1H-Indenes (1) react with dimethyl acetylenedicarboxylate (DMAD), unlike maleic anhydride and other ethylenic dienophiles, without prior isomerization to 2/f-indenes (2), giving a 1:1 Diels-Alder adduct (6) formed with destruction of the aromaticity of the benzene ring. This intermediate, not isolated in the present work, appears to serve as the precursor for all further adducts. Thus, lif-indene (la) and 1-methyl-lfl-indene (lb), but not the more sterically hindered 1-ethyl- -indene, react with DMAD in refluxing benzene (with la) or toluene (with lb) via a 1,2-cycloaddition to 6 to give solid 1:2 adducts (7a, 34%; 7b, 30%). In refluxing xylene the reaction goes further to give a 1:3 adduct (11a; 40% from la, 71% from 7a) formed by a Diels-Alder addition of a third molecule of DMAD across the remaining diene system of 7a. Reaction of 2-methyl-lfl-indene (lc) with DMAD in refluxing xylene gave the corresponding 1:3 adduct (11c, 5-6%), but an attempt in refluxing toluene to isolate a solid 1:2 adduct (7c) was unsuccessful. A 3-substituent in the 1/f-indene, which becomes a 4-substituent in 6, blocks the 1,2-cycloaddition (to give 7) and diverts the DMAD to the cyclohexadiene system of 6, where a Diels-Alder reaction occurs in refluxing xylene to give another type of 1:2 adduct (8). The following 3-substituted 1.H-indenes (1) gave 1:2 adducts of type 8: 3-methyl- (8d, 41%), 3-ethyl- (8e, 40%), 1,3-dimethyl- (8f, 31%), 2,3-dimethyl- (8g, 19%), 3-carboxy- (81, 74%), 3-(methoxycarbonyl)- (8m, 66%), and 3-cyano- -indene (8n, 63%). Alkaline hydrolysis of 81 and acidification to pH <2 gave the monosodium salt (91%) of the corresponding pentacarboxylic acid (131). Hydrogenation of 81 over Pt02 gave a tetrahydro derivative (141,100%). That maleic anhydride can take the place of the second (but not the first) molecule of DMAD in 8 is shown by the formation of a 1:1:1 mixed adduct (101,17%) along with the 1:2 adduct (81,18%) from reaction of 11, DMAD, and maleic anhydride in a 1:2:2 molar ratio in refluxing xylene. A similar 1:1:1 mixed adduct (10, 59%), but no 1:2 adduct (8n), was isolated from the corresponding reaction of In, DMAD, and maleic anhydride in a 1:1:1 molar ratio. Similarly, maleic anhydride can take the place of the third molecule of DMAD in the 1:3 adduct 11. Thus, reaction of 7a and 7b with maleic anhydride in refluxing xylene gave the corresponding 1:2:1 mixed adducts (12a, 69%; 12b, 32%), formed by Diels-Alder addition across the cyclohexadiene system of 7a and 7b. Reaction of 1- methyl-lff-indene (lb), DMAD, and maleic anhydride in a 1:2:1 molar ratio in refluxing xylene also gave 12b (31%) but no 1:3 adduct (lib). Methyl esterification of 12a gave the corresponding hexamethyl ester (15a, 52%). On the basis of the shielding effects of neighboring ethylene groups on the methylene bridge protons, an NMR rationale has been developed for assignment of stereochemistry to the adducts 8-12.