An atom-economical approach to conformationally constrained tricyclic nitrogen heterocycles via sequential and tandem Ugi/intramolecular Diels-Alder reaction of pyrrole

An efficient approach to rigid tricyclic nitrogen heterocycles via sequential and tandem Ugi/ intramolecular Diels-Alder (IMDA) cycloaddition of pyrrole is described. The one-pot Ugi four-component condensation (4CC) reaction was used as the key transformation to prepare trienes with a carboxamide substituent on the tether. The use of acrylic acid (21) and N-propyl- and N-benzylmaleamic acids (24b and 24C) as the acid components provided trienes 22, 25b, and 25c, respectively, which upon heating at 120degreesC for 12 h yielded the corresponding [4 + 2] cycloaddition products. In the case of maleic acid derivative 24a, heating the reaction mixture at 60degreesC for 6 h promoted the cycloaddition reaction and provided the desired product 26a in 78% yield. In contrast, fumaric acid monoethyl ester (27a) and 3-acetyl- and 3-(4-methylbenzoyl) acrylic acids (27b-c) directly yielded the corresponding Ugi/IMDA cycloaddition products 29a-c in high yields at room temperature without any trace of initially formed trienes 28a-c. The IMDA cycloaddition reactions proceed with excellent stereoselectivity with the formation of five stereogenic centers and three rings.