Methylpyrrol-2-yl propenal

The empirical formula contains five double-bond equivalents. In the H NMR spectrum a doublet signal at Sh = 9.55 stands out. This chemical shift value would fit an aldehyde function. Since the only oxygen atom in the empirical formula is thus assigned a place, the methyl signal at Sh = 3.80 does not belong to a methoxy group, but rather to an iV-methyl group. 

The coupling constant of the aldehyde doublet (7.8 Hz) is repeated in the doublet of doublets signal at Sh = 6.3. Its larger splitting of 75.(5 Hz is observed also in the doublet at Sh = 7.3 and indicates a CC double bond with a trans configuration of the vicinal protons. 

The coupling of 7.8 Hz in the signals at Sh = 9.55 and 6.3 identifies the ( )-propenal part structure. The large H shift (Sh = 9.55) thus reflects the -A7 effect of the conjugated carbonyl group. 

Apart from the A-methyl group, three double-bond equivalents and three multiplets remain in the chemical shift range appropriate for electron rich heteroaromatics, Sh = 6.2 to 6.9. A-Methyl-pyrrole is such a compound. Since in the multiplets at Sh = 6.25 and 6.80 the Jhh coupling of 4.0 Hz is appropriate for pyrrole protons in the 3- and 4-positions, the pyrrole ring is deduced to be substituted in the 2-position. 

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In contrast, 2-(l-methylpyrrol-2-yl)propene 42e afforded an ene adduct 241 [85JCR(S)12]. 

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