Direct observation of .alpha.-oxo ketenes formed from 1,3-dioxin-4-ones and the enols of .beta.-keto esters

Acetylation products of pentacyclic triterpene glucosides from Combretum psidioides

“…The IR spectrum of compound 1 exhibited an absorption band at 1750 cm -1 for the ester carbonyl groups. The molecular formula C 56 H 78 O 21 was revealed by DEPT 13 C NMR data, and by the positive APCI (atmospheric pressure chemical ionisation) mass spectrum which displayed a pseudo-molecular ion at m/z 1109 ([M+Na] + ). The peak at m/z 331 suggested the presence of a tetraacetylated hexopyranosyl moiety.…”

“…Indeed it has been shown that acetylketene can be generated by thermolysis of ethyl and t-butyl acetoacetate at 92-106 °C and made to react in situ with nBuOH and various tertiary alcohols to give the corresponding acetoacetate esters. 20,21 In contrast to Ac 2 O acetylketene now may enter into the interior endo-face of the cis-fused bicyclic D-E ring system where it reacts with the hindered 19-OH group to form a β-ketoester intermediate; eventually this β-ketoester transforms into the geometric isomers 1, 2 after further O-acetylation of the corresponding enol forms. Acetylation using acetic anhydride and DMAP at room temperature for 48 hours also resulted in isolation of compounds 1 and 2.…”

On the thermal stability and thermal decomposition kinetics of some mono- and disubstituted furan-2,3-dions

“…The resulting biradical can be stabilized either by dimerization to the cyclic dimer: The second possibility seems more probable [15].…”

A simple and an efficient approach to the synthesis of a special tautomer of 1,3-thiazinones and 1,3-oxazinones

“…α-Oxoketenes are highly reactive molecules which usually cannot be isolated under ordinary reaction conditions, although several examples have been reported that the ketene bands of the α-oxoketenes have been detected at low temperature by infrared spectroscopy technique. [1][2][3][4] However, some of these ketenes can be stabilized both sterically and electronically. For example ketenes containing carboxylic acid derivatives such as chlorocarbonyl ketenes (CCKs) are extraordinarily stable.…”