1-Methylindene, 3-methylindene, 1,3-dimethylindene, and 3,4,7-trimethylindene react with dimethyl acetylenedicarboxylate at room temperature (20 d) to give mainly the 1 : 1 adducts (5). The adduct from 4,4â ², 7,7â ²-tetra-methyl-3,3â ²-bi-indene (1) is reformulated as (4); it is accompanied by a stereoisomer and a product derived by addition of three molecules of the dienophile to (1).