Pyrazolines dehydrogenation

Moffett and coworkers203 reported the synthesis of several 4-/3-D-ribofuranosylpyrazoles, such as 284(a-c), by 1,3-dipolar cycloaddition of diazoalkanes to the alkenic C-glycosyl compound 283, followed by dehydrogenation of the resulting pyrazolines. In view of the known biological activities of several nucleosides containing the... 

A2-Pyrazolines are converted into pyrazoles by oxidation with bromine or Pb(OAc)4 and they can also be dehydrogenated with sulfur. 3,5-Diphenylpyrazoline (419) on heating with platinum disproportionates to the pyrazole (420) and the pyrazolidine (421) (66AHC(6)347). 

Another interesting reaction of benzoxazines 114 (115) is the reductive opening of the oxazine ring with simultaneous dehydrogenation of the pyrazoline moiety [170]. This process is carried out in a KOH suspension of a mixture of dimethyl sulfoxide and dimethylformamide. For instance, this treatment involving 2,5-diary l-l,10b-dihydro-177-pyrazolo[l,5-c]benzo[e]-l,3-oxazines 116 leads to the formation of pyrazoles 117 (Scheme 2.32). Similar disproportionation reactions have also been described for some bezopiranes, for example, pyrazole derivative 118 [91, 170]. 

Photoreaction of 5-(2-iodophenyl)-l,3-diphenyl-A2-pyrazoline (184) produces 1,3,5-triphenylpyrazoline (185) (24%) and 2-phenylpyrazolo[l,5-/]phenanthridine (186) (39%) (equation 139)558,559. The dehydrogenation to 186 takes place during the irradiation. 

Diosgenin has been converted into [3,2-c]pyrazole and [2,3-d]isoxazole derivatives of types (452) and (453). Pyrazolines (454), prepared from the corresponding 2-benzylidine-3-oxo-steroid and phenylhydrazine, were dehydrogenated to give the substituted pyrazole (455). ... 

For dehydrogenation of pyrazolines with clayfen see Bougrin, K., Sonfiaoni, M. and El Yazid, M., Tetrahedron Lett, 1995, 36, 4065. 

Compound 12 cyclizes with hydrazine to give a pyrazoline which can be dehydrogenated to afford pyrazole 12. Many trifluoromethylated pyrazoles are known as described in an excellent review (ref. 27). Alcohols U can be methylated using NaH/TfOMe. These methyl ethers could not be hydrolyzed to the corresponding Mosher acid. As the hydroxyl is now protected by an alkyl group, the thioamide moiety can be chlorinated to give the amide chloride 14- This unstable compound was directly cyclized with benzothiazole (Scheme 35). 

Dehydrogenation. Pyrazolines, available from the reaction of hydrazones and conjugated esters, undergo dehydrogenation readily on exposure to H2O2 in the presence of iron(II) chloride. 

A diverse range of carbocycles and heterocycles has been aromatized by aerobic dehydrogenation under AC catalysis [80]. Substrates include 9,10-dihydroanthracenes [81], a l,8-diaza-9,10-dihydroanthracene [82], Hantzsch 1,4-dihydropyridines [83], pyrazolines [83], 2-arylimidazolines [84, 85], indolines [86], 3,4-dihydropyrimidin-2(l//)-ones [87, 88], and various... 

Dehydrogenation of pyrazolines. Pyraz.olcs (2) are obtained in almost quanti-lalivc yield by liealmcnl of pyrazolines (I) in olhci with this high-potential diiiiione. I lie reaclioii is rapid al 20... 

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