Diazoalkanes, cycloaddition with diazomethane preparation

Another method used to prepare dialkyl-substituted diazomethanes involves the photolysis of 2-alkoxy-2,5-dihydro-1,3,4-oxadiazoles (209), which can be prepared by the oxidative cyclization of A(-acetyUiydrazones. The diazoalkanes are trapped in situ by cycloaddition with dimethyl acetylenedicarboxylate (54) (Scheme 8.49). The resulting pyrazoles 210 are converted into cyclopropenes 211 by continued irradiation. 

Reactions offluorinated dipoles. In recent years, much effort has been devoted to the preparation of tnfluoromethyl-substituted 1,3-dipoles with the goal of using them to introduce trifluoromethyl groups into five-membered nng heterocycles Fluorinated diazoalkanes were the first such 1,3-dipoles to be prepared and used in synthesis A number of reports of cycloadditions of mono- and bis(tnfluo-romethyl)diazomethane appeared prior to 1972 [9] Other types of fluonne-substi-tuted 1,3-dipoles were virtually unknown until only recently However, largely because of the efforts of Tanaka s group, a broad knowledge of the chemistry of tnfluoromethyl-substituted nitrile oxides, nitnle imines, nitnle ylides, and nitrones has been accumulated recently... 

The 1,3-dipolar cycloaddition reaction of diazoalkanes with alkenes has also been reported (395). Kanemasa and Kanai (395) used the chiral DBFOX-Ph ligand with various metals such as Ni, Zn, and Mg for the preparation of 255a-c. The reaction of TMS-diazomethane 171 with alkene 241 was catalyzed by 10 mol% of 255b to afford the 1,3-dipolar cycloaddition product 296 in good yields and enantioselectivities of up to 99% ee (Scheme 12.96). Also, the Ni-catalyst 255a and the Mg-catalyst 255c were excellent catalysts for the reaction, resulting in >90% ee in both cases. 

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