Diazoalkanes, cycloadditions with

The energetic 1,3,4-oxadiazole (22) is synthesized from the reaction of the tetrazole (20) with oxalyl chloride. In this reaction the tetrazole (20) undergoes a reverse cycloaddition with the expulsion of nitrogen and the formation of the 1,3-dipolar diazoalkane (21) which reacts with the carbonyl groups of oxalyl chloride to form the 1,3,4-oxadiazole rings. 

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. 

If the C=N function is attached to an electron-withdrawing group, 1,3-dipolar cycloaddition with diazoalkanes occurs leading to 1,2,3-triazoles (5, 276). When diazomethane is used, the initially formed NH-triazole is not isolated due to a rapid subsequent NH deprotonation followed by N-methylation. Consequently, a mixture of the three Wmethyltriazoles is formed when methyl cyanoformate (71) (216) or trichloroacetonitrile (276) (217) is treated with excess diazomethane (Scheme 8.51). Huisgen and co-workers found that methyl diazoacetate reacts with TCNE by a 1,3-dipolar cycloaddition at the C=C bond and not, as published earlier by other authors, at one of the nitrile functions (72). 

Benzo[6]thiophene 1,1-dioxides undergo [2+4] cycloaddition with 1,3-dienes such as cyclopentadiene, anthracene, etc. (70AHC(11)177>. Cycloaddition with dipolar species such as diazoalkanes (74M550), nitrilimines (74M869) and nitrile oxides (79TL4845) have also been described (Scheme 192). 

According to the nature of the HOMO of diazoalkanes one expects the formation of complexes like X or XI by the formal way of a 1.3-dipolar cycloaddition with... 

Diels-Alder reactions of thiete 1,1-dioxides occur readily as exemplified by the syntheses of 151 ° and 152. Adducts of thiete 1,1-dioxide with tetraphenylcyclopentadienone or a-pyrone ° are thermally unstable. Thiete 1,1-dioxides also undergo 1,3-dipolar additions with diazoalkanes, (e.g., the formation of 153 from which the strained bicyclic thietane sulfone 154 is obtained) " nitrile oxides, and cycloadditions with the MA -dimethylenamine of isobutyraldehyde (e.g., the formation of 155). ° ... 

Occasionally, the term 1,3-dipolar cycloaddition is also used for the reaction of diazoalkanes with transition metal complexes, e. g., (6-9), investigated by McCrindle and McAlees (1993). This is not, however, a 1,3-dipolar cycloaddition as coined by Huisgen. This term should not, therefore, be used for reactions that involve a dipolar reagent, but only in a cycloaddition with a dipolarophile, as shown in Huisgen s mechanism (Schemes 6-5 and 6-6). They may be called [3 + 1] dipolar cycloadditions, however, in order to underline the difference to the [3 + 2] reactions. 

Regio- and Diastereoselectivity of Dipolar Cycloadditions with Diazoalkanes... 

If the dipolarophile is assymmetric (d e in d=e. Sect. 6.2, Scheme 6-5), there are two alternatives for the diazoalkane and all other unsymmetrical 1,3-dipoles in their cycloadditions with that dipolarophile. For example, diazomethane and an ethene derivative with an alkyl substituent R may yield the 3- or the 4-alkyl-l-pyrazolines (6-19), or a mixture of both. These primary products rearrange at higher temperature or in the presence of base to give the corresponding alkyl-2-pyrazolines. 

Some years ago, Huisgen et al. (1987 b) mentioned that diazoalkane cycloadditions to electron-deficient CC multiple bond compounds and other dipolarophiles represent the class of [3 + 2] cycloadditions that has been most intensively studied for synthetic purposes. This is also evident from the reviews that concentrate on preparative aspects of diazoalkane cycloadditions (Regitz and Heydt, 1984 Stanovnik, 1991). Yet, in contrast to these statements, not a single 1,3-dipolar cycloaddition with a diazoalkane is reported in Organic Syntheses... 

Double bonds of benzene and related aromatic compounds do not react with diazoalkanes in 1,3-cycloadditions. The corresponding benzo-annellated dihydro-pyrazoles such as (1 a,6a)-6,9,9-trimethyl-7,8-diazabicyclo[4.3.0]nona-2,5,7-triene (6.98, R = H) can be synthesized, however, from methyl cyclohexa-l,4-dienecarboxy-late (6.97, R = H), as shown by Klarner et al. (1990). The reduction of the ester group to a methyl group was carried out with di-isobutylaluminum hydride (DIBAL-H), esterification of the OH group with methanesulfonyl chloride and reduction with lithium triethyl borohydride. The second double bond was introduced by bromina-tion with Br2 on a polymeric carrier after Bongini et al. (1980). Cycloaddition with 2-diazopropane in ether at -- 5 °C and the following steps gave 6.98 in a yield of 58%. 

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