Alkylation of enolate anions with dimethyl 3-bromo-2-ethoxypropenylphosphonate. A convergent cyclopentenone annulation method

A new, convergent synthesis of 2-cyclopenten-1 -ones is reported. Treatment of dimethyl 2-oxopropylphosphonate (21) with excess triethyl orthoformate in the presence of ferric chloride hexahydrate provides the enol ether 22, which upon allylic bromination with N-bromosuccinimide yields the allylic bromide 19. Alkylation of the ketones 24–28 with 19, followed by mild acid hydrolysis of the resultant products 29–33, affords the diketo phosphonates 34–38. When the latter substances are treated with sodium hydride in dimethoxyethane, the corresponding 2-cyclopenten-1 -ones 40–44 are formed in good yields.