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(A) Phosphine-catalyzed [3 + 2] annulation of unsymmetric isoindigo 100 with allene in the preparation of spiro adduct 103 [46]. (B) The proposed catalytic cycle. (C) Application of phosphine-catalyzed asymmetric [3 + 2] annulation to prepare the chiral adduct 105 with symmetric isoindigo 104 in the formal synthesis of (−)-ditryptophenaline (12).
