D Pyrazolines

Unfortunately, 4,5-dihydro-32/-pyrazoles can not only produce cyclopropanes but also tautomeric 4,5-dihydro-l//-pyrazoles (d -pyrazolines) and acyclic constitutional isomers of the cyclopropanes. This is illustrated for the 4,5-dihydro-3//-pyrazoles 11 which result from [3-1-2] cycloaddition of a-diazo esters to electron-deficient alkenes. If = H, they undergo rapid tautomerism to form 4,5-dihydro-l//-pyrazoles 12 or 13, both of which are not subsequently... 

The reactions of diazomethane with C- and X-substituted alkenes are much slower, and consequently there are fewer known examples. The slower rate of reaction is explained easily by the larger energy separation in the frontier orbitals (10 and 9.8 eV, respectively, in Fig. 6.42). The regioselectivity, however, is the same d -pyrazolines like 6309 and 6311 with the substituent at C-3 are obtained with both C- and X-substituted dipolarophiles.848 This at first sight surprising observation can be explained by the change from dipole-HO control in the cases of the Z- and C-substituted alkenes 6307 and 6308 (supported, incidentally, by the positive Hammett p-value for the latter) to dipole-LU control 6310 in the case of the X-substituted alkene ethyl vinyl ether.849... 

Cj3Hj,BrN.0)S 1 - C4- Brom-benzolsalfony]]-S-methyl-S- iaopropyI-d -pyrazolin 28 II 29. 

MethyI-l-aoety].d -pyrazolin-oarbon> >4ure.(3)-methyle8teT 26 II101. 

MethyI-3-phenyI-d -pyrazolin-oacboiv saure-(l)>amid 23 II170. 

CuHigBrN) 1,6-Dipbenyl-3-[3-brom-4-ammo-phenyl]-d -pyrazolin 26 II321. 

Diazoacetyl compounds s. a. a-Diazoketones 3-Diazoacetyi-d -pyrazoline ring... 

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