Abstract
Organo-catalyzed aldol condensation of 2-hydroxyacetophenone and 4-nitrobenzaldehyde underwent regio- and enantioselective cyclization via intramolecular 5-exo-trigonal cyclization to give 4-nitroaryl-3-benzofuranones. The product formation indicated that the proline bases (organocatalysts) suppressed the effect of carbonyl group by iminium ion formation and hence Michael addition is not possible. The conjugation effect moved toward 4-nitroaryl moiety suitable for 5-exo-trig cyclization (Baldwin’s rule). Thus, the non-equilibrating intermediates (NEIs) which gave in enantio-enriched 3-benzofuranones. A high enantioselectivity (80–96.6%) was obtained with (S)-pyrrolidinyl-tetrazole V under a metal-free condition.
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