A Comparison of Phosphonium and Phosphonate Carbanion Reagents in
Reactions with 1,3-Diphenyl-2-(hydroxyimino)-1,3-propanedione

Abstract

1,3-Diphenyl-2-(hydroxyimino)-1,3-propanedione 1 reacted with phosphonium ylides 2a,b to give mainly the substituted 1-hydroxy-2,3-dihydropyrroles 6a,b along with the novel ylides 10a,b whereas the phosphono substituted-N-heterocycles 11a,b and 14a,b were obtained from the reaction of 1 with α-phosphoryl carb­anion counterparts 3a,b. Reaction of 1 with cyanomethylene(tri­phenyl)phosphorane 2c afforded the Wittig product 17 and the oxazole derivative 18 while the phosphono 2,2-disubstituted dihydrooxazole 20 and the diolefin 19 were produced in the reaction with the phosphonate ylide 3c. Application of the unsaturated carb­anions 2d and 3d on 1 yielded the pyrrole 23 and the phosphono substituted 2H-1,4-oxazine 25, respectively.

Key words

α-imino ketones - oximes - Wittig reagents - Wittig-Horner synthons - N-heterocycles - phosphonates

References

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8

Typical procedure: To a stirred solution of alkoxycarbonylmethylene(triphenyl)-phosphorane 2a,b (6.32 mmol) in 20 mL dry CHCl₃ was added a solution of oxime 1 [7] (0.8 g, 3.16 mmol) in 15 mL of the same solvent. The reaction mixture was boiled under reflux for ˜24 h (TLC). Removal of volatiles followed by column chromatography on silica gel, gradient eluting from 2% to 10% CHCl₃ in hexane, yielded compounds 6a,b and 10a,b.
Oxidation of 6a: N-Bromosuccinimide (NBS) (28 mg, 0.15 mmol) and benzoyl peroxide (3.6 mg, 0.02 mmol) were added to a solution of 6a (60 mg, 0.15 mmol) in 15 mL of dry CCl₄. The mixture was heated under reflux for 2 h and filtered while hot. Evaporation of the solvent left a residue, which triturated with diethyl ether, and crystallized from acetonitrile to give the dehydrogenated derivative 7.
Dehydration of 6a: Compound 6a (0.13 g) in polyphosphoric acid (PPA) (5 g) was heated at 120-130 °C for 1 h. The cooled reaction product was poured into ice-water (20 mL), and then extracted with CHCl₃. After evaporation of dried solvent, the residual solid was recrystallized from benzene to give the dehydrated product 8. Percentage yields, physical and spectral data of compounds 6a,b, 7, 8 and 10a,b are listed in Table [1] .

11

Typical procedure: To a stirred solution of oxime 1 (0.8 g, 3.16 mmol) in EtOH (20 mL), was added the appropriate phosphonate 3a,b (9.5 mmol) and an equimolar amount of LiOH (0.5 M, 20 mL). The reaction mixture was refluxed for ˜18 h (TLC), cooled to r.t., poured into a small amount of water and extracted twice with CHCl₃. Removal of the volatiles followed by chromatography on silica gel, gradient eluting from 2% to 10% CHCl₃ in hexane, yielded compounds 11a,b and 14. Oxidation of 14a to 15 was carried out as previously described for the conversion of 6a to 7 (cf. Table [1] ).

14

Typical procedure: A solution of cyanomethyl(triphenyl)phosphonium chloride 16 (1.14 g, 3.4 mmol) and the monoxime 1 (0.8 g, 3.16 mmol) in EtOH (40 mL) was stirred by means of an efficient magnetic stirrer. Freshly prepared aqueous LiOH (0.5 M, 15 mL) was added in one portion to the mixture and the system was further refluxed for 12 h. The combined organic extracts were washed with H₂O (30 mL), dried with CaCl₂, and the solvent was removed under reduced pressure. The residue was purified by chromatography on silica gel using hexane-EtOAc to give compounds 17 and 18 (cf. Table [2] ).

15

Typical procedure: A solution of the appropriate Wittig-Horner reagent 3c or 3d (9.48 mmol) and the monoxime 1 (0.8 g, 3.16 mmol) in EtOH (40 mL) was treated with LiOH (0.5 M, 20 mL), and the mixture was refluxed for 10 h (with 3c); or 6 h at r.t. (with 3d). The reaction mixture was worked up as described for the reactions of 3a,b, and separated by column chromatography using hexane-EtOAc yielding compounds 19 and 20 (with 3c) or 26 (with 3d) (cf. Table [2] ).

16

Typical procedure: A solution of vinyl(triphenyl)phosphonium bromide 21 (1.22 g, 3.3 mmol) and the oxime 1 (0.8 g, 3.16 mmol) in DMF (40 mL) was treated with LiH (500 mg) for 3 h at r.t. The reaction mixture was worked up as described for the reaction of 16 and separated by column chromatography using hexane-EtOAc (1:4) to give the pyrrole 23 (cf. Table [2] ).