TOTAL SYNTHESIS OF (-)-SILPHIPERFOL-6-ENE AND (-)-METHYL CANTABRADIENATE

(-)-Silphiperfol-6-ene (10) and (-)-methyl cantabradienate (11) have b een prepared in seven steps from (R)-3-methyl-1-cyclopentenecarboxalde hyde in 13 and 9% overall yield, respectively. Addition of isopentenyl magnesium bromide to imine 22 afforded 65% of aldehyde 14. Oxidation p rovided acid 15, which was converted to ketene 20, which underwent an intramolecular [2 + 2] cycloaddition to afford cyclobutanone 13. Mn(II I)-based oxidative fragmentation-cyclization of ethynyl cyclobutanol 2 5 provided an efficient route to the key methylenecyclopentanone 12. A ddition of methyllithium and Birch reduction completed the synthesis o f (-)-silphiperfol-6-ene 10. Formation of dienyl triflate 29 and palla dium-catalyzed carbonylation concluded the first synthesis of (-)methy l cantabradienate (11).