Synthesis of 1,2,3,4,5‐Penta(methoxycarbonyl)cyclopentadienides through Electrocyclic Ring Closure and Ring Contraction Reactions

Abstract

The mechanism for the reaction of dimethyl malonate with dimethyl acetylenedicarboxylate in the presence of pyridine and acetic acid to form isomeric octa(methoxycarbonyl)cycloheptadienes was established. The formation of intermediary N ‐[di(methoxycarbonyl)vinyl]pyridinium acetate was proven by means of NMR spectroscopy. On the basis of the established mechanism, dimethyl dibromosuccinate was proposed to be used instead of dimethyl acetylenedicarboxylate. A novel method for the synthesis of penta(methoxycarbonyl)cyclopentadienylsodium through the reduction of hepta(methoxycarbonyl)cycloheptatriene with sodium borohydride, electrocyclic ring contraction and retro‐[2+2]‐cycloaddition was developed.