SYNTHESIS AND THERMOLYSIS OF KETAL DERIVATIVES OF 3-HYDROXY-1,2-DIOXOLANES

3-[(Trimethylsilyl)oxy]-3,4,4,5-tetramethyl-5-phenyl-1,2-dioxolane (2), 3-methoxy-3,4,4,5-tetra-methyl-5-phenyl-1,2-dioxolane (3), and 3-acetoxy-3,4,4,5-tetramethyl-5-phenyl-1 (4) were synthesized from the corresponding 3-hydroxy-1,2-dioxolane (1a) under basic conditions. 3-Acetoxy-4,4-dimethyl-3,5,5-triphenyl-1,2-dioxolane (5) was also synthesized via this approach. Under acidic conditions, 3-hydroxy-1,2-dioxolane la underwent quantitative decomposition to phenol and 3,3-dimethyl-2,4-pentanedione. This competing degradation was dependent on the nature of the substituents at position-5. Methyl groups at position-5 slowed the degradative rearrangement whereas phenyl groups favored it. 3-Methoxy- and 3-(allyloxy)-4,4,5,5-tetramethyl-3-pheny dioxolanes (6, 7) were synthesized under acidic conditions from the appropriate 1,2-dioxolane precursors and the corresponding alcohols. At 60 degrees C, derivatized 1,2-dioxolanes 2-7 were found to be more stable than the corresponding 3-hydroxy-1, 2-dioxolanes. The first order rate constants for the thermolysis of 1,2-dioxolanes 2-7 were determined. Product studies showed that thermolysis of 2-5 yielded pairs of ketones and derivatized carboxylic acids. In addition to R-group migration products, an acetoxy migration product was observed for the thermolysis of 4. Thermolysis of 6 at 60 degrees C in benzene yielded methyl benzoate and pinacolone, quantitatively. Thermolysis of 7 yielded products analogous to those for 6. No evidence for internal trapping of radicals by the carbon-carbon double bond of the allyloxy group in 7 was found. The thermolyis appeared to proceed with peroxy bond homolysis as the rate-determining step. Subsequent beta-scissions of the intermediate 1,5-oxygen diradical with interesting rearrangements that show a high preference for alkyl vs phenyl migration account for the observed product distributions. The results suggest that the beta-scission/ rearrangement mechanism may not be concerted but rather stepwise to yield 1,3-diradical and carbonyl fragments.