Catalyst-free [3 + 2] cyclization of imines and Morita–Baylis–Hillman carbonates: a general route to tetrahydropyrrolo[2,1-a]isoquinolines and dihydropyrrolo[2,1-a]isoquinolines†
We have developed a mild and efficient catalyst-free [3 + 2] cyclization of isoquinoline imines and Morita–Baylis–Hillman carbonates. Various tetrahydropyrrolo[2,1-a]isoquinolines can be prepared in acceptable to excellent yields (up to 96%). The treatment of tetrahydropyrroloisoquinolines with DDQ afforded 5,6-dihydropyrrolo[2,1-a]isoquinolines in good yields (up to 90%). A one-pot synthesis of 5,6-dihydropyrrolo[2,1-a]isoquinolines has been achieved successfully (up to 86% yield). It should be noted that all the [3 + 2] cyclization, oxidation and one-pot reactions can be easily scaled up to the gram scale.