Inter‐ and intramolecular hetero diels‐alder reactions, XXI. Intramolecular hetero diels‐alder reaction of alkylidene‐1,3‐dicarbonyl compounds. Experimental evidence for an asymmetric transition state

The intramolecular hetero Diels‐Alder reaction of heterodienes 13 is described. The heterodienes, which are obtained in situ by Knoevenagel condensation of aldehydes 12a–e with dimethylbarbituric acid (2), yield the cycloadducts 14a–e/15a–e and the ene adducts 16a, c–e/17a, c–e. The ratio of 14/15 was determined by HPLC. The resulting data are interpreted as experimental evidence of an asymmetrical transition state of the Diels‐Alder reaction.