METHYLENOMYCIN-B - NEW SYNTHESES BASED ON BETA-KETO AND GAMMA-KETO PHOSPHONATES AND GAMMA-KETO PHOSPHINE OXIDES

Methylenomycin B has been synthesized from diethyl 2-oxobutanephosphonate (4) in three steps in 39% overall yield. Starting from diethyl 3-oxobutanephosphonate (10) and diphenyl(3-oxobutyl)phosphine oxide (13), methylenomycin B has been obtained in 34% and 27% overall yield, respectively. A characteristic feature of these syntheses of methylenomycin B is that the exo-methylene function is introduced via the Horner-Wittig reaction of formaldehyde (36% aqueous solution) with the corresponding alpha-phosphorylcyclopentenones 6 and 20. The latter were obtained from phosphorylated 1,4-diketones by intramolecular base-catalyzed cyclization. A brief discussion of some mechanistic aspects of the Conant reaction is also given.