Regioselective Nucleophilic Additions to Cross-Conjugated Dienone System Bearing β-Fluorine: A Versatile Approach to Highly Substituted 2-Cyclopentenones

3-Fluoro-5-methylene-2-cyclopentenone is treated with appropriate nucleophiles and Lewis acids to undergo regioselective 1,2-addition, exocyclic 1,4-addition, and endocyclic 1,4-addition, leading to 3-substituted 4-methylene-2-cyclopentenones, 5-substituted 3-fluoro-2-cyclopentenones, and 3-substituted 5-methylene-2-cyclopentenones in good yields, respectively.