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Synthesis of 5-hydroxycyclopent-2-enones from allenyl vinyl ketones via an interrupted Nazarov cyclization.

Treatment of an allenyl vinyl ketone with trifluoroacetic acid leads to Nazarov cyclization, and the intermediate carbocation is trapped efficiently by trifluoroacetate. Hydrolysis of the ester with methanol and basic alumina provides, in good to excellent overall yield, a 5-hydroxycyclopent-2-enone in which the alcohol is predominantly trans to a substituent at C-4.