l/í-Indene-3-carboxylic acid (lb) and its methyl ester (lc) react when heated via intermediate 2H-indenes (isoindenes) with the more reactive ethylenic dienophiles, giving the corresponding 1:1 Diels-Alder adducts as l,2,3,4-tetrahydro-l,4-methanonaphthalene-l-carboxylic acid 2,3-derivatives 3. Thus, 1:1 adducts were obtained with lb in refluxing xylene with maleic anhydride (3b, 71%) and V-phenylmaleimide (3g, 46%) and in refluxing 1,2-dichlorobenzene (but not in xylene) with dimethyl fumarate (5c, 19%). The less reactive lc gave a 1:1 adduct (3c, 43%) in refluxing xylene with maleic anhydride but not with N-phenylmaleimide. That the reaction is quite sensitive to steric hindrance is shown by the facts that lb failed to give 1:1 adducts in refluxing xylene with citraconic (methylmaleic) anhydride, di-chloromaleic anhydride,3-nitrostyrene, tetracyanoethylene, and diethyldiazenedicarboxylate, neat at 130 °C with phenylmaleic anhydride, or in refluxing 1,2-dichlorobenzene with cinnamic acid. The transformations which were carried out include hydrolysis of adduct 3b to the 2-endo,3-endo triacid 4b (64%), Fischer esterification to the trimethyl ester 4d (69% from 3b, 66% from 4b), thermal dehydration of 4b back to 3b, and methanolysis of 3b to a monomethyl 3-ester (4e, 83%) which underwent simple hydrolysis in 5% NaOH to 4b (22%) but epimerized during hydrolysis in methanolic 5% KOH to the 2-endo,3-exo triacid 5a (83%; similarly from 4d, 60%), which at 300 °C was reepimerized and dehydrated to 3b (22%). Hydrolysis of the 2,3-diester adduct 5c with both 5% NaOH (68%) and methanolic 5% KOH (60%) also gave 5a, showing that 5c has the same stereochemistry as 5a. With diazomethane, 5a gave its trimethyl ester (5b, 59%). Hydrolysis of adduct 3c with 5% NaOH gave 4b (68%), while neutral hydrolysis gave the 1-methyl ester 4c (78% as a hydrate), which at 150-160 °C was reconverted to 3c (99%). Epimerization also occurred during the acidic hydrolysis of adduct 3g, giving 5a (11%). With diazomethane, 3g gave the 1-methyl ester 3h (72%), which was not obtained directly by reaction of lc with iV-phenylmaleimide.