Diazo compounds pyrazoline ring

Diazo compounds also undergo cycloaddition with fullerenes [for reviews, see (104),(105)]. These reactions are HOMO(dipole)-LUMO(fullerene) controlled. The initial A -pyrazoline 42 can only be isolated from the reaction of diazomethane with [60]fullerene (106) (Scheme 8.12) or higher substituted derivatives of Ceo (107). Loss of N2 from the thermally labile 42 resulted in the formation of the 6,5-open 1,2-methanofullerene (43) (106). On the other hand, photolysis produced a 4 3 mixture of 43 and the 6,6-closed methanofullerene (44) (108). The three isomeric pyrazolines obtained from the reaction of [70]fullerene and diazomethane behaved analogously (109). With all other diazo compounds so far explored, no pyrazoline ring was isolated and instead the methanofullerenes were obtained directly. As a typical example, the reaction of Cgo with ethyl diazoacetate yielded a mixture of two 6,5-open diastereoisomers 45 and 46 as well as the 6,6-closed adduct 47 (110). In contrast to the parent compound 43, the ester-substituted structures 45 and 46, which are formed under kinetic control, could be thermally isomerized into 47. The fomation of multiple CPh2 adducts from the reaction of Ceo and diazodiphenylmethane was also observed (111). The mechanistic pathway that involves the extrusion of N2 from pyrazolino-fused [60]fullerenes has been investigated using theoretical methods (112). 

It should be noted, however, that the 1,3-dipolar cycloaddition chemistry of diazo compounds has been used much less frequently for the synthesis of natural products than that of other 1,3-dipoles. On the other hand, several recent syntheses of complex molecules using diazo substrates have utilized asymmetric induction in the cycloaddition step coupled with some known diazo transformation, such as the photochemical ring contraction of A -pyrazolines into cyclopropanes. This latter process often occurs with high retention of stereochemistry. Another useful transformation involves the conversion of A -pyrazolines into 1,3-diamines by reductive ring-opening. These and other results show that the 1,3-dipolar cycloaddition chemistry of diazo compounds can be extremely useful for stereoselective target-oriented syntheses and presumably we will see more applications of this type in the near future. 

Diazo compound 2 is obtained by thermal ring opening of the bicyclic pyrazoline 1." Catalytic decomposition of 2 with copper(I) iodide in refluxing benzene yields the bicyclo[1.1.0]butane 3. The same result is achieved with bis(acetonitrile)palladium(II) chloride as catalyst, whereas rhodium(II) acetate dimer and copper(I) cyanide are unreactive. 

An important synthetic use of pyrazolines is in the formation of cyclopropanes. This particular ring system is often accessible by addition of diazo compounds to alkenes (subsection 6.1.2). Some examples of both thermal and photochemical processes of this sort are given in Scheme 6.12. 

From Diazo Compounds. Addition of diazomethane or its substituted derivatives to double bonds is a standard approach for the preparation of cyclopropyl compounds. Pyrazolines may be intermediates in such reactions, but are often decomposed in situ without isolation. An example is the addition of diazomethane to the bis-ketal (92). The initially formed pyrazoline extrudes nitrogen at 450 C to give the cyclopropane (93 93%) which can be converted into the bis-aldehyde, or pyrolysed to ring-opened (94) and ring-expanded (95) products, ... 

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