A Novel [3+2] Cycloaddition Approach to Nitrogen Heterocycles via Phosphine-Catalyzed Reactions of 2,3-Butadienoates or 2-Butynoates and Dimethyl Acetylenedicarboxylate with Imines:  A Convenient Synthesis of Pentabromopseudilin

The reactivity of a new three carbon synthon, generated in situ from the reaction of 2,3-butadienoates or 2-butynoates with an appropriate phosphine as the catalyst, toward the electron-deficient imines is described. Triphenylphosphine-catalyzed reaction of methyl 2,3-butadienoate with N-sulfonylimines gave the single [3+2] cycloadduct in excellent yield; tributylphosphine-catalyzed reaction of methyl 2,3-butadienoate or 2-butynoate with N-tosylimines afforded the corresponding [3+2] cycloadduct as the major product along with a small amount of the three components adduct. Aliphatic N-tosylimines gave moderate yield for this reaction. In addition, a new phosphine-catalyzed cyclization reaction of dimethyl acetylenedicarboxylate with N-tosylimines is also described. A reaction mechanism is proposed. Further elaborations of the cycloaddition products and the synthesis of pentabromopseudilin using this method are exemplified.