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10.1.1.1.4.1.2 Variation 2: By [2+2] Cycloadditions with Ketenes: Perkin Conditions

DOI: 10.1055/sos-SD-010-00002

Dell, C. P.Science of Synthesis, (20011049.

Although more limited in scope than the reaction described in Section 10.1.1.1.4.1.1, a superior alternative for the generation and intramolecular cycloaddition of ketenes from (2-acylphenoxy)acetic acids as a route to benzofurans, e.g. 106, involves classical Perkin reaction conditions (sodium acetate/acetic anhydride) (Scheme 52).[‌291‌] The intermediacy of ketenes in the reaction has been indicated by appropriate experiments and the observation that ketones consistently give better yields than aldehydes. This is inconsistent with the classical Perkin carbanion mechanism but thoroughly consistent with a ketene [2+2] cycloaddition via a dipolar intermediate. The reaction is applicable to a range of 2-substituted, 3-substituted, or 2,3-disubstituted benzofurans. Yields are typically above 50%.

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