Cycloadditions, 25. – Synthesis and Reactions of sec ‐ N ‐Aryl‐2‐methyl‐2,3‐butadienamides

The reaction of 2‐methyl‐2,3‐butadienoyl chloride 1 with the N ‐aryl‐ or the N ‐hetarylamines, 2a–f and 2g , h , respectively, in the ratio 1 : 2 (or 1 : 1 in the presence of triethylamine) furnishes the corresponding allenamides 3a–h , accompanied by the HCl adducts, the 3‐chloro‐2‐methyl‐3‐butenamides 4a–g , or the 3‐butynamide 5h . Heating of the allenamides bearing a benzenoid ( 3a–e ) or thiophene ( 3h ) nucleus does not generate any defined products; only the amides bearing the 1‐naphthyl ( 3f ) and the furfuryl ( 3g ) moiety undergo an intramolecular Diels‐Alder reaction to give the oligocyclic compounds 6 and 7 . However, heating of the benzenoid allenamides 3a–c in the presence of tolyl isocyanate furnishes, via the N‐carbamoyl‐amides 10a–c , the intramolecular Diels‐Alder products 11a–c , whereby the monocyclic arene has functioned as diene. Reaction of 1 with the 2‐aminofuran derivative 2i leads directly to the Diels‐Alder product 9 , a 2 : 1 adduct, whereby the initially formed amide 3i and the intramolecular Diels‐Alder product 8 are not isolated.