The addition of 2-diazopropane to methyl acrylate and its methyl-substituted analogues normally (i.e., when electronic effects are dominant) gives methyl 1 -pyrazoline-3-carboxylates, of which those bearing a hydrogen atom at C(3) tautomerise readily to the A2-isomers. The orientation of addition is reversed in the reactions of methyl 3-methylbut-2-enoate with 2-diazopropane and diazoethane. Methyl 4.4-dimethylpent-2-enoate undergoes reverse addition with 2-diazopropane, but not with diazoethane or diazomethane. The reversals are attributed to the presence of unfavourable eclipsing interactions in the transition state for normal addition.IN the 1,3-dipolar addition of diazoalkanes to ap-unsaturated carbonyl compounds, the diazo-carbon atom generally becomes bonded to the carbon atom p to carbony1,l though cases are known where for steric reasons diazoalkanes add in the reverse sense to conjugated acetylenic 2 and allenic esters. Analogous effects have been observed with allenic nitriles? and a@-unsaturated sulphones 4 and nitro-compound^.^ We report here similar reversals in the reaction of simple, substituted acrylic esters with diazoalkanes.When treated with ethereal 2-diazopropane, methyl acrylate gave an almost quantitative yield of the 2-pyrazoline (I), formed by tautomerisation * of the initially formed l-pyrazoline (IV) under the reaction conditions. The structure of the adduct (I) was confirmed spectroscopically. It had U.V. absorption a t A,,, 276 mp (E 11,200), characteristic of the N=C-CO,R chromophore, and i.r. bands at 3380 (N-H) and 1570 (C=N) cm.-l. The n.m.r. spectrum comprised three singlets at T 8-70, 7.35, and 6.25 and a broad peak at 3.5 (intensity ratio 6 : 2 : 3 : l), corresponding to the gemdimethyl group, the C(4)-methylene protons, the ester methyl, and N-H, respectively. Similarly, methyl crotonate gave, via the 1-pyrazoline (V), the 2-pyrazoline (11) (80%). Traces of many other products were formed in this reaction; but, although a careful search was made, no compounds isomeric with the pyrazolines (11) and (V) could be detected.Methyl methacrylate gave with 2-diazopropane a nearquantitative yield of the l-pyrazoline (VI), which displayed the usual spectroscopic features of l-pyrazolines.7 The geometric isomers methyl tiglate (2-methylcrotonate) and methyl angelate (2-methylisocrotonate) reacted stereospecifically with diazopropane to give high yields of the isomeric l-pyrazolines (VIII) and (IX), respectively. The differences between the n.m.r. spectra of these two compounds were distinct and unambiguous; in par-* Presumably acetone azine, formed by the decomposition of 2-diazopropane,6 acts as a basic catalyst in the reaction solutions.
