AbstractThe reaction of a series of 4,4‐dimethyl‐3,5,5‐(R3, R2, R1)‐3‐hydroxy‐1,2‐dioxolanes1a–d[1a(R1= R2= R3= Me);1b(R1= R2= Me, R3= Ph);1c(R1= Ph, R2= R3= Me);1d(R1= R3= Ph, R2= Me)] with trimethyl phosphite or triphenylphosphine produced theβ‐hydroxy ketones,2a–d[(R1R2C(OH)CMe2‐COR3)], and the corresponding phosphoryl compounds in high yield. The reactions were slow (∼24 hours), requiring excess of the trivalent phosphorus compounds for optimum results. The reaction was found to be of the second order overall, first order in peroxide and in phosphorus reagent. Triphenylphosphine was found to be more reactive than trimethyl phosphite. Formal substitution of a phenyl group for a methyl group on the hemiperketals yielded a reduction of reactivity of ∼2.4‐fold at the 3‐position but an increase in reactivity (∼1.5‐fold) at the 5‐position. The rate constants for tautomerization of1cin benzene were determined. The activitation parameters for reaction of1cwith triphenylphosphine in benzened6were determined [ΔH‡= 15.0 ± 1.5 kcal/mol; ΔS‡= −21.7 eu; ΔG‡= 21.7 kcal/mol; k34°= 2.4 × 10−3M−1s−1]. The data are consistent with direct insertion of the phosphorus compounds into the peroxide bond to yield phosphorane intermediates. Rapid intramolecular hydrolysis of the intermediates woul