Cyclic enediol phosphoranes obtained from the reaction of trimethyl phosphite with biacetyl and hexafluorobiacetyl give strikingly different acylation products

Cyclic enediol phosphoranes (2,2,2-trimethoxy-2,2-dihydro-l,3, 2-dioxaphospholenes), prepared from α-diketones, RCOCOR, and trimethyl phosphite (TMP), give strikingly different acylation products depending on the electronic properties of the groups, R, that occupy the 4,5-positions in the dioxaphospholene ring with pentacovalent phosphorus. Reaction of acetyl chloride or bromide with the hexafluorobiacetyl-TMP phosphorane gives exclusively the product of $\text{endo}$cyclic O-acylation, dimethyl(2-acetoxy-$\text{cis}$-1,2, -$\text{bis}$trifluoromethylvinyl) phosphate. Acylation of the biacetyl-TMP phosphorane gives mixtures of the product of $\text{exo}$cyclic O-acylation (a cyclic enediol phosphate) and the product of C-acylation (an α-hydroxy-β-diketone phosphate), the proportions depending on the structure of the acyl halide and the solvent. The parent 1,3,2-dioxaphospholene, i.e., the glyoxal-TMP phosphorane where R = H, gives exclusively the product of $\text{endo}$cyclic O-acylation with both acyl halides in all solvents.