Unusual ring transformation of N-hydroxy-3 , 5-dinitro-4-pyridone Affording a polyfunctionalized pyrrole

Polyfunctionalized pyrrole 6 is synthesized in the ring transformation of N-hydroxy-3,5-dinitro4-pyridone 4 with enolate 2 derived from diethyl 3-oxogulutarate. The present reaction proceeds with C-N transfer from 4 to 2, which is hitherto unknown manner in similar reactions using dinitropyridone series.

This result prompts us to study the ring transformation of 4 with bidentate enolate 2a.As a result, we found an unusual and new ring transformation leading to polyfunctionalized pyrrole 6.
In the reaction of pyridone 4 with sodium enolate 2a, a trace amount of crystalline product 6 is isolated in addition to usual ring transformed product, 1 N-hydroxy-3,5-bis(ethoxycarbonyl)-4pyridone.The observation of two unequivalent ethoxy signals in the 1 H NMR of 6 suggests that diester 2 acquires unsymmetrical framework.On the basis of spectral and analytical data, the structure of 6 is determined as 2,4-bis(ethoxycarbonyl)-3-hydroxypyrrole, which is finally confirmed by X-ray crystallography (Figure 1).Acetonitrile is found to be the suitable solvent, and heating is also necessary (Table 3).Using triethylammonium enolate 2b is effective to improve the yield of 6 up to 30 % with easier experimental manipulations.Pyridone 4 has an acidic hydroxy group enough forming ammonium salt 7 with amines, and whole salt 7 is returned to 4 under acidic conditions without forming any by-products.On the other hand, O-protected dinitropyridone 8 only affords complex mixture under the same conditions used for the reaction of 4. Taking these experimental facts into consideration, we suggest a plausible mechanism as illustrated in Scheme 2.
The acidic N-hydroxy group is considered to play an important role for causing unusual ring transformation.Initially formed salt 7' undergoes ring opening reaction leading to nitroso compound 9 prior to the attack of enolate 2 to 4 that is prevented by anionic property of the pyridone ring.After addition of enolate 2 to 9, regenerated enolate 10 constructs a five membered ring by intramolecular cyclization.The following aromatization of 11 furnishes pyrrole 6. Detailed study on this reaction (determination of the mechanism and application to other active methylene compounds) is in progress, and new results will be shown in due course.

Experimental Section
General Procedures.Melting points (uncorrected) were determined on a Yanaco micromelting-points apparatus. 1H NMR and 13 C NMR spectra were recorded on a Bruker DPX 400 spectrometer at 400 MHz and at 100 MHz with TMS as an internal standard.Coupling constants are given in Hz and without sign.The IR spectra were recorded on a Horiba FT-200 IR spectrometer.Elemental analyses were performed using a Yanaco MT-3 CHN corder.

Materials.
All the reagents were commercially available and used as received.Solvents were dried and distilled according to usual methods.

Table 1 .
Intramolecular bond angles involving the nonhydrogen atoms

Table 2 .
Intramolecular bond length involving the nonhydrogen atoms

Table 3 .
The effects of solvents and counter cations of enolate