Synthesis of 2-Alky1-2-cyclopentenones from 2-Cyclopehtenone

Abstract

2-Alkyl-2-cyclopentenones [5] were synthesized in four steps from 2-cyclopentenone [1]as described in scheme 1.
Compound [1] was epoxidized in dichloromethane with hydrog en peroxide to 2, 3-epoxycyclopentanone [2] (91% yield) which then reacted with alkylmagnesium bromides in tetrahydro furan below -40°C to give 1-alkyl-2, 3-epoxycyclopentanols [3] selectively (85∼86% yield). Teh epoxide ring of compound [3] was opened by reaction with carbonyl compounds, such as acetone to give 1, 3-dioxolane rings (86%yield). By treatment of the products thus obtained with an aquseous mineral acid, the mixture of [5] and 2-alkylidenecyclopentanones [5'] were obtained. When a 1-butanol-hydrochloric acid solution was used instead of an aqueous mineral acid, [5] was obtained selectively in 83∼85% yield. Overall yields of 2-hexy1-2-cyclopentenone ap, 2-pentyl-2-cyclopentenone[5a], 2-butyl-2-cyclopentenone [5b] and 2-butyl-2-cyclopentenone [5c]from [1] were 57, 57 and 55%, respectively.