Methylenomycin B has been synthesized from diethyl 2-oxobutanephaphonate (4) in three steps in 39% overall yield. Starting from diethyl 3-oxobutanephosphonate (10) and diphenyl (3-oxobuty1) 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 ...