and 21 . In fact, on heating a 9:1 mixture 20/21 , indanone is obtained quantitatively. With 3‐oxocyclopent‐1‐ene‐1‐carbonitrile ( 15 ) in place of 17 , indanone is formed in lower (8%) yield besides much tars.