Selective oxidations of methyl ricinoleate:: Diastereoselective epoxidation with titaniumIV catalysts

Conditions were developed for the selective epoxidation of the double bond of methyl ricinoleate (1) with ethylmethyldioxirane (EMDO) to give the homoallylic epoxyalcohol, methyl (Z)-9,10-oxido-12-hydroxyoctadecanoate (2) in high yields but in poor enantiomeric excess. The diastereomeric ratio for epoxyalcohol 2 was improved modestly when t-butylhydroperoxide, coupled with a titanium catalyst and a D-tartrate ligand, was used as oxidizing agent. Reaction of 1 with excess EMDO resulted in the concomitant epoxidation of the double bond and oxidation of the hydroxy group of 1 to give methyl (Z)-9,10-oxido-12-oxo-octadecanoate (4), along with methyl 8-(5-hexylfuran-2-yl)octanoate (5). Alternatively, ketoepoxide 4 was prepared by dioxirane oxidation of methyl 12-oxo-(Z)-9-octadecene (3) or by treating epoxyalcohol 2 with sodium hypochlorite. The ketoepoxide 4 is acid-labile and rearranges with loss of water to give furan 5 in high yield.