Thiele's acid revisited: Isolation and characterization of two minor products formed by carbonation of cyclopentadienide anion

Article abstract of DOI:10.1016/S0040-4020(98)00770-4

Carbonation of sodium cyclopentadienide leads to the formation of 3aα,4α,7α.7aα-tetrahydro-4,7-methano-1H-indene-2,6-dicarboxylic acid [i.e., 'Thiele's acid', 2i (R = H), 60%] along with several isomeric C₁₂H₁₂O₄ minor products. The dimethyl esters of two of these minor products, i.e., dimethyl 3aα,4α,7α,7aα-tetrahydro-4,7-methano-1H-indene- 2,4-dicarboxylate [2a (R = Me), 13%) and the corresponding 3a,6- dicarboxylate [2n (R = Me), 1%] have been isolated and characterized. Intramolecular [2 + 2] photocyclization of 2a (R = Me) afforded 3a (R = Me) (30%), whose structure was established unequivocally by application of X-ray crystallographic techniques.