Reaction of dimethyl acetylenedicarboxylate with derivatives of malonic acid: pentamethoxycarbonylcyclopentadienide anion, tetramethoxy-carbonylcyclopentadienone, cyanotetramethoxyfulvenolate anion, and related compounds

Abstract

The revised formulation of the adducts of dimethyl malonate with dimethyl acetylenedicarboxylate in the presence of pyridine and acetic acid as tautomeric octamethyl cycloheptadienoctacarboxylates [(III) and (IV)] is confirmed. Both tautomers, or the analogues from other esters of malonic acid, are converted by potassium acetate into Diels' potassium pentamethoxycarbonylcyclopentadienide (VIa). Diazomethane methylates the free cyclo-pentadiene, which is a very strong acid, to the C-methyl derivative (Vb), easily hydrolysed to the 1,2,3,4-tetramethoxycarbonyl-5-methylcyclopentadienide anion (VIII). The C-chloro- and bromo-derivatives (Vd and e), formed by treatment of the salt (VIa) with chlorine or bromine, are strong halogenating agents.

The yellow pyridinium 1-cyano-1-ethoxycarbonyl-2,3,4,5-tetramethoxycarbonylpentadienide (XIIIa) produced by reaction of cyanoacetic ester with dimethyl acetylenedicarboxylate in the presence of pyridine and acetic acid [corresponding to our original formula for (III)] easily loses a mole of methanol to form a blue dye, pyridinium 2-cyano(ethoxycarbonyl)methylene-3,4,5-trimethoxycarbonylcyclopentadienolate (XIVa). The colourless pseudo-acids reversibly formed by treatment of the blue dyes (XIV) with acid are dimers, but the absence of an infrared carbonyl stretching band at high enough frequency makes it unlikely that they are Diels–Alder dimers of the cyclopentadienones (XVIII). The bridging carbonyl group of tetramethoxycarbonylcyclopentadienone dimer (XXIVc), for example, absorbs at 1835 cm.^–1.

The blue dye (XIVd) from p-bromobenzyl cyanoacetate adds ethanol at the cyclopentadienone carbonyl group to give a colourless product (XXVIa) that regenerates the blue dye with elimination of ethanol on reaction with pyridine. The analogous methanol derivative (XXVIb) is the conjugate acid of the intermediate anion (XIIIa) in the usual synthesis of the blue dye.