Regioselective Oxidation of 3-Substituted Pyridinium Salts

(1'R)-(+)-3-Hydroxymethyl-1-(1'-phenyl-ethyl)-pyridinium chloride (1), 1-benzyl-3-[1', 3']-dioxolan-2'-yl-pyridinium chloride (2) and (2'S, 4'S, 5'R)-(-)-1-benzyl-3-(3',4'-dimethyl-5'-phenyl-oxazolidin-2'-yl)-pyridinium bromide (3), were transformed by oxidation with potassium ferricyanide into the corresponding 1H-pyridin-2-ones in excellent yields with high regioselectivity.


Introduction
The oxidation of chiral pyridinium salts is an area of research with wide interest because enantiopure 1H-pyridin-2-ones obtained can be used in the asymmetric synthesis of alkaloids.The synthesis of chiral 1H-pyridin-2-ones is useful, because the starting material is easily obtained and the regioselectivities of the reactions can attain high values depending on the substituent at position 3. [1,2,3,4,5,6,7,8].In a preliminary communication [9], we reported the oxidation of 3-(methyl and ethyl) pyridinium salts, where in all cases, the oxidation at the 2-position in the starting material was favored.Now, we report three different examples of pyridinium salts differently substituted at position 3, incorporating chiral substituents in the quaternary nitrogen or at position 3, which were oxidized with potassium ferricyanide.
We observe an increasing percentage of oxidation at position 6 when the bulkiness of the substituent was increased.The products obtained were characterized by NMR and in one case, by X-ray diffraction.

Conclusions
These results show that the steric hindrance exerted by the substituent at position 3 plays a key role in the extent of 6-oxidation in the starting material.In particular, we found that the size of the substituent in

General.
Melting points were determined in open capillaries and are uncorrected.IR spectra were recorded in KBr on a Nicolet Magna-750 spectrophotometer.NMR spectra were measured on a Jeol 400 MHz.spectrometer, using TMS as the internal standard.
1 H-NMR assignments were confirmed by extensive use of 13 C-1 H correlation techniques.Optical rotation was measured on a Perkin-Elmer Polarimeter M241.X-ray diffractions were measured on a Siemens P4/PC diffractometer.