Synthesis and solution structure of 3,5-dioxopimelic acid diesters—stable 1,3,5,7-tetracarbonyl derivatives

Abstract

A variety of 3,5-dioxopimelic acid diesters, stable 1,3,5,7-tetracarbonyl derivatives, were prepared by catalytic condensation of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with methyl malonyl chloride. The keto-enol tautomerization of these compounds has been investigated by NMR spectroscopy. One keto and up to four enolic tautomers could be detected in chloroform solution and the influence of the substituents on the tautomeric equilibria has been studied.

Experimental and Theoretical Study of the Keto–Enol Tautomerization of 3,5‐Dioxopimelates

“…We have previously reported [5] that 3,5-dioxopimelates 3a-e exist in five tautomeric forms (Scheme 2). For reasons of comparison, the tautomeric ratios are again listed in Table 1.…”

“…The synthesis of derivatives 3a-e has been previously reported. [5] Compounds 3a and 3d are unsubstituted, whereas 3b,c,e contain a substituent located at the central carbon atom (C-5, for numbering see Scheme 2). Derivatives 4a-e contain a substituent located at a lateral carbon atom (C-7, for numbering see Scheme 2) located next to an ester group.…”

“…The NMR signals of the tautomers of 4 were assigned based on their intensities and chemical shifts and couplings as well as additional proton-carbon chemical shift correlation experiments (HSQC, HETCOR, HMBC spectra) and by comparison to the known [5] NMR spectroscopic data of compounds 3. For example, a typical region in the 1 H NMR spectra is the one for the enolic OH group signals with a characteristic downfield shift of the enol-I signal and with two equally intensive signals for the dienol (Figure 1).…”

“…Because of their two terminal ester groups, which are less electrophilic than keto groups, 3,5-dioxopimelates do not undergo intramolecular aldol reactions under the conditions of their formation. In our first report in this field, [5] we studied the synthesis of unsubstituted 3,5-dioxopimelates and derivatives containing a substituent located at the central 1,3-dicarbonyl unit. Herein, we report, for the first time, the synthesis of 3,5-dioxopimelates containing a substituent located at a lateral 1,3-dicarbonyl unit (located vent on the keto-enol tautomerization of these compounds was investigated by NMR spectroscopy.…”

“…[4] Recently, we have reported the synthesis and first detailed spectroscopic characterization of 3,5-dioxopimelates by condensation of 1,3-bis(silyloxy)-1,3-butadienes with methyl 3-chloro-3-oxopropanoate. [5] 3,5-Dioxopimelates are of fundamental chemical interest as they represent rare examples of stable 1,3,5,7-tetracarbonyl compounds. Because of their two terminal ester groups, which are less electrophilic than keto groups, 3,5-dioxopimelates do not undergo intramolecular aldol reactions under the conditions of their formation.…”