Synthesis and Thermolysis of Ketal Derivatives of 3-Hydroxy-1,2-dioxolanes

Article abstract of DOI:10.1021/jo00101a030

3-<(Trimethylsilyl)oxy>-3,4,4,5-tetramethyl-5-phenyl-1,2-dioxolane (2), 3-methoxy-3,4,4,5-tetramethyl-5-phenyl-1,2-dioxolane (3), and 3-acetoxy-3,4,4,5-tetramethyl-5-phenyl-1,2-dioxolane (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 1a 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-phenyl-1,2-dioxolanes (6, 7) were synthesized under acidic conditions from the appropriate 1,2-dioxolane precursors and the corresponding alcohols.At 60 deg 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 deg 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 thermolysis appeared to proceed with peroxy bond homolysis as the rate-determining step.Subsequent β-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 β-scission/rearrangement mechanism may not be concerted but rather stepwise to yield 1,3-diradical and carbonyl fragments.