The starting materials of the formula (IV), i.e., N-mesyl-O-(p-alkoxybenzyl)hydroxylamines or N-mesyl-O-(2,4,6-trialkylbenzyl)hydroxylamines can be easily prepared by a well-known technique, for example, by reacting carboethoxyhydroxylamine with a p-alkoxybenzyl halide or a 2,4,6-trialkylbenzyl halide of the formula Y--CH2 Z wherein Y is as defined above and Z represents a halogen atom such as a c