THE SYNTHESIS OF ALKENES VIA EPI-PHOSPHONIUM SPECIES - 1 - AN ANTI-WITTIG ELIMINATION

Anti-1,2-phosphinyl alcohols 11 and their corresponding syn-isomers up on treatment with phosphorus trichloride and triethylamine give E and Z alkenes respectively, by an anti elimination. This is in marked cont rast to the syn Horner-Wittig elimination of the corresponding 1,2-pho sphinoyl alcohols. The 1,2-phospbinyl alcohols 11 were prepared by the reduction of 1,2-phosphinoyl alcohols with cerium(III) chloride/lithi um aluminium hydride. The anti elimination is explained by the formati on of a transient epi-phosphonium species. An unexpected E-selective H orner-Wittig elimination during the cerium(III) chloride/lithium alumi nium hydride reduction of a 1,2-phosphinoyl alcohol in which the diphe nylphosphinoyl group is adjacent to an aryl group is described. This l ed to the synthesis of the antimitotic agent E-combretastatin A-4. An alternative synthesis of the 1,2-phosphinyl alcohols from the correspo nding phosphine-borane complex is also described. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.