Dipivaloylketene and its dimers. [2+4] Versus [2+2] cycloaddition reactions of .alpha.-oxo ketenes

Synthetic and Theoretical Aspects of New Dimroth Rearrangement of6‐Aminopyran‐2‐ones to 6‐Hydroxypyridin‐2‐ones via Carbamoyl Ketenes

“…Formation of zwitterionic ketene-nucleophile adducts frequently have been postulated and recently confirmed by low-temperature NMR spectroscopy. [34,35] According to calculations, the stability of such zwitterions strongly depends on the solvent polarity. [35] Obviously then, thermal vs. nucleophile-initiated ring opening of pyranones 1 will depend on the nucleophilicity/basicity of the amine used in the respective reaction (Scheme 3).…”

Reactions of dipivaloylketene and its dimer with C-nucleophiles

“…Elemental Analyses were obtained from a CNN Elemental Analyzer Carlo Erba 1106. α-Oxoketenes 1 and 2 were prepared according to the literature. 1,2 General procedure for the reactions of α -oxoketenes 1 and 2 with C-nucleophiles The monosodium salts of 1,3-diketone and diethyl malonate were prepared 5 in situ by adding the active methylene compound (0.27 -0.71 mmol) to a stirred suspension of NaN (equimolar in 2 mL of acetonitrile) at rt during 10-15 mm. To this suspension the α-oxoketene 1 or 2 equimolar amount) was added at once, and stirring was continued at rt.…”

“…Dipivaloylketene 1 is obtained from preparative flash vacuum pyrolysis (FVP) of the corresponding furan-2,3-dione in excellent yield. 1 It slowly dimerizes to afford the 4H- [1,3]dioxin-4-one derivative 2 possessing also an α-oxoketene moiety 1,2 . Both compounds are remarkably stable and have been found to smoothly add N-as well as O-nucleophiles under very mild reaction conditions 3 .…”

Synthesis of 4H‐1,3‐oxathiin‐4‐ones by reaction of α‐diazo‐β‐diketone with thiones

“…The acylketenes are also generally generated in situ by flash vacuum pyrolysis of 2,3-dihydrofuran-2,3-diones [4]. The acylketenes are highly reactive and useful synthons for the synthesis of oxy-containing sixmembered heterocyclic compounds [3].…”