Cycloaddition Reactions of Indenes. Adducts of 1H-Indene-3-carboxylic Acid with Ethylenic Dienophiles

Article abstract of DOI:10.1021/jo01305a017

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