Preparation of substituted pyridine N-oxide compounds

A process for preparing substituted pyridine N-oxide compounds of the formula

• in which R1, R2, R3 and R4 are each independently H, a carboxyl group or a C1-C12-alkyl radical which may optionally contain one or more atoms from the group of N, O and S, or R1 and R2 and/or R3 and R4 together may each form an optionally substituted C4-C20-alkylene radical which may contain one or more atoms from the group of N, O and S, A is benzyl or a (CH2)m group where m may be an integer from 1 to 12, Z1 and Z2 are each independently O or S, and Y is H, a C1-C12-alkyl radical which may optionally contain one or more atoms from the group of N, O and S, a C6-C20-aryl radical or a C5-C20-heterocycle, and the radicals may optionally be substituted by halogen, C1-C6-alkyl, nitro, phenyl or tert-amine, or Z2 and Y together may form an optionally substituted ring or an optionally substituted ring system, and the ring or the ring system may contain one or more atoms from the group of N, O and S,
• which comprises reacting a 4-nitropyridine N-oxide compound of the formula
• in which R1 to R4 are each as defined in formula (I), in the presence of a phase transfer catalyst and in the presence of an acid, with an alkali metal halide to give the corresponding compounds of the formula
• in which R1 to R4 are each as defined in formula (I) and X is chlorine, bromine or iodine, and then afterwards, after isolating it from the reaction mixture, reacting the compound of the formula (II), in the presence of a phase transfer catalyst and of a base, with a compound of the formula
• HZ1-A-Z2-Y (III)
• in which Z1, Z2, A and Y are each as defined above, at a temperature up to the reflux temperature, to give the corresponding substituted pyridine N-oxide compound of the formula (I).