Diphenyldiazomethane, a red crystalline solid is a useful reagent for the protection of carboxylic acids by conversion to their diphenylmethyl (dpm, benzhydryl) esters. Dpm protecting group has been widely used due to its easy deprotection by mild acidic condition or hydrogenolysis, especially in the field of β-lactams and peptides. Diphenyldiazomethane has been prepared by the oxidation of benzophenone hydrazone with mercuric oxide, peracetic acid, iodosobenzene diacetate or OXONE.4d However, some methods suffer from a disadvantage such as toxic nature of reagent, strong oxidative conditions or incompatibility with certain functional groups. For example, OXONE may not be employed for the in situ protection of carboxylic acid containing sulfide group due to the possibility of the concomitant oxidation of sulfide group. Recently, Magtrieve, a magnetically retrievable oxidant (CrO2) has been shown to oxidize alcohols in high yields. We have already shown that this reagent can serve as an oxidant for the aromatization of 1,4-dihydropyridines in high yields. We now report that Magtrieve can also serve as an efficient reagent for the oxidation of benzophenone hydrazone to diphenyldiazomethane, enabling the one-pot protection of carboxylic acids including N-protected amino acids, as shown in Table 1. Treatment of benzophenone hydrazone with Magtrieve in dichloromethane gave a purple color characteristic of diphenyldiazomethane, immediately. The oxidation was completed within 15 min at room temperature, based on TLC analysis (Rf of hydrazone = 0.33, Rf of product = 0.60 in EtOAc : hexanes (1 : 5)). Then, a solution of acid was added to the purple solution and the progress of the reaction was monitored with TLC. The reaction time was in a range of 0.1 h ~ 5 h, depending on the substrate. After the reaction was finished, Magtrieve was retrieved with magnet and the solution was concentrated to give a crude product, which was purified by column chromatography. The yield was good, except for the entry 6, where the dpm ester was decomposed during the column chromatography.