Dedicated to Dr. G. Ohloffon the occasion of his 65th birthday (25.1.89) The [Co,(CO),]-catalyzed reaction between propyne and CO in acetone at 110" and 170 bar was reinvestigated. There are five major products: (E)-3,4'-dimethyl-2,2'-bifurylidene-S,S'-dione (4), 3,5,8trimethylcoumarin (8), 3a,7a-dihydro-2,4,7,7a-tetramethyl-lH-inden-l-one ( 9), 2,6-dimethyl-SH-furo[3,2-h]pyran-5-one (ll), and 2,7-dimethyl-SH-furo[3,2-h]pyran-S-one (12); of these, only one, 4, had previously been recognized. In parallel experiments were obtained 2,6-dipentyl-5H-furo(3,2-b]pyran-5-one (13), 2,7-dipentyl-5H-furo[3,2-h]pyran-5-one (14), 3a,7a-dihydro-2,4,7,7a-tetrapentyl-IH-inden-l-one (15), and 3a,7a-dihydro-2,4,6,7a-tetrapentyl-lH-inden-l -one (16) from hept-I-yne, and two further types of products from two atypical I-alkynes: 3,6,9-tri(rert-butyl)-I-oxaspiro[4.4]nona-3,6,8-trien-2-one (20) from (tert-buty1)acetylene and the em-dimer 21 of 2,S-bis(trimethylsilyl)cyclopenta-2,4-dien-1 -one (22) from (trimethylsily1)acetylene. Compounds 11, 12, and 20 were identified by X-ray analysis. ') Previous nomenclature: bifurandiones [ Ib-e1,octatrienediolides [ If-h]; systematic nomenclature: 5-(oxofuran-2(5H)-yl idene)furan-2(5H)-ones.