Catalyst-controlled formal [4 + 1] annulation of N-vinyl fluorenone nitrones and allenoates to prepare spirofluorenylpyrrolines†
We report a facile commercial gimeracil-catalyzed formal [4 + 1] annulation approach for the synthesis of spirofluorenylpyrrolines in good yields with high diastereoselectivity from easily available N-vinyl fluorenone nitrones and allenoates. The reaction showed broad substrate scope and tolerated a variety of functional groups on both N-vinyl fluorenone nitrones and allenoates. Mechanistic studies showed that the catalyst might play important roles in promoting enolation and selective cyclization of 1,3-dicarbonyl intermediates. More importantly, the obtained spirofluorenylpyrrolines were easily converted to various spirofluorenylpyrroline-based scaffolds.