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The 2-azadiene 5 was generated from the Schiff bases of serine methyl ester and cysteine methyl ester and 2-allyloxybenzaldehyde 1. Compound 5 failed to undergo an intramolecular Diels-Alder reaction. When it was generated in the presence of aluminium trichloride the pyridine 6a was isolated; the structure of this pyridine indicates that the catalyst has reversed the regiochemistry of the dimersiation of 5. The reaction of the activated allyloxybenzaldehyde 2 with cysteine methyl ester follows a different course from that of 1. The allylic function is transferred to sulfur and the Schiff base 9 is formed. This undergoes a rapid internal 13-dipolar cycloaddition at room temperature to give the thienopyrrole 10 the structure of which has been established by X-ray crystallography.