Reactions of dipivaloylketene and its dimer with C-nucleophiles

Dipivaloylketene 1 as well as its dimer 2 add 1,3-diketones as C-nucleophiles in strongly basic medium affording the open-chain tetracarbonyl compounds 3 . The dibenzoylmethane adduct 3a undergoes acid induced cyclocondensation, and under concomitant loss of one pivaloyl group the 4-pyrone derivative 4 is formed. Employing diethyl malonate as C-nucleophile provides the 2-pyrone 5 .

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 .The reaction of 1 with nucleophiles provides the corresponding dipivaloyl acetic acid derivatives in excellent yields.On the other hand, the products obtained from the reaction of dimer 2 with nucleophiles are 2,9-dioxabicyclo[3.3.l]nona-3,7-dienesfeaturing the rather rare molecular skeleton of a concave, bridged bisdioxine exhibiting axial chirality 3 .Furthermore, these compounds can be easily transferred by acid hydrolysis into functionalized 2,4,6, 8-tetraoxatricyclo[3. NuH These findings prompted the investigation of the reactivity of these stable α -oxoketenes 1 and 2 towards carbon-nucleophiles.

Results and Discussion
1,3-Diketones (dibenzoylmethane, dimesitoylmethane, dipivaloylmethane, and dimedone) were first treated with sodium hydride (50% suspension in mineral oil) 5 in acetonitrile to generate the corresponding carbanions.Subsequently, oxoketenes 1 or 2 were added at room temperature, and after 24 h adducts 3 were isolated.Although several tautomeric forms can be envisaged for these products, the tetracarbonyl structure 3 (as depicted in Scheme 2) is in agreement with the spectroscopic data: the dipivaloylmethyl moiety of compounds 3 is evident from the characteristic C-H signals in the range of δ 6.2 -6.8; these shift values indicate a rather strong deshielding due to the anisotropic effects of vicinal carbonyl groups.Products 3 do not show any tendency to form other enol tautomers (as evidenced from the NMR spectra and within the accuracy of this method).Earlier measurements of dipivaloylacetic acid derivatives in the solid state and in solution of different solvents demonstrated a similar behaviour.3b, 6 Further support comes from theoretical investigations on keto-enol tautomerism of β-tricarbonyl compounds. 6he observation of a single C-H signal in structures 3 is in agreement with the corresponding 13 C NMR signal of 3a (δ 64.2, d, 1 J= 125 Hz) and leads to the conclusion that only the monoenol forms 3 are resent.

or 2
(1) The experimental findings that compounds 3 were also obtained from the reaction of the xoketene 2 confirms earlier results 3b that strong bases (e.g.triethylamine 3b ) catalyze the dissociation of the dioxinone ring into two monomers of ketene 1. Attempts to cyclize the openchain compounds 3 were successful only in case of 3a; treatment with polyphosphoric acid at 90 0 C afforded the 4-pyrone derivative 4.Under the reaction conditions applied, one pivaloyl group was lost.Examples for similar deacylation reactions of polyacyl compounds in acidic media are well known. 7Allring carbon atoms of 4 are readily assigned from their 13 C-NMR signals.
Surprisingly, when oxoketenes 1 or 2 were treated with ethyl malonate and NaN, no open-chain adduct could be isolated, the 4-hydroxy-2-pyrone derivative 5 was the only reaction product obtained.Its structure was elucidated mainly by evaluation of its 13 C-NMR data.A plausible reaction mechanism for the formation of 5 is given in Scheme 3.

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.After 24 h the mixture was diluted with H20 (5 mL), neutralized with aqueous HCl, and extracted with dichloromethane (4 times S mL).The combined organic layers were dried over Na2SO4 for 4 h.After filtration and evaporation of the solvent the colourless crude residue was recrystallized from petroleum ether (60-80 0 C) or petroleum ether / ethyl acetate.
In summary, the reaction of the stable α-oxoketenes 1 and 2 with C-nucleophiles gives rise to C-C bond formation leading to tetracarbonyl compounds 3 and / or pyrone derivatives 4 and 5 in moderate to good yields.In order to generate the required carbanions, the reactions have to be performed in the presence of a strong base, which in case of the dimer 2 also a cycloreversion of the dioxinone ring 3b .Formation of bridged bisdioxines (see Scheme 1) was not observed in the course of this work.