Preparation of, and alkylation of enolate anions with, dimethyl 3-bromo-2 ethoxypropenylphosphonate. An efficient, convergent synthesis of 2-cyclo-penten-1-ones

The alkylation of enolate anions with dimethyl 3-bromo-2-ethoxypropenylphosphonate, followed by acid hydrolysis of the resultant products, affords diketo phosphonate compounds which can be cyclized efficiently to give the corresponding 2-cyclopenten-1-ones. This overall five-membered ring annelation process can be carried out without affecting acid-labile functional groups (e.g. ketal) in the enolate substrate.