THIELES ACID REVISITED - ISOLATION AND CHARACTERIZATION OF 2 MINOR PRODUCTS FORMED BY CARBONATION OF CYCLOPENTADIENIDE ANION

Carbonation of sodium cyclopentadienide leads to the formation of 3a a lpha,4 alpha,7 alpha,7a alpha-tetrahydro-4,7-methano- 1H-indene-2,6-di carboxylic acid [i.e., ''Thiele's acid'', 2i (R = H), 60%] along with several isomeric C12H12O4 minor products. The dimethyl esters of two o f these minor products, i.e., dimethyl 3a alpha,4 alpha,7 alpha,7a tet rahydro-4,7-methano-1H-indene-2,4-dicarboxylate [2a (R = Me), 13%) and the corresponding 3a,6-dicarboxylate [2n (R = Me), 1%] have been isol ated and characterized. Intramolecular [2 + 2] photocyclization of 2a (R = Me) afforded 3a (R = Me) (30%), whose structure was established u nequivocally by application of X-ray crystallographic techniques. (C) 1998 Elsevier Science Ltd. All rights reserved.